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UBC Theses and Dissertations

Life history of the creeping vole, Nicrotus Oregani Serpens Merriam Merry, Margaret Gertrude 1949

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L I F E HISTORY O P T H E C R E E P I J I G V O L E *  KECROTUS OREGOITI SERPENS KERRIAM "by Margaret Gertrude Kerry A Thesis Submitted i n Partial Fulfilment of the Requirements for the Begree of MASTER OF ARTS in the Department of ZOOLOGY The University of B r i t i s h Columbia April, 1949. LIFE HISTORY OP THE CREEPING VOLE, MICROTUS OREGONI SERPENS MERRtAM ABSTRACT A study of the biology of the vole, Hicrotus oregoni  serpens, was carried on on the campus of the University of British Columbia, Vancouver, B.C. It was undertaken for a f u l l calendar year and included f ie ld study consisting of an examination of the area for preferred habitats, for types of ctittings and for types of runways. This was supplemented by laboratory examination of snap trapped animals for ectoparas-ites and for the condition of the reproductive systems. Captive animals were studied to obtain additional information on their reproductive biology and ha,bits. In some respects, M.o.serpens was found to be similar to other species of I-Iicrotus, such a,3 M.agrestis, In both casea the female has a post parturient heat and a. lactation anestrum, and was also found to control the length of the breeding season. The age of reaching sexual maturity is somewhat similar. The activity pattern is also similar in that both show.increased activity at night. M.o. serpens differs from the other species of Microtus in having a longer gestation period and in having smaller l i t t e r s . In the area studied, this vole was found to be an animal of the forest edge and serai stages. TABLE OP COITTMS Page INTRODUCTION - 1 t ACKNOWLEDGEMENTS — — 3 METHODS 4 RESULTS Reproduction. Breeding season 8 Sex ratio - — —. 9 I-Iating 11 Gestation period------ 12 Average size of l i t t e r — 13 General behaviour of female •-- 15 Birth — 15 Reproductive potential 15 Effect of light - - 16 Growth of young • • • • — • - •• 18 Development • of young ---- — 19 Age of sexual maturity — 26 Age classes — - - — — —> 27 Habits — — — i - - - 34 Activity 35 Poods Cuttings —. > 38 Preferred foods • > 40 Habitats and Runways — > i - - - - - , — . 41 Parasites Pleas >---Ticks —• Kites Lice (Anoplura) Condition of Animals DISCUSSION Breeding season >—— Sex ratio — — . Age of Maturity and Oestrus Gestation period"----- — — Average l i t t e r si2e —-— Breeding potential - - — — Limiting factors Age Classes Activity Pood and Habitats — —•• suMaPY--.-.----LITERATURE CITED LIFE HISTORY OP THE CREEPING VOLE, MICROTPS OREGON! SERPENS MERRIAM SffiROBUCTION The role, Micro tug oregoni serpens Merriam, "belongs to the subgenus Chilotus of the genus Mierotus. As with a l l members of the subgenus, i t has the following characteristics: 5 plantar tubercles, small ears, close, dense fur without s t i f f hairs, flank glands when present, small. The species range i s from the Eraser Valley in southern B r i t i s h Columbia, south to the northern quarter of California, including the Olympic Peninsula and the Coast Range. The subspecies serpens i s found in the lowlands of southwestern B r i t i s h Columbia and northwestern Washington between the Cascade Mountains and Puget Sound (Hat-f i e l d and Hooper, 1935). In years of abundance, this mouse does considerable damage upon agricultural lands. This i s in large part due to i t s use of mole tunnels which form extensive networks. Consequently, a study of the biology of the vole was considered to be of importance as an aid to i t s more effective control. This study was based upon animals caught on the campus of the University of B r i t i s h Columbia. This i s on the edge of the j?uget Sound Lowlands and the Coast "Forest Biotic areas (Munro and Cowan, 1947), where the climax i s the cedar- hemlock association. In the actual study area, i&Ost of this ciimax type has been' replaced by deciduous forest. This forest represents a serai stage in the secondary succession of red alder (Alnus rubra) and broad leaf maple (Acer macrophyllumQ- towards the regional climax type. The climate in this region according to Thomthwaite (1931) i s humid, mesothermal, with r a i n f a l l adequate at a l l seasons. The average summer temperature i s 63*P. and for the winter months i t i s 38*P. The average yearly r a i n f a l l i s 57.13 inches, the average number of frost free days i s 242 and the hours of bright sunshine, 1582 per year. -3-ACIOsTOWttDGEtSEER S I wish to express my sincere gratitude to Dr. I. McT-aggart Cowan, under whose direction this study has been undertaken, for his assistance and valuable suggestions at a l l times; and to Dr. ¥. A. Clemens for his constant interest throughout the course of my work.- I am most grateful to Drs. V. S. Hoar and P. A. Larkin for their helpful advice and assistance; to Mr.-G, P. Holland for identification of the fleas; to Mr. J. Gregson for identification of the ticks; and to my fellow students for their interest and assistance in feeding the animals for certain periods. -4-METHODS Specimens of Mierotus pre-goni serpens were taken from March,--1948, u n t i l March, 1949, by the use of snap traps. At the same time, animals were l i v e trapped in the fieii. and a colony started in the laboratory for closer observation and study. Of the 107 mice captured in the wild, 102 were caught in small snapback mousetraps. Unbaited traps were placed cross-wise in runways, both surface and underground. A l l animals were removed from the traps and placed immediately in small paper bags to prevent loss of ectoparasites. In the laboratory, a few drops of chloroform were put into each bag, and a l l para-sites then removed into 70$ alcohol. The animals were weighed, measured and examined. In most cases, the skins of the"voles were made up as specimens and the skulls of a l l were kept. The internal organs were checked for any macroscopic abnormalities and the condition of the reproductive system noted. In males, the weight and volume of the testes were taken and a smear from the Cauda epididymis examined for -the presence of sperm. The colony was started from 5 animals, one taken alive in a snap trap and the other 4 caught in a drop trap. It was found to be very d i f f i c u l t to entice the voles into metal l i v e traps, possibly due to their lack of interest in bait. Consequently, i t was necessary to bury a drop trap in the middle of a runway. Since some of the larger animals can jump at least 12 inches, 18 inch lengths of stovepipe, with a piece of wire screening f i t t e d over the lower end were used* The l i v e animals were kept in wire cages. Small Cages of 15x8x8 inches were used for breeding and raising young, a maximum of 3 adult animals being placed in a cage at a. time. Larger cages of 18x12x12 inches were used to observe group behaviour. One of these larger cages was partitioned into two. This made i t possible for a male and female animal to become acquainted through the mesh, with the result that they would mate as soon as they were placed together. For the rearing of this mouse, cages opening-at the top were found to be prefer-able to those opening at the front. A cardboard nest box f i l l e d with non-absorbent cotton batt was placed in each cage. The four lower corners were cut off to provide openings for the voles. However, they 'spent a f a i r amount of time chewing additional openings and the boxes had to be replaced every few weeks. The top flaps of the boxes were l e f t loose to provide easy access to the nest.. Cleaning of the cages was simplified as the nest boxes could be removed from the cages complete -with nests and animals. It was found that i f the animals were held with their heads wedged between one's thumb and forefinger, there was seldom any tendency for them to back up, struggle or bite. Ranson (19*H) found i t possible to diagnose pregnancy in Hicrotus agrestis by pinching the abdomen with the thumb against the f i r s t finger held across the back, but, during this study, an earlier and more certain diagnosis of pregnancy was found to be possible on the basis of weight behaviour. In each cage a dish containing rolled oats and small rabbit pellets was present. The rabbit mash pellets, obtained from Buckerfield 1s, Vancouver, B.C., consist of J Protein - 14.5$ (min.^ Pat - 3.5% (min.