Foraging offsets declining fitness: Evidence of behavioural compensation for reproductive senescence

Animals face declining fitness contributions near the end of life, termed reproductive senescence. Though reproductive senescence frequently stems from physiological inefficiencies, animals making their final attempts at reproduction have the greatest incentive to succeed, raising the question of whether this deterioration is inevitable or if they can compensate to enhance fitness. Here, we examined whether foraging effort serves as behavioural compensation for reproductive senescence in female mule deer (Odocoileus hemionus). We expected that, as animals approach the end of their lifespans, they would increase foraging effort as a means of compensating for declining reproductive output. To assess compensatory foraging, we disentangled two components of lifetime fitness: energy acquisition and allocation. We used animal-borne activity sensors to quantify foraging effort (time devoted to energy acquisition) and followed animals across the annual cycle to measure reproductive output and seasonal fluctuations in stored energy (body fat). Then, we evaluated whether foraging effort mediated the effect of age on allocation to current reproduction and future fitness contributions. For senescing animals, foraging effort positively influenced newborn size, providing evidence of compensation for declining fitness contributions. Moreover, we detected a possible diversion of resources away from future survival and to current reproduction. Senescing animals with high foraging effort accumulated little fat over summer, likely diverting incoming energy to finance current reproduction (lactation) over future fitness. Our work demonstrates that, facing their final opportunities to reproduce, animals may use behaviour to compensate for the physiological declines associated with aging to improve their chances of reproductive success. By evaluating the role of behaviour in fitness components, we reveal processes underlying senescence, and its alleviation, in wild populations.