Seasonal Activity Patterns and Breeding of the Western Jumping Mouse (Zapus princeps) in Wyoming

The seasonal activity cycle and reproduction of the western jumping mouse (Zapus princeps) were studied at 8,500, 9,500, and 10,500 ft (ca. 2600, 2900 and 3200 m) elevations in the Medicine Bow Range of southern Wyoming in 1965-1966. Males first awoke from hibernation in May or June, approximately two weeks later from each 1,000-ft increase in elevation (around 16 May at 8,500 ft, about 1 June at 9,500 ft, and about 13 June at 10,500 ft). At each elevation, females awoke from hibernation 9 to 12 days later than the males. Aboveground activity of both males and females ceased around 1 September 1965 at 10,500 ft and 17 September at the two lower study areas, apparently in response to severe weather conditions. The apparent maximum length of summer activity in males was about four months at 8,500 ft elevation, and the minimum activity period was slightly more than 2 I/2 months at 10,500 ft. In females, the maximum duration of activity was about 3'/2 months at 8,500 ft, and the minimum was about 274 months at 10,500 ft. Mice hibernating in the laboratory averaged only one arousal every 38 days. Testes of adult males were well developed when the mice first appeared, but mature spermatozoa were not present for 7 to 12 days. Peaks in testis weight and development occurred approximately oine month following appearance of the mice. Testes regressed in late summer with deterioration of spermatozoa evident as early as 2 to 3 weeks after the occurrence of peak testis weights. Zapus that hibernated at 40 F in the laboratory during the winter exhibited a gradual increase in testis development through April. Changes in seminal vesicle length corresponded closely to testis weight, but the peak lagged behind testis development by about two weeks. Breeding of adult females occurred within the first week or so after they became active at each elevation. The length of gestation was calculated to be 18 days, and lactation about one month. Embryo counts indicated that only one annual litter of four to eight young (mean=5.4) was produced with very little preor post-implantation loss of embryos. Small females averaged significantly fewer embryos than did large females. The gonads of juveniles remained minute up to the time of hibernation; reproductive maturation appeared to occur the following summer. The short season of summer activity of the western jumping mouse in the Central Rocky Mountains would probably preclude the successful production and survival of more than one litter per year.