Age at first reproduction and senescence in Eastern chipmunks (Tamia striatus)

Senescence is the degradation of biological functions with increasing age. Its existence and relationship with life-history strategies remains poorly studied in short-lived wild vertebrate species. We investigated the relationships between age at first reproduction (AFR), reproductive senescence and longevity in an eastern chipmunk (Tamias striatus) population, where the first opportunity to breed is conditioned by pulses of seed production by trees (i.e., masts). We used 11 years of data from a longitudinal study, in which females breed for the first time at seven, 15, or 22 months of age and males at seven or 15 months of age. We first assessed the effect of age on three traits associated with breeding performance, namely the number of offspring produced, and the probability of weaning or siring a litter. We then tested whether an earlier AFR accelerated reproductive senescence and reduced survival of both males and females. We found sex-specific relationships between AFR and senescence. Females reproducing at 15 or 22 months of age showed reproductive senescence, but early-breeding females did not show any decline in reproductive performance at an older age. Also, although we observed reproductive senescence in males, it was not affected by AFR. Our results are consistent with studies highlighting the existence of reproductive senescence in small, wild mammals. Importantly, we provide the first evidence that AFR can strongly influence the patterns of senescence in small short-lived species and does it in a sex-specific way. Our results highlight the importance of studying life-history strategies in both males and females when studying senescence in the wild.

Methods
These are data from a long term study on a population of eastern chipmunks in southern Quebec. We have followed individuals over their lifespan, to measure age at first reproduction, reproductive status, the probability of reproducing, number of offspring produced. We used Generalized linear mixed models on different reproductive traits in females and males, and a quadratic function of age, to test whether we could detect senescence patterns and whether age at first reproduction was linked to senescence.