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Summary1. Asynchronous hatching has been studied extensively in altricial birds that feed their offspring, but this phenomenon is also known to occur in insects where parents provide food for their offspring. 2. We examined the proximate mechanisms underlying asynchronous hatching in the burying beetle Nicrophorus vespilloides , an insect species in which parents feed their offspring. 3. Mean hatching spread was 30 h. As expected if egg-laying was the major proximate mechanism underlying variation in hatching patterns, the duration of the egg-laying period had a highly significant and positive effect on hatching spread. The duration of the egg-laying period was unrelated to clutch size. 4. Hatching spread was slightly but significantly reduced compared with the duration of the egg-laying period, and the last eggs laid by females developed slightly but significantly faster than the first eggs. There was no significant difference in the size of the larvae that hatched from the first and last eggs laid by females. 5. We discuss the value of N. vespilloides and other insects where parents provide food for their offspring as alternative models to birds for the evolution of asynchronous hatching.

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

Asymmetric sibling competition, which occurs when some siblings hatch as stronger competitors than others, is an important component of avian reproductive strategies. Here, we report two experiments on the burying beetle Nicrophorus vespilloides investigating how parents might influence the outcome of asymmetric sibling competition. In this species, as in altricial birds, different-aged offspring compete for resources provided by the parents. However, unlike altricial birds, offspring depend only partially on their parents for resources, and parents adjust the brood size directly through filial cannibalism. In the first experiment, we compared the growth and survivorship of different-aged offspring when parents could and could not influence asymmetric sibling competition. In the second experiment, we recorded behavioral interactions between different-aged offspring and parents. We found that senior offspring (early-hatched) grew faster than juniors (late-hatched) when parents were present and could influence the outcome of sibling competition, whereas seniors and juniors grew at similar rates when parents were removed. Thus, seniors benefited more than did juniors when the offspring could obtain resources by begging from the female parent. There was no difference in the survivorship of seniors and juniors. We also found that seniors and juniors spent a similar amount of time feeding from female parents, but juniors spent more time begging and were less effective at begging than seniors. Interestingly, juniors spent more time begging only as long as seniors also begged, suggesting that juniors adjusted their begging effort in response to direct competition against seniors for resources provided by parents. Our study provides novel insights into the ecological significance of asymmetric sibling competition by showing that asymmetric sibling competition took place when parents were present and offspring could obtain resources by begging. In contrast, we found no evidence of asymmetric sibling competition when parents were absent and offspring obtained resources solely by self-feeding.

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