( - Pibre - 9.0% (max., In addition, fresh green plants were supplied every day. A sufficient- amount being given so that the majority was eaten. This amount was found to Vary with different individuals and -with the same animal at different times. During the summer and f a l l , this fresh food consisted of s Leaves and flowers of false dandelion (flypochaeria radicata) Branches of., red clover tTrifolium sp.) Chickweed (s t e l l aria media) Small amounts of grass occasionally. During the winter, the clover and chickweed died down and at this time the food supplied was : Hoots and leaves of false dandelion Leaves of plantain (Plantago major) Roots and leaves of aourgrass (Rumex acetosella) Pieces of carrot. Until the middle of September, the animals were -kept at approximately outside temperature. Prom that -time onwards, the heat was on during the day. Prom the middle of November, the temperature was controlled by means, of an electric heating element and varied between 57° and 68*> P. The cages were kept in a room with a window on the south and the west wails and thus received a normal amount of light except, perhaps, in the mornings. On October 16, some animals, 3 females and 2 males, were designated as controls and vrere given only the normal length of daylight. Others, consisting of 6 females and 4 males, were subjected to increasing periods of illumination. Regular daily increments of 3 minutes were made* Light was supplied by two goose-neck desk lamps with 40 watt bulbs, placed from 8 to 12 inches from the cages. • . An activity cage, similar to that described by Morris " (1944), was set up. Various ages cf animals in different con-ditions were placed in this cage for approplate lengths of time. It was found to be very useful for determining the exact time and duration of the birth of young, a, very characteristic pattern being produced at this time. -8-RESUMS Reproduction • - Breeding season,- In 1948, some females had mated at the beginning of March while a few had not mated by the middle of the month. However, by April 24, a l l adult females captured had had at least one li-tter, as shown by the presence of uterine scars, fto special examination-'of the male for sexual condition was made at this time. A comparison of the measurements" of the testes with those of animals known to be fecund, however, indicates that a l l male animals captured from March 14 were sexually active. The breeding season must therefore have started at the beginning of March and was well under way by the end of the month. Implantations were observed in young females caught July 22 and 24. After this, no young animals, although of a similar size to those bearing young earlier in the year, gave any sign of sexual activity. In the laboratory, females born July 12 and placed with a male August II, f a i l e d to have l i t t e r s . Thus i t seems that female animals not old enough to mate by the end of July, did not reproduce during 1948, although the females reaching maturity before that time continued to breed for at least two months longer. A similar tendency was evident i n the males. Young animals captured during the l a s t week of July, although the same size as fecund males caught one to two weeks previously, were not sexually mature. The youngest, mature males were found to be present during the f i r s t 3 weeks of June. Evidently, then, only the f i r s t half of the l i t t e r s born in a season w i l l breed during that same season. Adult females in captivity continued to breed until the middle of September. Females trapped on and after September 30 contained only^scars. However, a young male was captured December 6 that was about a month old, thus indicating a f e r t i l e mating i n the middle of October. It therefore appears that adult females continued to mate until the end of September and the f i r s t part of October in a few cases. "Fertile males were captured periodically throughout the f a l l u n t i l November 23. Apparently ovulation ceases i n females of this species at an earlier date in the season than does spermatogenesis in the male. It i s thus the female that controls the length of the breeding season. In the spring of 1949, breeding started in the wild earlier than i t did the previous year. A female that had just had young was taken on March 10. This mating must have occurred prior to February 16. Furthermore, a female was taken on March 9 which, judged by weight and the condition of the external genitalia, was juvenile. On the basis of weight, this animal was probably bom about the second week of February and conceived about the third week of January. These instances provide clear evidence of late winter breed-ing i n 1949, a condition that apparently did not exist in 1948. Sex ratio.- The data upon \tfhich the sex ratio was determined were derived from two sources. Sex ratio at birth Sex ratio at birth was obtained from 57 young born in -10-captivity (Table l ) . In this group i t i s 51$ males and 49$ females ('x*=»018). In 105 wild caught animals, there were 9 more males than females (Table 2). This gives a sex ratio of 54:46 (%*=.771). However, neither the sex ratio at birth nor that of animals of a l l ages-departs to a s t a t i s t i c a l l y significant degree from a 50:50 ratio. Table 1. Sex distribution at birth in captive animals. Jul Avig Sep Oct Hov Dec Jan Feb Mar Total Male 4 0 1 1 - 2 8 IS 1 29 Female 2 3 3 2 - 2 8 7 1 28 57* Table 2. Sex distribution in wild caught Mierotus sexually mature and sexually immature, regardless of age. Sexually mature Sexually immature Total Male 32 25 57 Female 31 17 _48 105 Table 3. Sex distribution of animals more than and less than 30 days of age. More than 30 days Less than 30 days Total Male 41 15 56 Female 34 14 48 104* If the juvenile animals i n the wild, caught sample are segregated (Table 3) by using as a criterion for juvenility weights less than 18 grams for males and 18.5 grams for (Fl\s>. 13. a n d 1 3 ) females/it can be seen that the slight excess of males, although not significant, i s due to immature animals over 30 days of age. The majority of these animals were captured -11-during the late summer and early autumn months* If these subadult males, at this time, have larger home ranges or more wandering tendencies than the other animals, they would he more subject to capture and would thus tend to distort the 50:50 sex ratio. Mating.- When a mated pair are together in a cage, the time of mating i s marked by several periods of vigorous squeaking. These last for 10 to 15 minutes at a time, and recur at half-hourly to hourly intervals for as long as 5 hours. Copulation takes place near the beginning of the series of vocal periods. It may be repeated several times during not more than two successive periods, but after this, the female rejects the male even though pursuit continues for some time. The vocal demonstrations are most pronounced during this period immediately following copulation. The actual coitus occupies not more than 2 or 3 seconds. The use of the divided breeding cage made possible closer observation of the details of mating. When the female was i n -12-heat, she would become more, active and both she and the male would climb up and down en opposite sides of the partition, • frequently squeaking at the same time. I f , then, the connecting s door was opened, one would enter, sniff about, and dieover the other animal. This was followed-by a few minutes of chasing each other* Copulation took place within 5 to 10 minutes and was repeated several times u n t i l , within about 30 minutes, the female began to fight off the male. At this time they were usually separated. Following i s an excerpt from the record made at the time of one mating: Female - A, Hale - K 22 2 2 2 2 2 2 2 2 2 3 3 20 - Door opened. 25 - K into A1 s cage, sniffing around. 26 - K into own cage, running rapidly around. 28 - It into A fs nest, squeaking, chasing, 29 - Mating. 31 - Mating. 32 - K into own cage, running rapidly back and forth. 36 - K back and forth from one side to the other. 40 - Mating. Then from'one cage to the. other. 56 - K chasing A, ho mating. 59 - A would not allow K! to enter her nest box. 03 - K into own nest. 09 - Door shut. Gestation •period.--The gestation fte'riod was determined to be from 23 to 25 days. Exact times obtained were 23&, 231/3» 24^, and 24 days. This gives an average of 23.8 or approximately 24 days. Lactation does not alter the length of gestation. Mating i s possible immediately following birth. However, i f this i s not successful, the females do not mate again until after the young are weaned. The winter anestrum i s accompanied by closure of the vulva. It may also close or remain open subsequent to mating, with or without pregnancy. -13- : Average size of litt e r ; . - Twenty-eight l i t t e r s were "born in captivity. The range in the size of these l i t t e r s was from 1 to 5 and the average l i t t e r size was 2.79. The number of young was also obtained from embryos and from uterine scars in trapped animals. Table 4 gives these figures, the number of scars was used only when no embryos were present that might obscure some of these scars. Table Number of embryos and scars i n trapped animals. Left uterine horn - Total -,1,0,0,1,3,2i1,0,2,1,0,1,3,1,1,2,3,0,1,2,0,3,-,1,0,0, 29 Right uterine horn --,3,4,5,2,1,1,4,3,1,3.,3,1,1,2,2,1,1,3.,2,0,2,0,-,1,1,2, 49 Total 3,4,4,5,3,4,3,5,3,3,4,3,2,4,3,3,3,4,3,3,2,2,3,3,2,1,2, 84 The range of l i t t e r size was the same as that found in the caged animals, from 1 to 5, and the average l i t t e r size was 3.11. The average of the two means was 2.95 or almost 3. which was found to be the most frequent l i t t e r size in both captive and wild animals. This i s illustrated in Pig. 1, B WILD Q C A P T I V E I 2 3 LITTER SIZE Pig. 1. Frequency of l i t t e r size. -14-The average number of young per l i t t e r increases with successive l i t t e r s bom t& the female. From Table 5, i t can be seen that there i s a steady increase in mean l i t t e r size from 2-3 in the f i r s t l i t t e r to 4-5 in the fourth l i t t e r . Table 5 . Number of young i n consecutive l i t t e r s . L i t t e r No. of Average Range number l i t t e r s x 1 12 2.3 1-4 2 8 3.1 2-4 3 4 3.3 3-4 4 2 4.5 4-5 However, an examination of the range in l i t t e r si2e for each group, indicates that the increase in the average i s due to a decrease in the number of smaller l i t t e r s i n subsequent births rather than to an increase in the actual number of young per l i t t e r . Only one animal has had f i f t h and sixth l i t t e r s to date, and these have both consisted of but 2 each. Thus i t i s possible that the average l i t t e r size would decrease as the females approached senility. Table 6. Mean l i t t e r size in the middle of and towards the end of the breeding season. June July Sept. Average l i t t e r size - 3.3 3.1 2.5 No. of females examined - 9 8 4 Jameson (1947) found that l i t t e r s in Microtus ochrogaster averaged smaller at the beginning and at the end of the breeding season. The average l i t t e r size in M. o. serpens also decreased towards the end of the breeding