Who pays? The cost of reproduction in mammalian life history
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-generations trade-offs) or between (inter-generations trade-offs) generations. Using lifetime survival and reproductive histories of 416 female golden-mantled ground squirrels (Callospermophilus lateralis) in Colorado, 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, yet, they did not experience a greater number of lifetime reproductive events, did not produce larger litters, nor did they have higher fitness than those females who attained reproductive maturity earlier. However, daughters had higher lifetime reproductive success if their mothers delayed age of first reproduction, if they were born into small litters, and if they were born in one of the first litters of the mother. Daughters also had higher individual fitness when their mothers waited to reproduce to an older age, and when they were born into one of the mother’s first litters. Thus, in our study system, daughters, not the mothers themselves, bear the cost of mothers’ earlier reproductive maturity. These findings suggest that costs of reproduction can transcend a generation, and that studies testing for costs of reproduction on survival or reproductive output within a short timeframe or those focusing within a generation will fail to detect inter-generational trade-offs.