Age-specific reproductive investment in female burying beetles:

19 1. How much should an individual invest in reproduction as it grows older? Answering 20 this question involves determining whether individuals measure their age as the time 21 left for future reproduction or as the rate of deterioration in their state. Theory 22 suggests that in the former case individuals should increase their allocation of 23 resources to reproduction as opportunities for future breeding dwindle, and terminally 24 invest when they breed for the last time. In the latter case they should reduce their 25 investment in reproduction with age, either through adaptive reproductive restraint or 26 as a passive by-product of senescence. 27 2. Here we present the results of experiments on female burying beetles, Nicrophorus 28 vespilloides, in which we independently manipulated the perceived risk of death (by 29 activating the immune system) and the extent of deterioration in state (by changing 30 age of first reproduction and/or prior investment in reproduction). 31 3. We found that the risk of death and state each independently influenced the extent of 32 reproductive investment. Specifically, we found a state-dependent decline in 33 reproductive investment as females grew older that could be attributed to both 34 adaptive reproductive restraint and senescence. A perceived increase in the risk of 35 death, induced by activation of the immune system, caused females to switch from a 36 strategy of reproductive restraint to terminal investment. Nevertheless, absolute 37 reproductive investment was lower in older females, indicating constraints of 38 senescence. 39 3 4. Our results show that a decline in reproductive investment with age does not 40 necessarily constitute evidence of reproductive senescence but can also result from 41 adaptive reproductive restraint. 42 5. Our results further suggest that the extent of reproductive investment is dependent on 43 several different intrinsic cues and that the particular blend of cues available at any 44 given age can yield very different patterns of investment. Perhaps this explains why 45 age-related reproductive investment patterns seen in nature are so diverse. 46 47