season (Table <o) • An examination of 3 animals caught in March, having ah average l i t t e r size of 2", indicates that the number of young per l i t t e r -15-i s possibly less also at the "beginning of the season. General behaviour of f emale>- Usually the animals leave the cotton in their nests in lumps, but a few days before the young are born, the female shreds the cotton and makes a spherical chamber about 6 to 7 cms. in diameter, with an entrance at either end. In this-hollow the young are bom and can be found there together u n t i l they become active. This makes i t easy to find the young and rearrange the nest after examination. Only one female was bothered by this and pushed the young out of the nest box for 5 to 10 minutes while she readjusted the cotton. It was found to be possible to handle and examine the young within an hour of b i r t h and every day following without.disturbing the mother unduly. When the young f i r s t venture out of the nest box, they are frequently carried in again, particularly i f any disturbance takes place near the cage. The female w i l l grasp them by that part nearest to her, l i f t her head, and proceed. Only one l i t t e r was eaten and this' was born in a cage with 3 other females and 2 males present. Evidently the disturbance caused by the other adults was too much. However, l i t t e r s have been raised successfully with the male and another adult female present. Birth.r The time and duration of b i r t h were determined by the use of the activity cage. Under normal conditions, a pattern i s produced showing alternate periods of rest and activity. During the birt h of young, a relatively inactive period of from to 5 hours i s recorded (j?ig. 17, patterns A and D). This time may not a l l he spent in the birth of a l i t t e r as a normal inactive period may occur either just before or after and this would obscure the true bir t h time. However, the average time of birth and consequently the average age of the young i s known. Table 7. Duration of birt h of l i t t e r s . Ho. of young jfo. of l i t t e r Time Duration 3 2 12:30p.m. 2 hrs. 3 3 . 7:00a.m. 2£ 3 2 11:00p.m. 5 5 4 10:30a.m. 5 3 2 3:30p.m. . l£ 4 2 1:00a.m. 4 It i s apparent that the birth of young does not take place at any particular time of the day. There i s also slight, but positive, correlation between the period of inactivity and the number of young. He-productive potential.- Most of the females were not l e f t with a male for a prolonged period. However, two animals were with males continuously for 131 and i32 days and both in that time produced 3 l i t t e r s . Following are their records: Date Time Nov 9 with male Dec 8 1 young 29 days Dec 8 mated Jan 2 4 young 25 days Feb 25 mated 54 days Mar 21 3 young 24 days Date Time Nov 5 with male •* -Jan 9 •* 4 young 65 days Jan 26 mated 17 days Feb 20 2 young 25 (24^ -) days Feb 20 mated Mar 16 2 young 24 days 132 rsr Thus both animals produced 8 young each in almost 4^  months. Effect of l i g h t . - Due to the small number of l i t t e r s born during the summer, i t was deemed necessary to try to bring -17-about winter breeding by an increase in the duration of iight.-Starting October 16, when the time from sunrise to sunset was 10 hours 48 minutes, lights were turned on for an additional 3 minutes every night* A daily increase of 3 minutes was chosen because the days were getting shorter by that amount at that time and i t was assumed that the reverse would be true in the spring. Pour males and six females were subjected to this additional li g h t while two males and three females were covered 20 minutes after sunset to act as.controls. Both groups were held in similar temperature conditions. Beginning January 20, a l l animals were given the same amount of ligh t consisting of 14 hours. Prom then on, the lights were l e f t on until 10j00 p.m. every day and this gave a slight increase as the days lengthened. Two females had had l i t t e r s prior to the start of the experiment. One of these was placed in the extra light and the other, in the normal light cages. The one receiving extra light was the f i r s t to have a l i t t e r , mating November 11, almost 4 weeks from the beginning of the experiment* Table 8. L i t t e r s born under normal and extra li g h t conditions. No. of Date No." of Pate Time from Time from' l i t t e r born young Oct". 16 previous l i t t e r 3 Dec 5 .4 Eaten 50 days 1 Dec 8 1 Died 53 days 1 Dec 12 1 Died 57 days 1 Dec 20 2 Died 65 days -1 Dec 21 4 Lived Mating controlled 2 Jan 2 4 Lived - 25 days 4 Jan 25 5 Lived - 51 days 2 Jan 26 2 Lived - 37 days 1 Peb 2 2 1 died 109 days 2 Peb 11 4 Lived - Mating controlled 5 Peb 20 2 1 died - 42 days L i t t e r s from animals given extra l i g h t . -18* L i t t e r s from animals receiving normal l i g h t . Ko. of \ Date jfto." of Fate Time from Time from l i t t e r horn young Oct. 16 previous l i t t e r 1 Dec 29 3 Died 74 days 1 Jan 8 1 Died 84 days 4 Jah 9 4 3 died 65 days(with male Nov 5) 2 Jan 22 3 Lived - 33^ - days 2 Feb 3 2 Lived - 25 days Table 8 shows the dates at which the experimental and control animals had l i t t e r s . A l l but one of the six experiment-al animals had produced a l i t t e r before the f i r s t control animal gave birth. Males in both the normal and extra l i g h t cages attempted to mate November 11. This resulted in one l i t t e r in the experimental-cages. In the control cage the mating was not f r u i t f u l and no-.more attempts were made until one on December 5, which was successful. Growth of young,.- The weight at ftirth and growth rate were recorded for fourteen l i t t e r s , including 4i individuals. The young at birth weigh between 1.5 and 2.2 grams, the aver-age being 1.7 grams. For th f i r s t 4 days the rate of gr&wth averages about 6.4 grams per day. After this until:.this animals reach the age of 20 days, their.growth is most rapid, averaging 0.7 grams per day. After 20 days, the rate begins to diminish and by 40 days they have- reached average adult size (Fig;. 2). This applies only i n the height of the breed-ing season, however, as increase in weight i s more rapid when the amount of lig h t i s greater. At other times, the females w i l l take up to 50 or 60 days to reach an adult size of 20 to 24 grams. The males reach 20 to21 grams in 40 days but remain at that weight until the breeding season is well advanced when they increase in weight to the 28 to 31 grams of the mature breeding male. j AGE IN DAYS  Big.2. Growth and development of young. This graph shows the mean growth curve for the average of 14 l i t t e r s and the standard deviation from the mean. An examination of S'ig.2 shows that the range of weight at birth i s very small,there being no apparent difference between weights of individuals in different sized l i t t e r s . Gradually, however, the range increases until , after about 36 days, the x^eight becomes more or less independent of age. • Development of young.- At birth the young are completely naked, pink, with eyes and ears closed and the pinnae folded forward. By the f i r s t 12 hours down-like whiskers have appeared and the back i s turning grey (j?ig.3). At the end of 24 hours, -20-the pigment on the hack has become black and the f i r s t fuzz has appeared on i t . By 2^ days, black pigment has spread to the upper surfaces of the legs and the t a i l , and at the same time, the pinnae begin to separate and to unfold. At the end of 3^- days , the color of the hair on the back has become evident and the incisors are visible although not yet erupted (Fig.4). The f i r s t sparse fuzz appears ventrally at abtout 3-ij}- days, 2 days later pigment has formed on the anterior region (Fig. 5), and by 8 days i t has spread posteriorly and the animals are completely furred (Fig.6). The incisors erupt at about 5-jjr- days, the lower ones f i r s t . The young can crawl slightly by 6fc days. The ears open at about 10 days (Fig.7). From 10 to ll£ days the eyes open (Fig.8), those of older individuals in a U t t e r opening f i r s t , and frequently the right eye preceding the l e f t . As soon as the eyes open, the young hegin to amn about outside the nest box: and their attitude changes from one of apprehension to curiosity. This is illustrated by a comparison of Figs. 8 and 9. The molars appear about ll£ days, the animals begin nibbling on greens at 12 days and are able to leave their mother by 13 days. However, at from 2 to 4 weeks, they are not as cautious as are the older animals. It i s possible, by moving slowly, to open the cage door and touch them before they dart away. If the animals raised in the wild react in a similar manner, they would be "toery vulnerable to predation immediately after leaving the nest. Fig. 4. Young at 3£ days. Average weight - 3.4 grams. F i g . 5 . Young aged 7 day3 8 hours. Average weight 5 . 5 grams. F i g . 8. Young, two w i t h eyes open. Age 11 days 10 h o u r s . Average weight 7.7 grams. -25--26-The fur of the young i s greyish in color^ dor sally, as compared to that of the adults which'is brovm* The post juvenal moult takes place at about 50 days and starts in the middle of the back spreading cephalad and caudad, the rump and the head being the last to change color.• Several young were born with a white patch of fur on the crown of the head. This was, in a l l cases, present only in the juvenal.: pelage and disappeared at about 3 5 days. Another characteristic noted in animals born in captivity was a kinked t a i l . This was present in two animals, one a male and the other a female* born to the same parents in subsequent l i t t e r s . Age of sexual maturity.