No limit yet to phenological plasticity of reproductive timing in a high-elevation hibernator experiencing earlier springs

Plasticity in the timing of reproduction may enable species to adapt to changing climates and match reproductive demands to shifting peaks of energy availability. At high elevations, hibernating mammals are often facing drier winters and earlier snowmelt, shifting the onset of the vegetative growing season. We expect substantial plasticity in reproductive phenology at these sites because inter-annual variation in snowmelt date is typically high. However, whether species are reaching limits of phenological plasticity with ongoing directional change has rarely been assessed for vertebrate populations; identification of phenological limits is important to predict further adaptive capacity. Here, we use 30 years of litter weaning dates of golden-mantled ground squirrels ( Callospermophilus lateralis ) from the Rocky Mountains in Colorado, USA, to assess individual plasticity in reproductive phenology, population trends over time, and whether the population has reached a limit of plasticity. On average, litter weaning dates advanced by 2.3days/decade over the study period. We found that individual reproductive timing was quite plastic and exhibited near-zero heritability. At the population level, a linear relationship between average date of litter weaning and date of snowmelt suggests further potential for phenological adaptation to earlier springs. However, litters emerge later in relation to vegetative phenology in early springs, generating a potential mismatch between peak vegetative abundance and energetic demands of mothers and pups.