Causes and consequences of asynchronous hatching in the burying beetle Nicrophorus vespilloides
Ford, Lucy Ella
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In this thesis, I explore the causes and consequences of asynchronous hatching (when the offspring form a single reproductive event hatch over an extended period of time) in the burying beetle Nicrophorus vespilloides, an insect with biparental care. Hatching asynchrony can be influenced by both intrinsic and extrinsic factors and may be adaptive or a consequence of constraints on egg laying. The four potential causes of asynchronous hatching I focus on here are 1) sexual conflict over parental care, 2) physiological constraints on completing reproduction quickly, 3) inbreeding and 4) ageing. The sexual conflict hypothesis for the evolution of asynchronous hatching suggests that females adjust hatching patterns in order to increase male parental effort relative to female effort. As predicted, I found that males provided care for longer to asynchronous broods whereas the opposite was true of females. However, I did not find any benefit to females of reducing their duration of care in terms of their mass change or lifespan, and I found substantial negative effects of hatching asynchrony on offspring fitness. The hurry-up hypothesis suggests that completing reproduction quickly is beneficial when the quantity or quality of breeding resources declines over time and this may lead to asynchronous hatching if there are physiological constraints on laying. My results suggest that, although asynchronous hatching might emerge as a by-product of parents attempting to complete reproduction sooner, there is no evidence that females attempt to do so sooner under conditions where this would be favourable. Inbred females may be constrained in their laying patterns if they are of poorer quality, which might cause them to lay more asynchronously. Conversely, I found that inbred mothers produced clutches where egg laying was less skewed towards the early part of laying, improving larval survival. Inbred females may facultatively adjust their laying patterns to compensate for the detrimental effects of maternal inbreeding on offspring. If selection on laying patterns is weaker at older ages, females breeding at older ages might lay more asynchronously. Instead, I found that females breeding at older ages produce clutches with a lower within-brood variance in laying times and that were less positively skewed. Age-related reductions in clutch size were also associated with reduced within-clutch variance in laying times and a less positively skewed distribution of laying times. Thus the indirect effect of female age mediated through clutch size reinforces the direct effect of female age on the variance and skew in laying times. I conclude that asynchronous hatching in N. vespilloides is unlikely to be explained by a single cause.