Does Brood Parasitism Induce Paternal Care in a Polygynous Host?

Physiological responses of host parents to rearing an avian brood parasite: An experimental study

Raising an obligate avian brood parasite is costly for host parents because it redirects valuable parental resources from one's own offspring to genetically unrelated young. The costs of raising a brood parasite may be mediated by physiological stressors for foster parents if it requires greater or biased parental effort compared to raising non-parasitized broods. For example, upregulating glucocorticoid hormones or reducing immune responses may mediate a trade-off between resource allocation to a current brood versus future reproductive opportunities, but published data on parasitized hosts' physiology are scarce. Here we used an experimental approach to test if host parents respond to the presence of brood parasitic young through dynamic changes in their own physiology. We conducted our experiments with parasitic brown-headed cowbirds (Molothrus ater) fostered into nests of host prothonotary warblers (Protonotaria citrea). Relative to parents caring for non-parasitized control broods, parasitism increased baseline corticosterone levels and reduced body mass in adult male, but not in female, hosts. Immune responses to a novel antigen were depressed in both parents of parasitized broods compared to parents of non-parasitized broods. Additionally, we found that immune function increased along the breeding season regardless of treatment. These experiments also confirmed prior observational data that parasitized adult males have reduced return rates to breeding sites in years subsequent to raising cowbirds. The findings demonstrate diverse physiological effects of parasitism on the foster parents in our particular host-brood parasite system, yet we found no evidence of individual trade-offs between endocrine and immune responses of adult hosts.

Interspecific interactions explain variation in the duration of paternal care in the burying beetle

Why is there so much variation within species in the extent to which males contribute to offspring care? Answers to this question commonly focus on intraspecific sources of variation in the relative costs and benefits of supplying paternal investment. With experiments in the laboratory on the burying beetle, Nicrophorus vespilloides, and its phoretic mite Poecilochirus carabi, we investigated whether interactions with a second species might also account for intraspecific variation in the extent of paternal care, and whether this variation is due to adaptation or constraint. In our first experiment we bred beetles in the presence or absence of phoretic mites, using a breeding box that mimicked natural conditions by allowing parents to leave the breeding attempt at a time of their choosing. We found that males abandoned their brood sooner when breeding alongside mites than when breeding in their absence. Female patterns of care were unchanged by the mites. Nevertheless, in this experiment, no correlates of beetle fitness were affected by the presence of the mites during reproduction (neither paternal life span after reproduction nor brood size or average larval mass). In a second experiment, we again bred beetles with or without mites but this time we prevented parents from abandoning the brood. This time we found that both parents and the brood suffered fitness costs when breeding alongside mites, compared with families breeding in the absence of mites. We conclude that males adaptively reduce their contributions to care when mites are present, so as to defend their offspring's fitness and their own residual fitness. Interspecific interactions thus account for intraspecific variation in the duration of paternal care.

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The extent of paternal care varies greatly within species.

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Some of this variation might be due to interspecific interactions.

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We investigated how mites influence paternal care in the burying beetle.

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We found that males leave their brood earlier when mites are present.

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We show that this is adaptive because it enhances offspring fitness.