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Adaptations to arid environments in Perognathus parvus (Peale) Iverson, Stuart Leroy

Abstract

In the upper Sonoran and transition zone areas of southern British Columbia populations of Perognathus parvus living in different habitats exist within a few miles of each other. The area was deglaciated about 10,000 years ago setting a maximum time for occupancy by small mammals. This study was initiated to compare the adaptations of individuals of these populations to the different habitats and, if possible, comment on their evolution and distribution. Animals from an environmental gradient were examined in the field and laboratory. One end of the environmental continum (low area) was characterized by low rainfall, high temperatures and a long summer, the other (high area) by high rainfall, low temperatures and a long winter. Specimens from kill trapping indicated that the subspecies under consideration was morphologically variable with characteristics distributed in a checkerboard pattern. Analysis of stomach contents indicated that food during the summer was about equally divided between seeds, green vegetation and animal material. Seeds were the major item stored for winter food. Low area animals ate comparatively more green vegetation, possibly in response to greater water loss. Intensive live trapping and dissection of specimens indicated that high area females came into reproductive condition earlier in the spring and ceased reproducing earlier in the summer, producing fewer litters than the low area females. Young-of-year females in the low area reproduced while those in the high area did not. Average litter sizes (4.85) were the same. There was no postpartum estrus. Home ranges sizes of males (895 m²) in the high and low areas were the same. The ranges of the females (656 m²) were smaller. Burrows and home range centers were apparently randomly distributed and ranges of both sexes overlapped. Density was highest in the low area and decreased with altitude. Long term survival rates were high in all groups except low area young animals. Short term survival rates were highest in the winter, lowest in the spring and intermediate in the summer and fall. High area animals entered torpor earlier in the fall than did low area animals. Adults apparently entered torpor before young animals. In the laboratory animals tended to enter torpor during the dark period and leave torpor during the light period after 3-168 ([formula omitted] = 46) hours in torpor. Percent of time spent in torpor increased with time at 5 C and levelled off at about 60%. When maintained at low temperatures high area animals had significantly longer torpor periods and spent a greater proportion of time in torpor than did low area animals. Subjection to water stress indicated that the low area animals were better able to conserve water by reacting more quickly to dehydration. When dehydrated low area animals were able to maintain allow plasma osmotic concentration while high area animals were not. The production of highly concentrated urine appeared to be the main reaction to dehydration. Fecal and evaporative rates of water loss were similar to those found in other small desert rodents and did not consistently decrease with dehydration. Both high and low area animals maintained their weight on a dry diet at 76% humidity at 20 C but lost weight at 42% humidity. It is suggested that the northern distribution of P. parvus is limited by the short summer season available for birth and establishment of the young. Analysis of morphological characters shows that genetic differences exist between individuals of the two populations. Concrete evidence of genetic differences in physiological characteristics is lacking but a strong circumstantial case for the existence of such differences can be built. It is suggested that high selection pressures have been more responsible for the differentiation of the populations than has restricted gene flow.

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