Kin-selected siblicide and cannibalism in the European earwig

Abstract

Aggression levels among individuals can severely increase under high density or shortage of crucial resources, sometimes resulting in individuals killing conspecifics. This is not uncommon in family groups of diverse taxa, where the dependent offspring compete for the limited resources provided by their parents. Killing a nest mate can relax the level of competition, and cannibalism provides a direct nutritional benefit. However, nest-mate killing bears the risk of reducing indirect fitness if the victim is related (i.e., siblicide), imposing selection on kin-recognition abilities. Based on this hypothesis, we predicted that first instar juveniles (nymphs) of the European earwig (Forficula auricularia) kill and cannibalize unrelated nest mates earlier and more often than related nest mates and that cannibalism has a direct nutritional benefit in terms of survival. We tested these predictions experimentally by establishing related and unrelated pairs of nymphs and recorded survival, aggregation behavior, and cannibalistic outcomes in the absence of alternative food sources. In order to obtain expected survival probabilities of victims and survivors in the absence of any interaction, we simulated virtual nymph pairs based on survival data of singly held control individuals. As predicted, victims lived for less time and survivor for longer than expected from the simulated survival distributions, demonstrating nest-mate killing and cannibalism. Furthermore, unrelated individuals were killed significantly earlier and were more often cannibalized than related individuals. The survival patterns of victims and survivors were quantitatively consistent with the expectations of Hamilton's rule. Our study shows that earwig nymphs recognize kin and adjust their nest-mate killing and cannibalistic behavior as predicted under the hypothesis of kin-selected siblicide and cannibalism.

One clutch or two clutches? Fitness correlates of coexisting alternative female life-histories in the European earwig

“…Alternatively, it is possible that a higher cannibalism rate in second than first clutches may explain this result. Because mortality in earwig clutches is almost always associated with cannibalism (Dobler and Kölliker 2010), our finding of significantly lower survival rate together with higher weight gain of nymphs in second than first clutches are consistent with the hypothesis that cannibalism is higher in second clutches.…”

Paternal signature in kin recognition cues of a social insect: concealed in juveniles, revealed in adults

“…But consistent with the hypothesis that selection on mothers favoured concealment of paternally inherited signatures on her offspring, previous experiments showed discrimination in cannibalism among nymphs from different clutches (i.e. with different mothers) [29], but lack of effect of multiple mating on cannibalism rate within clutches [38].…”

“…The offspring (nymphs) live in family groups for several weeks, during which females provide multiple forms of care, such as egg and offspring attendance and food provisioning [27,28]. Previous work has shown that sibling competition and cannibalism are common in this species [29] and occurs significantly earlier and more often between unrelated nymphs from different clutches [29]. Thus, kin recognition cues seem to be present and used, and cannibalism is a potential form of nepotistic interactions among young nymphs.…”

The population determines whether and how life-history traits vary between reproductive events in an insect with maternal care

“…Furthermore, these findings could indicate that the benefits of maternal care are more important for second than first clutch nymphs and therefore select for higher expression of care towards late clutch nymphs across populations. In line with this idea, spring is often well advanced when second clutch eggs hatch, so that the environmental conditions experienced by the resulting nymphs are likely to increase (as compared to first clutch nymphs) their exposure to pathogens, such as fungi, or their predators, such as other arthropods and even the first clutch nymphs (Dobler and Kölliker 2010). Irrespective of the mechanism mediating the increased level of maternal care towards second clutch nymphs, the absence of a population-specific effect indicates that the environmental conditions experienced by females during first clutch family life (and egg care) are crucial in determining the subsequent expression of maternal care towards their second clutch offspring.…”