Balancing The Costs Of Inbreeding In Cooperative Crows
Because the cooperatively breeding American crow (Corvus brachrhynchos) exhibits natal philopatry of both sexes, related adults of the opposite sex interact, increasing the probability of matings between kin. Cooperative breeding based on natal philopatry of both sexes might be expected to persist as a strategy if inbreeding is avoided altogether, if inbreeding occurs with low or no costs, or if the benefits of living and breeding with kin outweigh inbreeding costs. In a suburban population of crows in Ithaca, New York, I found that 19% of social pairs were related at the level of first- or second-order kin, and that kin matings had severe costs for inbred offspring in terms of reduced body condition and immunocompetence, higher disease probability, and lower survival probability. Ithaca crows were not genetically monogamous: I found that 17% of offspring were sired by extrapair males, and that a male was more likely to lose paternity when he was injured, potentially due, in part, to a reduction in his functional fertility. I found no evidence that extrapair males were less related to a female than she was to her social pair male, no evidence that prospective extrapair males were more successful when they were less related to a given female, and no evidence that there were any genetic benefits gained by females through extrapair paternity. Furthermore, some within-group extrapair sires were sons of the breeding female, and incestuously produced offspring appeared to suffer the most severe costs. On the level of overall brood output, however, these genetic costs appeared to be outweighed by the parental contributions provided by within-group extrapair sires: overall provisioning rate and brood output was higher in broods associated with within-group extrapair auxiliary sires, suggesting that these extrapair sires provided direct benefits to all young from broods in which they shared paternity. Another potential reason for the persistence of natal philopatry in this suburban population, despite severe inbreeding costs, is that the costs of inbreeding are lower in adjacent, contiguous rural populations, dampening an evolutionary response to selection against inbreeding.
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