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Despite considerable research on reproductive timing, factors influencing intraspecific variation in gestation length in wild mammals have received limited attention, largely due to a lack of high-resolution data. We examined potential drivers of gestation length variation in a wild population of North American red squirrels ( Tamiasciurus hudsonicus Erxleben, 1777) in Yukon, Canada. Drawing on 27 years of conception and parturition data, we found that gestation length was highly conserved, with minimal variation (35.4 ± 1.49 days; mean ± standard deviation). Gestation length was unaffected by maternal age, litter size, or litter sex ratio. Likewise, the anticipation of an upcoming food pulse, caused by synchronous conifer seed masting, had no statistically significant influence on gestation length. This finding stands in contrast to other reproductive traits in red squirrels, which are known to shift in response to food pulses. Overall, our results suggest that gestation length in red squirrels is buffered against environmental variability, likely due to stabilizing selection or strong heritability. 

Abstract Understanding the causes and consequences of repeatable among-individual differences in behavior (i.e., animal personality) is a major area of research in behavioral and evolutionary ecology. Recently, attention has turned to understanding the processes behind changes in repeatability through ontogeny because of their implications for populations. We evaluated the relative importance of selective disappearance (i.e., differential mortality), an among-individual mechanism, in generating age-related changes in the repeatability of aggression and activity in juvenile North American red squirrels (Tamiasciurus hudsonicus). We observed age-related decreases in the repeatability of aggression across ages, arising from lower among-individual variance. Although we found evidence for directional selection on aggressiveness, it was insufficient to erode among-individual variance. Thus, ontogenetic decreases in the repeatability of aggression do not appear to be due to selective disappearance. In contrast, the repeatability of activity was higher across ages due to higher among-individual variance in activity, but there was no support for selective disappearance based on activity. Taken together, our results suggest that age-related changes in trait repeatability in red squirrels are not the result of selective disappearance and instead may be the result of within-individual developmental processes, such as individual differences in developmental trajectories.