How do caring parents respond to mate loss? Differential response by males and females

Game theory has been used to provide theoretical solutions for the evolutionary stability of biparental care given sexual conflict between parents. Existing models predict that, should one parent reduce its care, the mate should adjust its care facultatively to compensate partially. This has been tested mostly on birds with equivocal results. To examine the generality of this prediction, we tested for facultative adjustments in care in the burying beetle Nicrophorus vespilloides, where both parents regurgitate predigested carrion to their offspring. Females were considerably more involved than males in direct care of the larvae before mate removal. Males adjusted their care significantly after female removal, whereas females showed no response to male removal. Thus, only male behaviour was consistent with theoretical predictions. There was no evidence that male or female removal had detrimental effects on offspring fitness. We suggest that both the differential response by males and females to mate removal and the lack of a fitness effect of mate removal reflected the sex difference in the involvement in care before removal. Male removal had no effect on larval fitness simply because male involvement in care before removal was low, whereas female removal had no effect because males adjusted their behaviour after female removal. Our study reinforces the potential for alternative responses to mate removal in species with biparental care, and highlights the need for theoreticians to develop models that explore the conditions under which biparental care can be evolutionarily stable when parents engage in multiple parental behaviours.