- It was not found possible to t e l l the sex of the young untii they were from 22 to 26 days old. As i t was thought that the females, particularly, might reach sexual maturity before this age, a mature male was placed in the cage with a l l the young of one or several l i t t e r s when they reached the age of 18 to 20 days. At this time the young males were not old enough to bother the adult animal and they usually settled down together in 1 or 2 hours with very l i t t l e squeaking. One or two young males were placed with each of the mature females as sOnn as their young were weaned. Males placed together before one @f both are mature do not fight seriously, eaten in the presence of a female, either at the time of when they reach maturity. However, the adult females bite and chase the young, males when they are f i r s t placed together and i t i s usually -27-several "hours before the young ones can safely venture into the nest box. Young females were receptive to a. male when 22 and 24 days old, However* the youngest anmmal to become pregnant mated when 27 days old and other young females did not conceive until 35 and 36 days of age. Some animals received a. decreased amount of extra light and they were a l l over 7 weeks of age before they mated successfully. Young males were f i r s t observed mating when between 34 and 38 days old. None of these attempts *iere successful. Sperm began to appear in the epididymis as the aninmals reached the age of 6 weeks but did not become abundant until they were 7 to 8 weeks old. Age Classes.- J?or young anmmals, Up to about 20 grams, the approximate age can be obtained by reference to the growth curve, Figs. 12 and 13 give the mean and standard deviation curves of the growth of 10 male and 16 female animals of known age. These shov/ that when growth is relativ-ely rapid and the slope of the curve i s correspondingly steep, an animal of. a given weight can be aged to within about a week. As age increases, however, the growth curve flattens, the range increases, and anmmals over 21 grams may be any age cove"r; 3S;diays. This i s true of the curves for both males and females. - a s -io 20 30 40 50 A G E I N D A Y S Pig. 12. Average growth curve of 10 males. M e A n t w r v t a.tnd stand a.*- J c f e v r a f i o n . f r O f n t^e- m t a t i . After the animals reach 21 grams, individual variation becomes very marked. The mean.curve for the females reaches a plateau at about 40 days. The subsequent increase.1 was, in a few cases, due to pregnancy, but almost a l l of the animals began to gain weight after 50 days. This slow increase continues throughout'their liv e s with, of course, abrupt changes due to pregnancy and lactation. -29-j?ig. 13. Average growth curve of 16 females. Mean tuLv-vt and stand arc/ devia tion. Jrom 1"ke_ *nr>co-n. The males, on the other hand, gain weight steadier until mature. At this time i f they are isolated and not allowed to "breed, they w i l l not increase past a certain weight until placed v/ith a female, when they w i i l make a gain of 5 to 10 • grams. Upon subsequent isolation they w i l l lose their extra weight. This i s illustrated i n "Pig. 14. -30-I 151 | I I I I — 4 — + — H — I ! 0 10 20 30 40 50 60 70 80 90 100 110 | D A Y S  Pig. 14. Change in weight of male when isolated. This variation makes i t impossible to age the adults from weight alone. However, i t i s possible to distinguish the older, breeding-adults by means of their weights. That i s , these animals, during- the breeding season, are almost a l l over 25 grams. JFig. 15 gives the percentage of animals 25 grams and over caught each month. The number of older animals i s seen to decrease rapidly as the season progresses. -31-Fig. 15. Percentage of adults over 25 grams Caught each month. 18 CM. 3 0 21 2 2 £ 3 2 4 25 CONDYLO-BASILAR LENGTH Females mature. ? immature. Males mature, d* immature. Fig. 16. Skull measurements of trapped animals. lamination of the size of the skulls of mature animals shov/s that the youngest were caught in June. As a result of work on the age variation in Microtus -32-montanus yosemite, Howell ( 1 9 2 4 ) has stated that the condyle-b a s i l a r length i s the most satisfactory measurement. Taking this length for animals captured from May 20 to July 24 (the time during which the majority of animals were trapped) and plotting i t against the date caught, gives the pattern in "Fig. 1 6 . It i s apparent that 3 groups of animals can be identi-fied. If most of the females started breeding at about the same time, these groups would be from the young of certain l i t t e r s of the season. "From the weights of the younger animals, an animal with a condylo-basilar length of 1.9 cm. was deter-mined to be about 14 days old. Thus, the f i r s t group of animals would have been born about the middle of Ap r i l . Since breeding started at the beginning of March, the three groups would be from the second, third and fourth U t t e r s of the season. The third and subsequent l i t t e r s would probably tend to be obscured by females producing l i t t e r s Out of phase due to failure of adults to breed immediately after birth and some young females that would take longer than 24 days to reach maturity. However, i f "lines of best f i t " are plotted for these three groups, those of the f i r s t two have about the same slopes indicating about the same rate of development. They are also about 25J- days apart, which i s almost 23.8.days, the average length of the gestation period as determined from the captive animals. It therefore appears that this skuli measurement reflects the relative age during the l i f e of the animals. -33 However, since the skulls do not increase in size indef-initely and the growth slows with age, the means would not form a straight line hut would curve downwards. The degree of this curvature would have to be known in order to use this skull measurement to obtain absolute age. The weight and volume of the testes of both mature and immature animals were measured in an attempt to find a value which, taken in relation to the body weight* could be used to differentiate fecu?d from non-feculid males. There \>/as found to be a large difference in these values during the middle of the breeding season. However, at either end' of the season, there was very l i t t l e difference between the weights and volumes for mature and immature animals (Table 9). The ratios of t?.e weight of the testes to the body weight of the two classes overlapped to a slight extent. But the'ratios of the testes volume to the body weight of the two classes, while very close, were distinct. A value of 3.25 X 10~3 for Volume of testes (cc) .separated the 40 animals for which Body weight (gm) measurements were obtained into tv/o grpups. Table 9. Ratio of testes size to body weight for fecund and non-fecund males. Fecund males -range in size Volume .» Weight * body wt? l u body w t l P - u Jun-Jul 5.5-16.0 5.5-15.9 Aug-Oct 3.7-10-.0 3.5-10.1 Nov-Bec 3.5-4.9 3.1-4.6 March 3.3-7.2 3.0-7.3 Hon-fecund males -largest values at a l l seasons Volume .j^o' 5" Weight ^ Q 3 * body wt.' 3.2 3.0 2.9 2.3 1.9 1.7 ' 1.7 body wt." 3.1 3.2 4.1 1.8 1.8 2.0 1.4 -34-Habita The most noticeable feature of the Creeping Vole i s i t s activity and curiosity. When placed in a new cage, or nest box, or any different object other than food i s put i n i t s cage, everything i s examined several times. There i s no attempt to avoid a new object. In this connection, i t w%s found that, for some animals that were shy of strange articles, observation of the number oi?&ays needed to become used to a new object corresponded with the number of days traps needed to be set before the majority of animals were taken. As M.o. serpens shows no fear of a foreign body in i t s environment, i t would be expected that the majority of animals would be caught the f i r s t trapping night. One small experimental plot, 14 yds. by 14 yds., was-trapped completely during the f i n a l week in October. The .animals caught each night are shown in Table 10. Table 10. Animals caught on a 14 yd. square plot. Hicro'tug Night o.serpens fSownsendii Sorex J?eromyscus Neurotfichus Total 1- 4 - 2 3 2 1 12 2- 2 0 2 1 1 .6 3- e i e i e 2 4- 1 0 0 1 0 2 5- 0 0 0 2 i 3 6 - 0 0 0 1 1 2 7- _e_ _o_ • i _ _o_ _i > 3 3 . 5 ~9 4 28 . Pour of the seven Creeping Voles were captured.the f i r s t night. A l l Sorex were removed the f i r s t two nights-and both species of Microtus were trapped completely in 4 days. -35-Peromyscus and Neurotrichus* on the other hand* were caught periodically throughout the trapping period* This indicates that they wandered onto the plot and presumably either have much larger home ranges than the other animals or that the "new object" reaction was prolonged for these animals. Some animals were alarmed mors by certain stimuli than others. A l l darted for cover i f any rapid movements were made within about 6 f t . of the cage. Slow movements were not so effective unless very close to the animals. Possibly this may be partly due to the voles becoming used to people moving around near the cages. Loud talking and most noises had no effect but the rustling of dried grass brought about an instant reaction. In this connaection, Hatfield (1935) mention-ed that his catch of mice on,©, windy night was about 15$ less than the catch from a similar number of traps set out during a quiet night. Possibly fear of a rustling sound i s associated with escape from psedators* Vibrations caused by banging attracted attention but did not cause the animals to run into their nests as they usually do when startled. Some animals stored pellets and rolled oats in their nest boxes. Usually those l i v i n g alone collected a larger stock than did 2 or morefohen l i v i n g together. Activity Charts obtained from the activity cage gave a very general picture.of an animal's activity periods throughout a week. The scale was too small to record every separate