From cells to societies, cooperation occurs at all levels of biological organization. In vertebrates, the most complex societies occur in cooperative breeders where some group members (helpers) forego reproduction, sacrificing their immediate direct fitness to assist in raising the offspring of others (breeders). Individuals in cooperative breeding societies can gain indirect fitness benefits from passing on shared genes when they help the offspring of close genetic relatives (kin selection), such that cooperation is expected to correlate with genetic relatedness.
However, some cooperatively breeding societies include cooperation between nonrelatives. Cooperatively breeding societies range in complexity, from singular (one breeding pair) to plural (two or more breeding pairs). In the majority of singular breeding societies, helpers are relatives of breeders. Thus, kin selection is thought to underlie helping behavior in singular breeding societies. Plural breeding societies, such as in superb starlings (Lamprotornis superbus) inhabiting the East African savanna in central Kenya, involve multiple territory-sharing families raising offspring with helpers who can assist more than one family simultaneously. The superb starling’s complex and dynamic social system, mixed kin structure, relatively long lives, and stable social groups make them an ideal study species for investigating how patterns of individual decision-making have shaped and maintained cooperative societies. My dissertation research focuses on using long-term data on cooperatively breeding superb starlings to explore how temporally variable environments, such as the East African savanna, influence individual decisions across lifetimes, and subsequently how individual behavior shapes the structure and organization of the society.
In Chapter 1, I apply a Bayesian approach to the animal model to estimate how genetic versus nongenetic factors influence among individual variation in the social roles: “breeder”, “helper”, and “non-breeder/non-helper”. Non-breeder/non-helper indicates that the individual maintained membership in the social group but did not breed or help during that season. I then estimated heritability and found, as predicted, overall low heritability of traits responsible for each role. This result is consistent with the findings of other studies on the heritability of social behavior, which tends to be low compared to non-social traits, primarily because the social behavior of an individual is highly influenced by interactions with other individuals.
In Chapter 2, I show that superb starlings (i) are nepotistic, and (ii) switch between the social roles of “helper” and “breeder” across their lives. This role switching, which unexpectedly includes breeders going back to helping again, is linked to reciprocal helping between pairs of helpers and breeders, independent of genetic relatedness. Reciprocal helping was long thought to be irrelevant for cooperative breeders because most helping is assumed to be unidirectional, from subordinate helpers to dominant breeders, and reciprocal helping is often measured on short timescales. These long-term reciprocal helping relationships among kin and nonkin alike may be important for the persistence of this population because previous research has demonstrated that enhancing group size by immigration from outside groups, while reducing group kin structure, is necessary to prevent group extinction.
Finally, the results of Chapter 3 reveal how social and ecological factors shape role switching across individual lifetimes. Overall, my dissertation highlights the remarkable flexibility of superb starling cooperative behavior and the crucial role of mutual direct fitness benefits from reciprocal helping, which may help promote the stability of cooperative group living among nonkin as well as kin group members, contributing to the resilience of this population within a harsh and unpredictable environment.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/1q8e-nh55 |
| Date | January 2024 |
| Creators | Earl, Alexis Diana |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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