Dispersal patterns of juvenile Townsend's ground squirrels in southwestern Idaho

Understanding the effects of habitat reduction or fragmentation on animals requires some knowledge of their dispersal patterns. We used radiotelemetry to examine dispersal characteristics of 59 (37 male and 22 female) juvenile Townsend's ground squirrels (Spermophilus townsendii) on the Snake River Birds of Prey National Conservation Area in southwestern Idaho from April to June in 1993 and 1994. We tested for differences between years, sexes, and habitats in the rates and distances of dispersal. We compared the fates of dispersers and nondispersers and tested whether the direction of dispersal differed from random and whether the distribution of dispersal distances could be fitted to a two-parameter exponential function. Of the 38 animals for which dispersal status could be determined, 16 dispersed. The proportion of dispersers was greater for males than for females, but these rates did not differ by year or habitat type. We found no differences between habitats in dispersal distance. Survival rates through immergence into estivation did not differ between dispersers and nondispersers, suggesting that dispersal is not risky over the short term. Direction of dispersal did not differ from random, and the distribution of dispersal distances was adequately fitted to a truncated exponential distribution with a truncation distance of 118 m. Dispersers tended to end up in the habitat type that they started in, suggesting the possibility of habitat imprinting.

Can shrub cover increase predation risk for a desert rodent?

Previous research indicates that predation risk may influence activity patterns, habitat partitioning, and community structure of nocturnal desert rodents. Shrub microhabitat is typically considered safer than open microhabitat for these small mammals. We investigated predation risk for Townsend's ground squirrels (Spermophilus townsendii), which are diurnal desert rodents that detect predators visually and use burrows for refuge. Our results suggested that shrub cover may increase risk for these squirrels by decreasing their ability to escape from predators. Our field experiment indicated that running speeds of juvenile squirrels were lower in shrub (Ceratoides lanata) habitat than in open areas. Shrub cover was also associated with shorter predator-detection distances (mammalian and avian) and fewer refuges (burrow entrances per hectare) than in open areas in one year but not in another. Our study demonstrated that the visual and locomotive obstruction of vegetative cover may increase predation risk for diurnal desert rodents and that elements of habitat-dependent risk may be temporally dynamic.

Annual cycles of activity and body composition in Spermophilus townsendii mollis

Data on activity scheduling and body composition in Spermophilus townsendii mollis Kennicott were collected over a 4-year period in northwestern Utah. Yearly variation in emergence scheduling was associated with variation in late winter warming patterns. Sex differences in patterns of growth and seasonal body composition were attributed to reproductive biology. These included (1) a decline in female lipid during reproduction, (2) a 2- to 3-week delay in the onset of female fattening relative to males, (3) a decrease in female water content during late lactation, (4) sex differences in rates of weight loss during dormancy, and (5) sexual dimorphism in juvenile growth and fattening phenology. The initiation of seasonal fattening coincided with the appearance of seeds in the diet and associated increases in dietary caloric value. Experimental supplements to the diet increased fattening rates in field populations. These observations support the hypothesis that yearly fattening and immergence schedules are influenced by variation in the quality of food resources.