movement as such and consequently many small inactive periods were obliterated. A l l animals tested were found to have short rhythms of 1 to 2 hours and a longer 24 hour ihythm. The shorter periods usually occurred at intervals of 1 or 2 hours and varied i n length c ohs icier ably. Vith a l l but one of the animals a period of reduced activity, consisting of shorter periods of move-ment and less strenuous movements, occurred during the middle of the day. The. length of this period of lowered, activity varied for different animals. In general males tended to show a larger decrea.se than the females, some of the l a t t e r showing hardly any change i n activity throughout the entire .24 hours, particu.la.rly when pregnant. One female reversed the usual rhythm and became more active between sunrise and sunset (Pattern C - Pig. 17). This pattern was constant except for one night when she was active almost constantly. During this period, -she was believed to be about to have a l i t t e r but when removed from the activity cage her weight had dropped again. It i s believed that her l i t t e r was aborted and eaten. In an attempt to develop an expression of total activity the active periods for 24 hours were added together and percentage* expressed as a of the whole time. In the males, total activity varied with different individuals and may be correl-ated with age. Two young males/6f 5 and 7& weeks (Pattern E) averaged about 50$ activity, one 9^- week old male averaged 60$ and an old, mature: .animal had about 80$ activity (Pattern p). Pattern E also illustrates greater activity following -37-sunset and subsequent reduction of activity during the night. This tendency was noticeable in a.few cases only. Similarly the total -activity of the females averaged between 40 and 50$, with an old female averaging between 55 and 58$. Pattern B shows the rhythm of a female 3^ months old. The activity of this animal suddenly decreased from 50 to 40$ in the middle of the pattern. The activity of pregnant females appears to be higher, ranging from 54 to 70$ with an average of 60$. There i s an increase in activity, up to 76$ for about one day preceding birth. The time of the birth of a l i t t e r i s indicated by X in Patterns A and D. Parturition i s marked by a period of relative inactivity broken by a few short movements only. The period of nursing i s characterized by f a i r l y regular 2 hour active periods of short duration. It may take up to 2 days from birth for the female to develop this pattern, however, with some animals, the rhythm changes immediately (Pattern D), while the rhythm of a few remains approximately the dame both before and after birth. The total activity in the day following birth i s between 49 and 70$ and this gradually decreases to the 40 to 60$ of the nursing period. -38-"Fig. 17. A c t i v i t y p a t t e r n s of a n i m a l s . Foods C u t t i n g s . - The C r e e p i n g V o l e , as o t h e r I l i c r e t u s . i s a w a s t e f u l f e e d e r . The an i m a l s c u t many l e a v e s and steins, c u t t i n g the l o n g e r p i e c e s i n t o s e c t i o n s of from 1 t o 14 cm. and a v e r a g i n g about 3 cm. Some of thes e a re e a t e n , some p a r t l y eaten and the r e s t are s c a t t e r e d a l o n g t h e i r t r a i l s . Only a few c u t t i n g s are fo u n d underground, t h e m a j o r i t y are l e f t on the s u r f a c e , o r , i f the s u r f a c e c o v e r i s not v e r y dense, may be c o n c e n t r a t e d i n the e n t r a n c e t o an u n d e r g r o u n d t u n n e l . The s u r f a c e runs a r e - s c a t t e r e d w i t h c u t t i n g s but they t e n d t o be c o n c e n t r a t e d at f e e d i n g s t a t i o n s , s l i g h t l y e n l a r g e d a r e a s i n t h e runs where most of the f e e d i n g appears to take p l 8.ce. These spaces are u s u a l l y f r o m 3 to 4 i n c h e s i n di a m e t e r and 2 to 3 i n c h e s deep, f o r n e d by the g r a s s and o t h e r c o v e r a r c h i n g over t h e space made by the a n i m p l s . The -30-ground i s covered \\tith cuttings and fecal pellets and many l i t t l e t r a i l s radiate- from these centres to the favourite food plants. T a l l stems are cut at an average of 3 inches from the ground, evidently about as high as the animal can con-veniently reach. In any given area the plants most eaten tended to be those most plentiful in that area. However, by far the favor-ed food was false dandelion (Hypochaeris radio at a). The flowers were the most preferred part but the young shoots and leaves were also important, and i n any place where these plants were growing, t r a i l s of cuttings of the stems l i t t e r e d the ground. In an area of second growth vegetation large clumps of false dandelion were ple n t i f u l . Of these plants, with a suitable amount of cover around the base, i t waa estimated that 70$ had been sought out and more than half of the flower stalks cut in each. Oniy a fev leaves had been partly eaten when the flowers were plentiful at the end of June. As the seasons progressed, and the flowers became more scarce, more leaves were cut. In this area during June and July, when the false dandelion growth was heaviest, no cuttings of any other plant were found. On a grassy path through young coniferous second growth the plant forming the bulk of the cuttings was Clover(Trifol-ium sp.). While in an area, containing mixed herbaceous growth, cuttings were of the following composition -Hypochaeris radi cat a - 42$ Red Clover (Triflolium sp.) - 27$ Grass Chickweed (Stell.aria-• media) - 25$ Grass cuttings were much more abundant in areas containing few other plants and became more common in the autumn. Also found eaten in small quantities where available were plantain (Plantago major L) and miner*s lettuce (Claytonia ;perf oliata). Due to the excessively wet summer* a l l these plants remained green and succulent throughout the season and were used for food until k i l l e d by the frost in the late f a l l . The false dandelion plants remained green a l l the year and the leaves of these and of grass formed most of the cuttings in the late f a l l , - During the winter, snow covered the ground until the beginning of March. Plants eaten under i t s protection included chickweed, leaflets of sword fern (Bolystichum muni tarn) the fronds of this being bent to the ground by the snow, leaves of t r a i l i n g blackberry (Rubus  macropetalus) and false dandelion. During the whole year a l l herbaceous roots and under-ground stems growing into used Underground tunneis were clipped off and cuttings of these were found in the under-ground runs. Particularly during the late autumn and winter these became an important source of food. During the winter, in patches where the snow had melted,it was possible to pull up the tops of many false dandelion plants and find the roots completely eateii. Preferred foods - Animals when presented with a choice of foods whould select the plants i n the following order --41-Plowers of Hypochaeris, leave's of Hypochaeris* clover* chick-weed, roots of Hypochaeris, dandelion leaves (Taraxacum sp.), plantain leaves, and sword fefih. The last was eaten .in a few cases and only as a lsjst resort. . Habitats and Runways. H.o. serpens was not found where coniferous trees formed a dense canopy nor in the open grasslands at any distance from the woods. The area i n which these animals were most abundant was a' second growth stand of cedar and ,kemlock, cleared about 25 years ago, i n which both deciduous and coniferous trees, some of the latt e r planted, have now come in. The ground cover i s mostly l i t t e r , chiefly deciduous leaves, bracken and local areas' of coniferous needles. The Kerb layer, comprising about 60% cover, consists of extensive clumps of sword fern (polystichum muni turn), salal (Caultheria  shall on), blackberry (Rubus mac r op e t al &us), a few Oregon grape (Mahonia nervosa), and grasses along the paths. In the shrub layer salmon berry (Rubus spectabilis), and thimble berry (Rubus parviflorua) formy&ense tangles and are present up to a height of 7 f t . throughout the whole area. Young Douglas f i r (Pseudotsuga taxif ol&a)? western red cedar (Thu.ja  pi i cat a) , western hemlock (Tisuga heterophylla) and occasional yew (Taxua brevifolia) are scattered through the open parts. Between the species forming the dense cover are regions of lighter cover consisting of braeken (Pteridium aquilinum), -42-f l o w e r i n g dogwood (Cornus I T u t t a l l i i ) , s p i r e a ( H o i o d i s o u s d i s c o l o r ) , r e d h u c k l e b e r r y ( V a c c i n i u m p a r v i folium)« e l d e r (Sambuc us racemosa), snowberry (Symphoricarpos a l b a ) , r o s e (Rosa Hutkana) , a l d e r ( A l n u s ru" ra) and maple ( A c e r macro-phyllum) s a p l i n g s . There are 3 t y p e s i n the t r e e l a y e r . F i r s t are about 25 f t . Douglas f i r and hemlock growing c l o s e t o g e t h e r and dense to w i t h i n a few f e e t of the ground. Second are mature a l d e r and maple, f o r m i n g a p a t c h y canopy. T h i r d are a number of mature f i r , b a l s am ( A b i e s g r a n d i s ) and c e d a r . I n the f r o s t f r e e season, the g r a s s y p a t h s i n the woods ( F i g , 18) and t h e g r a s s bands a l o n g the edges of the woods ( F i g s , 19, 20) are b o t h i m p o r t a n t f e e d i n g a r e a s . I n t h e w i n t e r , the an i m a l s are more numerous i n the woods. F i g . 18. W i n t e r and summer v o l e h a b i t a t . The a n i m a l s f e e d i n the g r a s s y p a t h i n the summer and - 4 3 -j ? i g . 19. Edge H a b i t a t . A n i m a l s p r e s e n t a t a l l seasons. P i g . 