Plasticity in parental care: Interspecific competitor cues shape biparental cooperation in a burying beetle.

Interspecific competition is an important evolutionary driver of many species' life histories and behaviours, arising wherever species come into conflict over limited resources. However, how such competition shapes the plasticity of social behaviours within a species, such as biparental care, remains less clear. The burying beetle Nicrophorus vespilloides is an insect that exhibits biparental care, relying on small vertebrate carcasses to breed on. Such carcasses are highly valuable but relatively rare in nature, and so are competitively sought out by many other species, including the bluebottle fly, Calliphora vomitoria. Despite the close association and frequent rivalry between the two species, how their interactions shape biparental care decisions in these beetles has received little attention. We investigated how perceived risk of interspecific competition affected breeding success and duration of male and female care by staging encounters between breeding pairs of beetles and fly intruders. We presented breeding pairs with dead flies, at varying times and densities, to assess changes in duration of care and fitness outcomes. We used dead flies to ensure the beetles' response was driven solely by cues associated with flies, rather than direct interaction or resource depletion. We found that the duration of both male and female care was plastic in response to the presence of competitors, as both increased their duration of care. However, these encounters also increased breeding failures, with the strongest effects occurring when encounters took place early in the breeding attempt. Our results demonstrate plasticity in parents' behaviour in response to a perceived competitive threat, but this plastic response was associated with increased breeding failures, suggesting a trade-off between dealing with competitors and completing other parental tasks.