Jan O. Murie

Annual Variation of Spring Emergence and Breeding in Columbian Ground Squirrels (Spermophilus columbianus)

Times of breeding were documented over 6 and 7 years for two populations of Columbian ground squirrels at different elevations in southwestern Alberta, Canada. The pattern of spring emergence from hibernation at the low elevation was typical for ground squirrels; adult males emerged first, adult females slightly later, and yearlings of both sexes last. Time of conception was highly correlated with time of emergence. Breeding was always later and year-to-year variation was greater in the population at the high elevation, where snow depth in spring, an index of the time of melt-off, was correlated with breeding dates. At this site reproductive success, but not survival, was greater in years when breeding was early. At the lower site, cold temperatures in spring appeared to delay emergence and breeding, and most females emerged during periods of relatively high temperature; neither survival nor reproductive success varied consistently with time of breeding. Overall, survival was greater and reproductive success was lower in the population at the higher elevation.

Delayed phenology and reduced fitness associated with climate change in a wild hibernator

The most commonly reported ecological effects of climate change are shifts in phenologies, in particular of warmer spring temperatures leading to earlier timing of key events. Among animals, however, these reports have been heavily biased towards avian phenologies, whereas we still know comparatively little about other seasonal adaptations, such as mammalian hibernation. Here we show a significant delay (0.47 days per year, over a 20-year period) in the hibernation emergence date of adult females in a wild population of Columbian ground squirrels in Alberta, Canada. This finding was related to the climatic conditions at our study location: owing to within-individual phenotypic plasticity, females emerged later during years of lower spring temperature and delayed snowmelt. Although there has not been a significant annual trend in spring temperature, the date of snowmelt has become progressively later owing to an increasing prevalence of late-season snowstorms. Importantly, years of later emergence were also associated with decreased individual fitness. There has consequently been a decline in mean fitness (that is, population growth rate) across the past two decades. Our results show that plastic responses to climate change may be driven by climatic trends other than increasing temperature, and may be associated with declines in individual fitness and, hence, population viability.

Interpretation of Intraspecific Life History Patterns: Evidence from Columbian Ground Squirrels

In populations of Columbian ground squirrels in the Rocky Mountains of southwestern Alberta, life history patterns consistently differ at different elevations. At present, these differences are not appropriately explained by the traditional models of life history evolution, r- and K-selection and bet-hedging. Nor do more-recent models based on the interspecific scaling of body weight appear to be appropriate. To explain different life history patterns of the ground squirrels, we present a simple qualitative hypothesis that invokes phenotypically plastic responses to the limitation of demonstrably influential environmental resources (food, and its availability) and to other environmental factors that may influence the acquisition and expenditure of resources. The principal strength of this resource-limitation hypothesis (and other hypotheses based on environmental factors influencing patterns of life history within species) is that it does not invoke genic differences among populations, though it need not preclude the influence of such differences. Because evidence indicates substantial plasticity in the life history patterns of ground squirrels and other vertebrates, different life history patterns within species might result from different phenotypic expressions of similar genotypes, either with or without different genotypic adaptations to different environments.

Litter Size in Columbian Ground Squirrels (Spermophilus columbianus)

Measures of litter size, based on counts of corpora lutea, placental scars, embryos, and juveniles emerging from natal burrows, were determined from 181 Columbian ground squirrels ( Spermophilus columbianus ) collected in three areas in southwestern Alberta, Canada, and from observations of 88 litters that emerged in these areas. Most measures of litter size differed between areas and were lower at a high elevation site (2,170 m) than at the other two sites (elevation 1,500 m). We suggest that differences between areas may be related to timing of snow melt and onset of vegetative growth. Columbian ground squirrels have smaller litters than most other North American species of ground squirrel, perhaps reflecting different life history tactics.

Inheritance of nest sites in female Columbian ground squirrels

SummaryHome ranges, areas of intensive use, and locations of nest sites were determined for female Columbian ground squirrels and their daughters over seven years from a low-density population in southwestern Alberta. Adult females usually retained similar home ranges from year to year. However, they moved their nest sites more often than expected in years when a yearling daughter was present. Core areas and centres of activity also changed when the nest site was moved. Most daughters settled on their natal areas. The relinquishing of nest sites by females to their daughters is seen as a form of parental investment.