20. Summer h a b i t a t (edge of woods). This area supported a'very l a r g e p o p u l a t i o n during the breeding season but very l i t t l e s i g n was present the follow-i n g s p r i n g . In the al d e r and maple areas, there i s m a E k e d seasonal aspection. Although the trees and shrubs are bare during the winter and s p r i n g , good cover i s provided by the m a j o r i t y of the herb l a y e r which i s evergreen. A l s o , at t h i s time, the animals appear to spend most of t h e i r time underground. The preference/of I I.o. serpens f o r the f o r e s t edge can be i l l u s t r a t e d by P i g . 21. This represents an experimental p l o t that was trapped. P o s i t i o n s where Creeping v o l e s were caught are i n d i c a t e d by a V, and a T stands f o r a Townsend vo l e ( I I . townsendii townsendii) . -45-FIR FIR M A P L E ^ r " v v1 HOLODISCUS < R U B U S DISCOLOR « P A R V F L O R U S M A C R O P E T A L U S . VV T T T G R A S S < IH- y d s . *• Eig. 21. Edge preference of M .C. serpens The main cover types are indicated. A l l M.o. serpens were caught in underground runways, and, of the 7 animals C a u g h t , 5 were on the edge. The M.  townsendii are essentially grassland animals and were captur-ed in surface runs formed under the long grass clumps. The Creeping Voles make two kinds of runways. Wnder-ground tunnel systems are not very abundant as these animals use mole runways t o a large extent. In sandy loam, part of a system consisted of a network of tunnels from 4 to 6 inches.deep and not more than 12 inches apart. There were a few short blind ends and many surgace openings. The area covered wets about 70 sq. f t . and the tunnels in i t consist-ed of about 70 linear feet. Usually, surface openings consist of a short extension from a through tunnel. If one of these i s opened and a trap aet in the inner tunnel, a vole i s usually caught the f i r s t or second night. Surface runways radiate from the openings. They are not usually well marked except that the grass i s pressed apart and a line of cuttings and fecal pellets covers the ground. In the denser grass cover, enlarged feeding stations are found at intervals. Of the animals trapped, about 40$ were caught on the surface and most of the traps were placed near the entrance to a tunnel. Runway systems on either side of a path are usually connected by an underground tunnel, usually that of a mole. These intercoimminieating runs are used continually by many small mammals. Parasites. Pleas. - The only f l e a found on the Vole and not on any other animal was One bird flea, Dasypsyllus galiinulae  perpinnatus (Bak.), accidental on Microtus. Those whose normal host i s Microtus are Atyphloceras multidentatus (Pox), Hystri- chopsylla occidentalis n.sp., Delotelis telegoni (Roths.), Megabothris abantis (Roths.), Peromyscopsylla selenis (Roths.^ Other flea,s normal on Peromyscus and insectivores were also present i n limited: numbers except for Oatallagia charlottensis (Bak:.), common on Peromyscus. tha.t was the third most plenti-f u l species on M.o. serpens. -47-Table 11. Pleas found on M.o, serpens. and i t s associates. . Pleas (see note) Hosts A. B. C. D. E. P. G. S. I. J. K. L. M. H. 0. M.o. serpens 1 4 1 - - - - - 1 - - - - 3 -Mar.14-Apr.24 M.o. serpens - 7 1 5 - 2 1 2 1 1 2 1 - 1 1 7 -May 11- Jun.21 M.o. serpens Jun.22-Aug.28 - 6 . 14 - 2 - - - 22 - 1 - - 19 -M.o. Serpens; Sep.25-Dec.6- 8 13 2 9 - - - - 35 - - - - 15 -M.t. townsendii - - - - 1 - - - 2 - - - - 1 4 -Zapus - - - - 1 - - - 3 - - - - - -Peromyscus - 4 - - 2 - - 1 8 1 - 35 1. 1 Sorex - - 1 - - - - - - 1 - - - . -lleurotrichus - 4 - .- 1 5 - - 1 - - - - 1 - -S cap anus, - 1 - - 5 - 9 - - - - - - - -ITote - A. Atyphloceras ttultidentatus (Pox) . B. Hystrichopsylla occidehtal'is n. sp. C. Catallagia, charlottensis (Bak.) D. Delotelis telegoni (RotKs.) E. Epitedia aoapani IVagn.) P. Micropsylla se"cTilis goodi Hubbard G. Corypsylla, ornata Pox H. Hearctopsylia j ordani Hubbard • I. Hegabothris abantis (Roths.) J. Monopsyllus wagneri ophidius (Jord.) K. Monopsyilus ciliatus protinus (Jord.) L. Dasypsyllus gallinulae p e rpi"nnatus (Bak.) M. Opisodasys Keeni (Bak.) '• ' ' H". Peromyscopsylla selenis (Roths.) 0. • Peroffiyscopsylla hesperomys pacigica n. ssp. It has been shown that fleas leave the bodies of their hosts after death to a limited extent only. I t .was- found (Elton, Pord and Baker, 1931) that the difference between the flea-rate on l i v i n g and dead mice during the colder months was smaller, possibly due to the fieas becoming sluggish. -48-Table 12. Plea-rate at different seasons. Date Total no. of mice Total ho. of fleas Plea-rate Mar.l4-Apr. 5 12 2.4 24 1 May. 11-Jun. 20 53. 2.7 21 Jun.22-Aug. 34 70 - 2.1 28 Sep.25-Dec. 27 81 3.0 6 Prom Table 12 i t can he- noted that the number of fleas per animal was smallest in the summer months and largest during the. f a l l . These results agree with the work of Elton, Ford.and Baker (1931). However the f l e a rate obtained by these workers was lower than that obtained for M.o. serpens. The largest number of fleas obtained from one vole of this species was 9. Ticks. - Microtus had very few ticks, 3 adults ( l male and 2 females), 7 nympha and 24 seed ticks were a l l that were found on the animals caught. These w?re a l l of one species Ixodes angustus Uevmiann. This species was the only one found on M. townsendii and wa3 also present on the other mammals trapped in association with the voles. The insectivores carried more ticks than any other ectoparasite and on them Ixodes angustus was very numerous. Mites. - Mites were numerous on a i l animals but Sorex. The following genera were found- on M.o. serpens - Echinohyssus Hirst, Eulaelaps Berlese, Haemogamasus Berlese, Laeletps Koch, Liponyssus Kolenati, Tetragonyssus Ewing. • Larval Srombiculidae were found encrusted in the ears - 4 9 -and on the skin of l a i c rot us, Zapus, trinotat.ua ,t., and iNTqurotrichus gibbsii g« Lice . (Anoplura). - Only one iouse was found on H.o. serpens during the f a l l . Perorayscus maniculatus was the only other animal with l ice and they were very numerous on i t . The l ice were a l l of one genus* Perrisella Ewing. Condition of Animals. The majority of animals captured were in good condition. A few had scars on the head, presumably from fighting. One male had an infected cheek and a female was found with a white gelatinous . mass (diam. 4 mm.) encapsulated between the skin and muscle of the shoulder. The only internal patholo-gical condition noted was in a large male. A large mass of encapsulated pus (diam. I cm.) was attached to the large intestine, and 2 folds of small intestine, partly closing off the lumen of one part of the small intestine. -50--DISCUSSION Breeding season In M.o. serpens, as in M. pennsylvanic.Us (Hamilton, 1941), the fecund period of the male persists later than that of the female. Thus i t is the cessation of oestrus in the female that terminates the breeding season. In an experimental study of the influence of temperat-ure and light upon reproduction in Mierotus agrestis, Baker and Hanson (1932 a and b) revealed that the influence of each of these factors, in depressing reproduction activity was, in the f i rs t instance, on the female. . Evidence suggests a similar- situation in M.o. serpens. In 1948, the breeding season began in March and contin-ued until October, a period of 7 months with the summer solstice as a mid-point. In 1949, on the other hand, breed-ing began about a month earlier. In comparing the climatic circumstances in the two winters, i t is found that 1949 was colder, with a long period of show cover and more sunlight than 1948. Baker and Hanson (1933) found a correla/tion s between the hours of sunshine and reproduction in E . agretis. In general, mice were breeding only in those months in which there was more than about 100 hours of sunshine. Hamilton (1941), however, found l i t t l e correlation between snowfall, jpe.HRsylvn .nims amount of light, and the breeding season of Mierotus*and concluded that an earlier onset of breeding was largely the consequence of population density. In the present study th.erel.has been no evidence of a - S i -higher population in 1949 as against 1948 and i t seems logical to conclude that some other factor* possibly climatic, was responsible. In an attempt to explore the influence of day length upon Microtus o. serpens, experimental animals were subject-ed to increasing periods of illumination, while controls were held at the same temperature,but normal lighting. Both the experimental animals and the controls were held at temperatures of 57° to 68 6 !F. and thus much higher than outdoor/temperatures, which, for the same period, were constantly below freezing. Both groups began breeding before the end of December, those subject to warmth alone, about 2 months ahead of animals in the wild, those subject to increasing periods of light, as well as temperature, about 3 months ahead, of the wild population. Baker and Hanson (1932 a), found that M. agrestis produced signicaritly fev/er l i t t e r s i f kept at summer temperature?but receiving only 9 hours of light a day^than those receiving 15 hours. Similarly, the animals having 15 hours of light a day and kept under winter temperature conditions had fewer l i t t e r s than those kept at summer temperatures, but under identical light conditions. In these Microtus, light and temperature play an important part i n the control of breeding, and the results obtained from the present work indicate that M.o. serpens i a affected in a similar manner. Also of possible importance in the stimulation of breeding in those animals subject to warmth alone, was the -52-mating excitement , with the accompanying vocalization,taking place in the neighbouring experimental light cages. Animals adjacent to a mating pair climb up the sides of their cage with apparent