Toward a Unifying Theory of Social Evolution: Testing Competing Models of Reproductive Skew in an Avian Cooperative Breeder

Cornell Univ - State: Awds Made Prior May 2010, Ithaca NY

Investigators

Abstract

TOWARD A UNIFYING THEORY OF SOCIAL EVOLUTION: TESTING COMPETING MODELS OF REPRODUCTIVE SKEW IN AN AVIAN COOPERATIVE BREEDER. STEPHEN T. EMLEN PROJECT SUMMARY (IN LAY TERMS) The family (defined as a multi-generation assemblage of genetic relatives) is among the least understood of all forms of social organization. Yet, because it is the basic social organization of our own species, it is arguably the most important for us to understand. Only about 3% of birds and mammals live in family-based societies, and only a few of these have been studied in detail. Families come in various sizes and forms. The most common form in both birds and mammals is the nuclear family, formed when grown young remain with their parents, but only the parents reproduce. Most so-called 'cooperative breeders' fit this category, with grown sons and daughters remaining as helpers at the nest or den of their parents. Less common, but of greater interest, is the extended family, formed when reproduction is shared among many family members. When multiple pairs within a single family breed, the result is a mixture not only of parents and offspring, but also of grandparents, uncles and aunts, nieces and nephews, and cousins as well as unrelated in-laws. Until very recently, we humans lived predominantly in groups comprised of such extended family units. The difference between extended and nuclear families is one of shared versus monopolized breeding. A rapidly growing number of evolutionary models, known collectively as reproductive skew theory, attempt to explain the variability of reproductive suppression found in nature. Skew models incorporate a small number of factors (two ecological, one genetic, and one social) to predict 1) when stable family groups will form, 2) what their stable size will be, and 3) how reproduction will be shared among family members. Reproductive skew they is generating considerable interest among behavioral ecologists and evolutionary biologists because it links many major features of animal societies to their ecological, genetic, and social underpinnings. Because of this breadth, some workers believe that skew theory may provide a unifying framework for viewing much of social evolution. However, the development of the theory has far outpaced the testing of its assumptions and predictions with real animals in the field. There is an urgent need for detailed observational and experimental studies of skew theory on birds and mammals that live in family-based societies. The proposed research will provide such a test. The study species is the Grey capped social weaver (Pseudonigrita arnaudi), a common inhabitant of the whistling thorn savanna of East Africa. This species was chosen because it lives in both extended and nuclear family units, background knowledge of its behavior and ecology is already well-known, and it possesses many features that make it logistically easy to collect data and perform experiments. A study population of over 50 family units has been captured and the birds individually marked. Molecular markers (microsatellite loci) have been developed that will allow genetic determination of family structures (e.g. who is the father, or aunt, of whom). The research program will help to train numerous Kenyan professionals in modern methods of behavioral research. It will also combine the talents of several senior investigators whose expertise ranges from field studies of behavior, to molecular studies of social organization, to theoretical modeling of social evolution. As such, it constitutes one of the first integrated tests of the applicability of recent skew models to a complex vertebrate society. The results should enhance our understanding of 1) the factors that favor the evolution of family groups, 2) the reasons why extended families develop when they do, and 3) the behaviors that family members use in their cooperative and competitive interactions with one another (so-called 'family dynamics'). This, in turn, should provide insights into the conditions that favored the evolution and maintenance of extended family groupings in our own species as well.