Who pays? Intra‐ versus inter‐generational costs of reproduction

Reproduction is thought to be costly for female mammals due to high energetic costs associated with pregnancy and lactation. Such costs of reproduction can be particularly high for younger females, who are less experienced and smaller than fully grown adults, and can manifest themselves within (intra‐individual or intra‐generational trade‐offs) or between (inter‐generational trade‐offs) generations. Using lifetime survival and reproductive histories of 416 female golden‐mantled ground squirrels (Callospermophilus lateralis) in Colorado, USA, we tested for the evidence of intra‐ and inter‐generational trade‐offs between age of first reproduction and fitness and its components. Females who waited to begin reproduction lived longer but they did not experience a greater number of lifetime reproductive events, did not produce larger litters, nor did they have higher lifetime reproductive success than those females who attained reproductive maturity earlier. However, delaying age of first reproduction substantially reduced individual fitness, quantified as the dominant eigenvalue of the population projection matrix, suggesting that fitness benefits of earlier maturity exceeded associated costs. Females born to mothers who delayed age of first reproduction had a higher probability of surviving to reproduce at least once during their lifetimes. Thus, in our study system, daughters pay for their mothers’ reproductive decisions, suggesting that some costs of reproduction can transcend a generation. Studies testing for costs of reproduction on survival or reproductive output within a short timeframe or those focusing within a generation may fail to detect inter‐generational trade‐offs.