Manipulation of Litter Size Reveals No Cost of Reproduction in Columbian Ground Squirrels

Survival and reproductive effort of female Columbian ground squirrels ( Spermophilus columbianus ) were examined for 1 year following experimental manipulation of litter size that either increased, decreased, or left their lactational cost of reproduction unmanipulated. Except for a trend towards females that incurred decreased costs in year 1 being heavier than other females at spring emergence in year 2, no short-term cost of reproduction was detected. The results fail to support the assumption of reproductive costs implicit in most life-history models, and are in accord with an earlier correlational study of reproduction of Columbian ground squirrels that found no trade off between current and future reproductive success.

Predation by Badgers on Columbian Ground Squirrels

Predation by badgers ( Taxidea taxus ) on Columbian ground squirrels ( Spermophilus columbianus ), primarily pre-emergent juveniles, was documented in eastern Washington during 6 years. Badgers dug up the nest burrows of female ground squirrels each year, but frequency of attacks was biennial and high attack rates occurred only in the 3 years that one adult and one or more juvenile badgers were on the study area. Female ground squirrels usually survived attacks, but one or more juveniles eventually emerged from only 41% of the nest burrows dug up by a badger, compared to 90% for undisturbed nest burrows. Females and juveniles that survived a badger attack did not differ from their undisturbed counterparts in survival to the following year, and females appeared not to shift nest burrow sites in the following year in response to a badger attack. Although loss of juveniles to badger predation accounted for up to 56% of the estimated production of juveniles in a year, the population did not decline markedly after years of heavy predation. Higher survivorship of the smaller number of juveniles in those years was largely responsible for maintaining numbers the following year.

Temporal overlap of female kin in Columbian ground squirrels (Spermophilus columbianus)

SummaryColumbian ground squirrels (Spermophilus columbianus) were live-trapped from 5 to 7 years at three sites in southwestern Alberta, Canada. The two most common adult matrilineal relatives for 2-year-old females were the mother and a 1 year older non-littermate sister. Co-occurrence of female kin depended firstly on size and sexual composition of litters and secondly on age-specific survival and recruitment rates. Adult matrilineal kin frequently coexisted among breeding females of this species, leading us to predict a social system strorgly influenced by nepotism. However, the life history traits of S. columbianus suggest an adult female kin cluster unlike that found for other species of Spermophilus in which littermate sisters are common. If availability of adult female kin influences kin-differential behaviour, then female S. columbianus should favour mothers, daughters and non-ittermate sisters.

Kin selection in Columbian ground squirrels ( Urocitellus columbianus ): littermate kin provide individual fitness benefits

Since W. D. Hamiltons seminal work on the evolution of sociality, a large body of research has accumulated on how kin selection might explain the evolution of cooperation in many group-living species. Our study examined the evolutionary basis of philopatry and cooperation; specifically, whether individuals benefit from the presence of close kin. We applied an individual fitness approach to a 16-year study of Columbian ground squirrels (Urocitellus columbianus) to investigate potential causal paths by which the presence of kin might act on individual fitness. Our results indicate that individual fitness benefits resulted from associations of philopatric female kin, and support the hypothesis that increased tolerance of proximity of kin is a proximate mechanism for these benefits. The major life-history influence of kin on individual fitness was through improved reproductive success, and this benefit may have been owing to philopatric settlement of kin that were recognized through familiarization in the natal burrow. Thus, we demonstrated an evolutionary basis necessary for ongoing kin-selected cooperation in Columbian ground squirrels, though the mechanism of familiarity may determine which kin individuals benefit from cooperative behaviours.

Maternal influences on reproduction in two populations of Columbian ground squirrels

In this study, we examined influences of maternal traits on offspring birth mass, growth rate, and weaning mass for two populations of Columbian ground squirrels (Spermophilus columbianus). We tested relationships between maternal body condition, structural size, change in mass (during gestation, during lactation, and during the entire reproductive period), timing of reproduction, and litter size on offspring traits using path analyses. To assess whether maternal investment in offspring traits extended beyond the period of direct maternal care, we examined associations between offspring traits and overwinter survival of pups. In general, females in better condition raised pups that were heavier at weaning and that had faster growth rates during lactation. Litter size had a negative effect on mass and growth rate, and only litter size had a significant effect on birth mass. For both populations, the average weaning mass of pups within a litter had a positive effect on the number of pups that survived to yearling age. In a population for which birth masses and growth rates were available, pups with faster growth rates survived better to yearling age, whereas birth mass had no effect on the number of surviving offspring in litters. We found substantial maternal influences on offspring growth and size, and evidence that these influences may extend beyond the juvenile period and constitute influences on fitness. The key to arriving at these conclusions was to take the number of offspring into account before testing for maternal effects.