Population ecology of Columbian ground squirrels in southwestern Alberta

Abstract

The characteristics of a large population of Columbian ground squirrels (Spermophilus columbianus), the numbers of which varied between ca. 700 and 1000 animals emerging in the spring, were studied over a 7-year period in southwestern Alberta. Over time, the density of nonjuvenile squirrels varied between 11.6 and 16.1/ha, with a sex ratio that consistently favoured females; the density of juveniles varied between 4.6 and 20.7/ha, but with a sex ratio that was usually balanced. Spatially, the density of squirrels also varied with a certain part of the study area (17%) annually supporting proportionately more animals of each sex than the remaining 83%. Dispersal was recorded primarily among yearlings, with more males than females involved in this movement. Most dispersal occurred in late June and early July, centred on the time juveniles emerged from natal burrows. We recorded squirrels dispersing up to 6.7 km. Survival of squirrels in their 1st year of life was lower (females 40%; males 35%) than in any subsequent year (females 50–64%; males 37–54%). These attributes of Columbian ground squirrels are compared with those of other members of the genus, and the evidence they provide for population regulation is discussed.

Maternal influences on reproduction in two populations of Columbian ground squirrels

“…Mating date was determined by examination of external reproductive morphology, observation of a copulatory plug, presence of sperm in vaginal smears (Murie and Harris 1982), direct observation of copulations above ground (at Turnbull), or by observations of precopulatory behavior, such as male interest in the female prior to, but not following, copulation, or males entering burrows with estrous females. Survival of young to yearling age can be ascertained because juveniles do not disperse prior to their first hibernation (Boag and Murie 1981, Murie and Harris 1984, Risch et al 1995. Thus, juveniles that emerge from their hibernaculum in the following spring are the ones that have survived their first hibernation.…”

Milk matters: offspring survival in Columbian ground squirrels is affected by nutrient composition of mother's milk

“…Survival of juveniles overwinter was determined based on emergence the following spring. Columbian ground squirrels exhibit male-biased dispersal, which does not occur until after the first hibernation (Boag and Murie, 1981;Murie and Harris, 1984;Dobson and Murie, 1987). Thus, juveniles that did not emerge the following spring were assumed to have died during the hibernation period.…”

A Consideration of Bet-Hedging in Spermophilus columbianus

“…(1) Powell and MacIver (1977) found that a 3-10-year period was as adequate as a long-term period (>30 years) for describing the summer temperature and precipitation at sites within and adjacent to our study region, and (2) Boag and Murie (1981), Bronson (1977Bronson ( , 1979, and Murie et at. (1980) found small or nonexistent interyear differences in ground squirrel life histories during several multi-year studies.…”

“…(1980) found small or nonexistent interyear differences in ground squirrel life histories during several multi-year studies. One of these studies spanned 7 years (see Boag and Murie, 1981;Murie et al, 1980) and concerned two populations in our study region where litter size and its variance, adult survival, and age at maturity did not significantly change among years. Notwithstanding, juvenile survival within our study region may vary among years (Boag and Murie, 1981;Murie, 1985;Murie and Harris, 1982), so we examine several measures of immature survival to help compensate for possible biases.…”