interest. The absence of winter breeding during periods of low density i s believed by Hamilton (1941) to be largely, due to the lack of a suitable mate for any individuals that should chance to come into oestrus. The lack was not experienced by the females i n the control cages, as both males> judging from their high breeding weight, were fecund. Thus the more Important effects due to low density would not be apparent to the animals in the winter light cages and the commencement of breeding does not seem unreasonable. It appears that: light, temperature and density are a l l important factors in controlling the length of the breeding season- in M.o. serpens and doubtless many other factors such as available food supply, amount of snow as related to protection from predators, disease etc. a l l exert varying amounts of influence. Sex ratio The ratio of male to female voles born i n the laborat-ory was 51.: 43, or a slight excess of male animals. However . the sex ratio of adult animals caught in the f i e l d was 54:46. The extra males captured were, at least in part, probably the result of the larger home ranges or more wander-ing tendencies of the males. However, they might also be the result of a lower survival rate for females than males, since -53-few of the females captured had very mature skulls as compared to those of the males. Age of Maturity and Oestrus Young males attempted to mate at 34 days hut spermatozoa were not present in the epididymis until about 6 weeks and did not become numerous for at least another week. Young femal' es were observed mating at 22 days but the earliest f e r t i l e mating was at 27 days. However, from the weights of animals caught during June* fecundity appears to be reached at an earlier age than this in animals in the wild at this season. For instance, a young female, captured July 3 wigsh 3 implanta-tions was', by interpolation from the growth curve, not over 23 days old. The age at f i r s t oestrus increases as the season advances u n t i l , when the breeding season ends,- many animals of a mature size have not manured sexually. Thus the factors controlling the length of the breeding season also appear to be important in determining the age at which sexual maturity i s reached. Some l i t t e r s were deprived of extra light f or about 2 nights a week during the winter experimental period. These • animals took 2 to 3 weeks longer to reach maturity than those that received continuous doses of extra l i g h t . Consequently, i t seems that an important factor in the control of the onset of maturity i s the total amount of light received. The nest boxes occupied,by the voles were f a i r l y light proof .and i t seems evident that the effect of light must be exerted during periods of activity. These feeding periods do not f a l l at the same time on successive days. However, the -54-number of active periods for a given length of time i s approximately the same for different days. It i s thus possible that the number of illuminated periods i s responsible for the effect of the lig h t on the animal's reproduction. The length of the oestrus cycle appears; to vary considerably in these animals. Matings at intervals of 2 days followed by a space of 10 days were common between young, not f u l i y mature males and older females. However, matings 6 and 8 days apart were also recorded. A post parturient heat i s present in M.o.. serpens and implantation i s not delayed mor gestation prolonged in lactating females. If the animals did not mate following birth, they apparently did not come into heat again until after the young were weaned,and under such circumstances several weeks might elapse before they again conceived. One animal, however, mated 6 days following a s t i l l b i r t h . A lactation anestrum i s therefore indicated for this species. This seems to be opposed to the findings of Hamilton (1941) for M. pennsylvanicus which, he stated, conceived 5 or 5 days following parturition. However, he does not mention i f the young of the previous l i t t e r were nursing or whether •a lactation anestrum was present. Gestation period The average length of the gestation period in M.o. serpens i s 24 days, whereas, in a l l other species of Microtus for which data i s available, i t i s usually about 21.' Haefield (1935) found that lack of use of an exercise wheel increases -55-the gestation time of M. californicus to 22 days and several hours, and, in as much as no exercise devices were used in the present study, it. i s possible that the gestation period of the animals in the wild i s something slightly less than 24 days. " ' The length of the gestation period within a species, and perhaps also a genus, i s said to be related to the size of the l i t t e r , speci,es characteristically producing larger l i t t e r s having a shorter gestation time (Asdell, 1946), Most species of Mierotus have up to 8 or 10 young at once while 5 was the most found in M.o.'serpens. On this basis the gestation period in M.o,., serpens might have been expected to be some-what longer than'"the more fecund species of Mierotus. Average..litter',size ' • ; The average l i t t e r size in the f i e l d was 3.11, and the average size for l i t t e r s born in captivity was 2.79. This gives an average of 2.95, or almost 3, rahich was the most frequent number in both cases, the range being from 1 to 5. The average for f i r s t l i t t e r s was smaller than for subsequent ones, and-it increased with each succeeding l i t t e r . This increase was not the result of the production of larger l i t t e r s by older females but, rather, resulted from fewer 3 m a l l l i t t e r s being produced in subsequent births. During the middle of the brdeding season, in June, females apparently gave birth to larger l i t t e r s than they did at the beginning or at the end of the reproductive period. The increase in average l i t t e r size might be due to increased population density as the year*s young begin to mature. This i s probably at least partly* the case, as the average l i t t e r size for any year is.said to be dependent on the density of the, mice Hamilton (l941). However this would not explain the decrease in l i t t e r size as the season ends. Possibly the factors controlling the end of the breeding season are also responsib l e for the. l i t t e r size. Thus the number of young i s dependent on the number of the l i t t e r and time of the breeding season at which i t i s produced. Breeding potential In a bree.ding season lasting from March to September, i t would theoretically be possible for a mature female to have 10 l i t t e r s . However, the females did not always mate again immediately following birth. In captivity, this happ-ened qhite frequently and in this event, mating was usually delayed some weeks after the young were weaned. This i s Illustrated by two females, both of which had 3 l i t t e r s in 131 and. 132 days. In these periods i t should be possible for an animal to hage 5 l i t t e r s . In addition, no mature animals that had started breeding in the spring lived throughout the whole season. Consequently the rapid, theoretical rate of production might take place at the peak of the breeding season in May, June and early July, but could hardly be realized through-out an entire season. -57-Limiting factors Among the factors that tend to iimit the Vole population can also he included some discussed under the reproductive "biology* These include decreased growth and slower develop-ment as the summer proceeds and the various factors limiting the breeding season. Lactation anestrum* factors limiting l i t t e r size* pooqibly the olight 0 : 1 0 0 0 0 of maloa at birth, and the possibility of a lower survival rate for females are a l l important. Another limiting factor in reproduction i s the absorp-tion of embryos. In the light and temperature experiments conducted on H« agrestis* Baker and Ranson (1932, za'.and b) reported a large proportion of pregnancies ending in absorp-T J i i i o m o u . n J " c < { to tioh.^about 46$ for the controls kept under summer conditions. Prom an examination of the records of M.o.. serpens, i t seems probable that several l i t t e r s were similarly lost. However, absorption was only recognized once in the 18 trapped pregnant females examined. Thus the incidence of absorptions in the wild appears to be lower than i t i s in captive animals. An important limiting factor, at least, in the laboratory, was the death of the Majority of f i r s t l i t t e r s . The mothers had a l l been born and raised in captivity and a l l their second l i t t e r s were successfully raised. Thus i t seems unlikely that the cause of the deaths was due to the effect of captivity on the female or "roguenes3" of either parent (Ranson 1941). These U t t e r s were born in December and the - 5 8 -heginnihg of January, when only a few green plants were avail-able. However, 2 l i t t e r s of. 4 each were raised during this time.Consequently i t seems unlikely that there was a defic-iency in the diet. A-possible explanation i s that due to the disturbance caused by remating activity, f i r s t l i t t e r s were neglected. Young animals, after they become independent, until they reach the age of 3 or 4 weeks, are exceedingly curious and apparently indifferent to many stimuli that send the adults scurrying. Hamilton (1941) found an apparently higher mortal-i t y among immature M« pennsylvanicus of between 3 and 4 weeks, than among older groups. It i s probable that this higher mortality i s due to behaviour of the young voles similar to that found in M.o. serpens. Adult voles were also curious regarding a foreign object in their rf?ages and investigated i t at once. M* agresi&s, on by 15. Ckitty (w.npu.tliikccl) the other hand, was fouiid Ato be very