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The reproductive biology of the western red squirrel. Millar, John Steven


The reproductive condition of male and female red squirrels (Tarniasciurus hudsonicus) in southern British Columbia was studied in relation to age, season, relative population level, and foods utilized. The reproductive cycle of males and females is described on the basis of gross anatomy and histological characteristics. A low population on Vancouver Island was studied in 1966 and 1967 and a high population was studied near Princeton, B.C. in 1967. The habitats are similar in both regions. The low 1966 population had a late breeding season in which all females sampled produced two large litters. The ovulation rate for the two litter season averaged 5.12 per litter. The second ovulation occurred shortly after the birth of the first litter, before the uteri and vaginae came into full estrus condition. In 1967 both the low Vancouver Island population and the high Princeton population had early breeding seasons when nearly all adults bred yet only one third of the yearling females participated. Only single litters were produced in both areas. The ovulation rate was 4.11 per litter in the low Vancouver Island population and 3.58 per litter in the high Princeton population. Preimplantation losses of about 11% were fairly constant in both years. Postimplantation losses were not present in the low 1966 population, were about 19% in the high 1967 population, and were not determined in the low 1967 population. Similarities between the high and low 1967 populations and differences between the low 1966 and low 1967 populations on Vancouver Island suggest that density was not the primary force Influencing productivity. Another explanation is found in the evaluation of the primary food sources. Conifer cones are the main winter food of the red squirrel. Lodgepole pine cones were the main food source before the highly productive breeding season in I966 while Douglas fir cones made up the diet before the less successful breeding season in 1967. The fruiting habits of the conifers is discussed and It appears that it is advantageous for the squirrel to feed on lodgepole pine cones. A good winter food supply is particularly important to the red squirrel since it breeds during the late winter and it's already high metabolic rate increases still more during cold winter weather. It is inferred that the winter food supply may influence the relative capacity of the female to produce young. The timing of the breeding season is quite variable and seems to be affected by the severity of the winter weather.

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