“Kinship and incest avoidance drive patterns of reproductive skew in cooperatively breeding birds”

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Christina Riehl

Why is reproduction unequal among members of social animal groups? New research suggests that incest avoidance is key

Family matters: incest avoidance and reproduction in social birds

A communal nest of greater anis (Crotophaga major) containing 10 eggs laid by 3 unrelated females.
(Credit: Christina Riehl)

Cooperatively breeding animals vary in how reproduction is divided among group members. In some species, several individuals all reproduce and share care of the mixed brood; in others, a dominant individual or pair monopolizes reproduction and is assisted by non-breeding “helpers.” Although the degree of reproductive sharing, or “skew,” has been a popular subject of theoretical models, researchers still disagree about the reasons for this variation. In the first large-scale analysis across species, a new study in The American Naturalist shows that patterns of genetic relatedness among members of cooperative groups are the best predictors of whether reproduction is shared. Using data from over 80 species of cooperatively breeding birds, Christina Riehl of Princeton University has found that reproduction tends to be shared when group members are unrelated, whereas one pair tends to dominate in family groups.

This general pattern—high skew in families and low skew with non-relatives—has been proposed before, and is supported by prominent theoretical models. But why? What prevents relatives from breeding together and producing offspring in the same nest? Two different mechanisms have been proposed. First, dominant group members might actively prevent related subordinates from reproducing; for example, a dominant female might prevent her younger relatives from laying eggs in her nest. Alternatively, given that inbreeding can have fitness costs, helpers might be less likely to reproduce if their only potential mates are relatives. In support of the latter hypothesis, Riehl finds that incest avoidance is the most likely reason that relatives don’t breed together. Helpers in cooperative groups were more likely to breed if they were unrelated to opposite-sex group members, whereas relatedness to same-sex group members had no effect. These results provide the first wide-scale evidence that incest avoidance constrains reproduction in cooperative groups, and they suggest new lines of inquiry for theoretical models. Read the Article