Routes to fitness in cooperatively breeding brown jays

Dean Alan Williams, Purdue University

Abstract

Debate about evolutionary pathways to sociality has focused on the importance of genetic relatedness and indirect enhancement of biological fitness through nonreproductive aid-giving. Only recently, however, have the molecular tools been available to directly determine genetic similarity of apparent altruists to recipients of their aid, and only a few long-term studies have generated the equally important empirical foundation of behavioral and demographic data. Cooperative breeding in birds occurs in hundreds of species, and is a heterogeneous phenomenon defying simple explanation. Brown jays (Cyanocorax morio ) of Central America are obligately social, always breeding in large groups, and they represent one of the most complex forms of vertebrate sociality. Hypotheses concerning the decisions individuals make about alternative routes to biological fitness include predictions that genetic parentage should be strictly associated with investments in cooperative behavior. Alternatively, natural selection may be more likely to favor cooperative strategies that maximize genetic returns on cooperation over the lifetimes of individuals, not necessarily over short time periods. I have employed DNA “fingerprinting”, along with behavioral measures of cooperation, demographic measurements of the ecological consequences of group living, and long-term data on dispersal among groups, to evaluate hypotheses explaining delayed breeding, philopatry, and cooperation. Most brown jays live a decade or more, and most delay breeding for half that time. Single breeding females almost completely dominate reproduction in a group, but paternity is shared in nearly half of broods, often including males from other groups. Females form philopatric matrilines that determine parentage within groups, and their principal route to fitness is natal group inheritance. Males pursue the very different strategies of intergroup mixing and dispersal, reversing the normal sex-bias among birds, and male participation in rearing young is not the best predictor of reproductive success. Neither paternity nor collateral kinship predicts male cooperation, particularly among consorts that invest considerable time and energy in attending breeding females, without immediate fitness gain. Results of this study, including patterns of aid-giving and dispersal among groups, refute the hypothesis that cooperation maximizes immediate fitness gain, and support the view that cooperation represents investment in the long-term benefits of group living, both ecological and social, including alliance formation and improved eventual access to breeding status. Better tests of this hypothesis will require longer-term studies that can fully describe lifetime fitness strategies.

Degree

Ph.D.

Advisors

Rabenold, Purdue University.

Subject Area

Ecology

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