suspicious of something different and required about 3 days to become used to i t . This "new object" reaction was made use of in catching the animals,' that i s , traps were prebaited for 3 nights prior to being set. In this way, the heaviest catch was on the f i r s t trap night. This procedure i s not necessary for M.o. serpens as the largest catch i s Usually the f i r s t trap night. The external parasites do not appear to affect the health of the animals. However, i t was noticed that older, less sleek individuals Carried more parasites than the younger ones. This agrees with the findings concerning other -59-species of Microtus* in which the number of parasites per animal increased with age (Jaiaesoh,i947), (Elton* Ford and Baker, 1931). A l l species Of ectoparasites were f a i r l y well-distributed among the vole and i t s associates. Thus any arthropod-borne epidemic arising in any one of these animals • would l i k e l y be transferred to any other susceptible species. Ho animals of known ages have yet died in captivity due to natural causes. Two females, captured as adults, are at least 320 days old and are s t i l l breeding. A male, ca*ture-ed when a juvenile, i s approximately 300 days of age and i s s t i l l fertile.,Thus, at least under laboratory conditions, these animals vri.ll l i v e 10-^  months, and probably longer, and remain capable of reproducing; whereas the average length of l i f e , for M. agrestis i s 260 days (Leslie and Ranson, 1946), However, an examination of Fig. 15 shows that the number of ma.ture animals over 25 grains decreases during the summer and f a l l , and increases again in the spring as the young animals mature. This could possibly be due to the loss of/wei'ght of non-breeding animals during the winter anestrum. However, an examination .of the skulls- shows that the animals caught in the autumn were not over a few months old. Since animals born in early spring would be expected to have reach-ed a weight of 25 grams by mid-summer, they apparently do not survive even one winter. It therf ore appears that these voles do not li&e more than a year i n the wild state and the average l i f e expectancy i s probably from S to 10 months. -30-Age fllaases. Due to individual variation no single factor was found that would give the absolute age of M,o, serpens. Until- the age of 30 days,, they Can be aged within a-f ew days by- means of their- weights. The condyle-basilar length of the skulls can be ;used to give a relative idea of the age of a group of animals. Skulls of very old animals can be separated flrom those of younger ones by the lack of ridges and the rounder appearance of the lat t e r . By the comparison of a l l these attributes, i t was found possible to age an animal approx-imately. Activity Activity of M.o. serpens consists of a short 1 to 2 hour rhythm due to feeding and a long 24 hour rhythm. This longer rhythm has a higher average of activity at night and a period of ]e ast activity around midday. This agrees with the activity of M. agrestis as recorded by Davis (1933) for a single female. However, this animal showed a peak of activity following sunset with a decrease throughout'.'the night and a secondary peak just prior to sunrise. This pattern was found only for a very few of the Creeping Voles used. Individual variation and different physiological conditions produced various differences in the activity patterns. Total activity appeared to increase with age. The total activity during pregnancy was' higher than for non-• -61-pregnant females. This is the reverse condition to that found in rats, where activity is less during pregnancy (Richt er, 1927). It is possible that the increase in total, activity immediately before and after birth may be connected with the postparturient heat. The overall pattern of activity for 11.o. serpens shows i t to be primarily a nocturnal animal. Food and Habitats On the University campus, the densest populations of Creeping Voles were found in open brush land and on the junction of dense brush and grassland. From the underground runways under the brush, surface paths led out through the grass marked by cuttings of herbaceous plants and by fecal pellets. In the winter, the animals tended to concentrate more in the woods, in those parts that were partly deciduous and thus fairly open. At this time their food consisted main-ly of roots, although sword fern and trailing blackberry leaves were eaten extensively. The most favoured food at a l l seasons was the false dandelion. The yellow flowers i</ere much'preferred. In smaller plants frequently a l l the flower stalks would be cut near -the base but a few were always left on the larger clumps. From observations of the captive voles, i t seems likely that once a stem is cut, the animal seizes the loose end, turns, and attempts to dztag i t away to its feeding station. The cutting of this stem into sections does not aeem to have any significance but captive animals also do i t with plants placed in their cages. M.o. serpens -,is usually found in association with mole in this area Scapanus orarius schefferi, using their exten-sive runway systems and eating roots projecting into these runs. In this way, the voles are able to cover larger areas than i f they ha,d to dig their own tunnels. In this area, the Creeping Voie i s an animal of the ' serai stages rather than of the climax types. Thus i t has probably increased greatiy in number following the settling and consequent opening up of the country. SUMMARY From March, 1948* to March* 1949* specimens of Microtus  oregoni serpens were snaptrapped and a captive colony studied.- These animals were captured on the campus of the University, of British Columbia* The breeding season in 1948 extended from the beginning Of March to the end of September. In the spring of 1949 breading started in February. The length of the^season was found to be controlled by the female. Important factors- affecting this length were deemed to be light* temperature and density. The sex ratio of young animals was found to be 5| males to 41 females* Females reached sexual maturity at 3 40 4 weeks of age with the majority conceiving at about 30 days. Males, however, did not become mature until at least 5 to 7 weeks of age. The females experience a post parturient heat, and a, lactation anestrum. The average length of the gestation time wa.s almost 24 days. Most frequent i i t t e r size was 3, with a range of from 1 to 5. First l i t ters averaged smaller and the number Of small l i t ters decreased with subsequent births. • Probable length of i i f e in the wild is about 8 to 10 months. Weights can be used to age an animal until i t is 30 days old. After this, relative ages can be determined "by comparison of the cond-yio-basiiar lengths of the skulls» A short 1 to 2 hour activity rhythm and a long 24 hour rhythm were found for these animals. There was marked variation, usually within these patterns, for different individuals. In this area M.O* serpens was found to favour the forest edge and open brush in the summer. During the winter, they were found more frequently in the open, mixed deciduous and coniferous habitats. Both surface and undergrstound runways were used, except in the winter, when the animals were chiefly under-ground. Ectoparasites common on the vole were found on i t s associates. Thus an epizootic i n one species of animal would probably be transmitted to other susceptible species. -65-LITERATURE CITED Asdell, S. A. 1946. Patterns of mammalian reproduction* Comstock.- Ithaca* Hew York. Baker. J. R. and R. M. Ranson. 1932a. Factors affecting the breeding of the f i e l d mouse (Mierotus agrestis). Part I. - Light. Proc. Roy. Soc. London, B. 110:313-322. April, 1932. • - 1932b. Factors affecting the breeding of the f i e l d mouse (Mierotus agrestis). Part II. - Temperature and food. Proc. Roy.Soc. London, B. 112: 39-46. Mov., 1932. 1933* Factors affecting the • breeding of the f i e l d mouse (Mierotus agrestis)-. Part III. - Locality. Proc. Roy. Soc. London, B. 113:486-495. Davis, B. H. S. 1933. Rhythmic activity i n the short-tailed vole, Mierotus. J. Animal Ecol., 2(2):232-238. Mov., 1933. Elton, C , E. B. Ford and J. R. Baker. 1931. The health and parasites of a wild mouse population. Proc. Zool. Soc. .London, 1931:657-721. Hamilton, W. J., Jr. 1941. The reproduction of the f i e l d mouse, Mierotus pennsyivanicus (Ord). Cornell Univ. Agric. Exp. Sta., Memoir 237:1-23. May-, 1941. Hatfield, D. M. 1935. A natural history study of Mierotus . c a l i f ornicus. J. Mamm., 16(4):261-271'. Mov., 1935. • and E. T. Hooper. 1935. Motes on the voles of1 the species Mierotus oregoni. Murrelet, 16(2):33-34. May, 1935. • Howell, A. B. 1924. Individual and age variation in Mierotus  montanus yosemite. J. Agric. Research, 28(l0):977-1016. June, 1924. Jameson, E. V/., Jr. 1947. natural history of the prairie vole (Mammalian genus Mierotus). Univ. Kansas Publ. Mus. Hat. Hist., 1(7):125-151. Oct., 1947. Leslie, P. H. and R. "M. Ranson. 1940. The mortality, f e r t i l i t y and rate of natural increase of the vole (Mierotus  agrestis) as observed in the laboratory. J. Animal Ecol., 9(1):27-52. . -66-Morris, R. F. 1944* A simple activity cage for small mammals. Acadian Hat., 1(4).153-156. Hov., 1944. Munro, J. A. and I. MoT. Cowan. 1947. A review of the bird fauna of B r i t i s h Columbia. B. C. Prov. Mus., Dept. Educ-ation. Victoria, B. C. Sanson, R. M. 1941. Pre-natal and infant mortality in a lab-oratory population of voles. Proc. Zool. Soc. London, A. 111:45-47. Richter, C. P. 1927. Animal behaviour and internal drives. Quart. Rev. B i o l ; , 2(3):307-343. Thornthwaite, C. ¥. 1931. The climates of North America according to anew classification. Geog. Rev., 21:633-655. 

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