Sexual Differences in Body Weight Patterns of Richardson's Ground Squirrels during the Breeding Season

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Abstract
During a 4-year study in southern Alberta, weight changes of adult Richardson's ground squirrels were determined by reweighing individuals at 1–3-day intervals following emergence from hibernation until the end of the breeding season. Temporal patterns in weight changes relative to reproductive events were compared between sexes. On average, males were 137–171 g heavier than females on emergence from hibernation. Females typically bred 4 days after resuming aboveground activity, and all females in the population emerged and bred in a 3–5-week period. Females gained mass prior to breeding, but the majority (65%) exhibited a 1 day interruption in weight gain on the day of insemination. Mass increased again after insemination, and weight gain during pregnancy exceeded that attributable to developing embryos. Males lost mass during the breeding season. Maximum loss by males occurred in the 1.5–2-week period in which the number of females inseminated per day was highest. The longer period of weight loss by males compared with females reflected the difference in effective length of the breeding season for the sexes (3–5 weeks for males versus <1 day for females).