Seeking common principles of social evolution in different taxonomic groups, the contributors to this volume discuss eighteen groups of birds and mammals for which long-term field studies have been carried out. They examine how social organization is shaped by the interaction between proximate ecological pressures and culture"--the social traditions already in place and shaped by local and phylogenetic history.
Originally published in 1987.
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Ecological Aspects of Social Evolution
Birds and Mammals
By Daniel I. Rubenstein, Richard W. Wrangham
PRINCETON UNIVERSITY PRESSCopyright © 1986 Princeton University Press
All rights reserved.
Socioecology: Origins and Trends
DANIEL I. RUBENSTEIN AND RICHARD W. WRANGHAM
Understanding why species differ in their social behavior has fascinated biologists for years. Yet the search for explanations has never been more vigorous than it is now. In the past decade theories of the way evolutionary forces affect social behavior have been developed and tested in substantially greater detail than ever before. In the same period there has been a significant increase in our knowledge of the social behavior of wild animals. These developments have paved the way for two kinds of advances in our understanding of social behavior.
First, diverse patterns of behavior, from foraging to mating, are being understood in terms of their individual selective benefits. General rules are emerging that cut across different species and tie behavior to fundamental principles of evolutionary theory. Some rules are concerned with adaptations to the nonsocial environment, while others are related solely to the social strategies of conspecifics. Excellent reviews of these rules are available, particularly in Krebs and Davies' Behavioural Ecology (1984), where selected rules are applied to a wide variety of species. These examples show many elegant matches between theory and data, and thereby promise a remarkably tidy future for the analysis of social behavior.
Second, the adaptive significance of whole social systems is being dissected, analyzed, and understood in relation to environmental pressures. Remarkably, however, no recent book has systematically compared social systems as wholes. It is therefore often difficult to appreciate how different components of the system fit together, or why similar principles yield different results in different species. Nor is it yet clear what the major principles of grouping are in the vertebrates, or the extent to which they vary in different taxa. Accordingly, the present collection reviews the relationships between ecology and social behavior in a variety of birds and mammals. By comparing social ecology in different species groups, this book is intended to contribute to understanding the broad patterns of higher vertebrate social evolution.
The first pieces to the puzzle were discovered by John Crook over twenty-five years ago. Crook began in the late 1950s by comparing the behavior of different species of African weavers, birds that occupy a variety of habitats from primary rain forest to grassland savannah. He collected data on habitats, diets, nesting habits, and group sizes of approximately one hundred species, and found strong superficial correlations between ecology and social organization (Crook, 1964, 1970). This was exciting because it suggested that if the right variables were found, and if appropriate groups of species were compared, then the ways in which environments shape social systems could be identified. After a flurry of activity, the comparative method faltered. At this time social systems were described not in terms of the types and strength of social interactions, but in terms of the outcome of these interactions. Correlations between the outcomes, such as group size, dispersion pattern, mating system, and ecology, were of little use in showing how environments shaped social structure. What the comparative approach needed was a good theory. It got two: one from the social theorists and one from the ecologists.
The twin pillars of the modern theory of social behavior are the principles of individualistic reproductive maximization (G. C. Williams, 1966) and kin selection (W. D. Hamilton, 1964). With the development of these principles, animals came to be viewed as individuals who were armed with many behavioral options in their struggle for maximizing either their own reproduction, or that of their relatives. Extensions of the theory showed which options were better than others, in particular circumstances. By analyzing the behavior of individuals, the foundations for a comprehensive theory of social behavior were laid. Perhaps the most striking insight was that of Trivers (1972). He proposed a relationship between parental investment and sexual competition. He argued that when one sex invests more in the rearing of offspring than the other, members of the latter will compete for members of the former. The implication was simple: the sexes tend to invest their reproductive effort differently. When females invest heavily in parenting, males should invest heavily in mating, and vice versa.
The comparative method got an additional theoretical boost from the ecologists as well. Orians, Verner, and Willson analyzed polygamy and monogamy in red-winged blackbirds and proposed a general model for the evolution of polygyny (Orians, 1969). Like Trivers's theory, it was based on the idea that ecology influenced each sex differently. It suggested that when ecological conditions were just right, some males could defend large amounts of resources that females required. The model required that the habitat be divided into patches of vastly different qualities. When this occurred, many females could rear chicks in the best areas, whereas in the less rich areas, only one could succeed. In their model females, by attempting to maximize their reproduction, showed how they could induce the coexistence of both polygyny and monogamy in a population.
At this period, both social theorists and the ecologists were expanding their theories and attempting to account for the evolution of mating systems other than polygyny. A major breakthrough occured when Maynard Smith (1977) viewed the mating system problem as a game in which the optimal behavior of one parent depended on the behavior of the other. He argued that whether or not one parent should withhold parental care depends on how likely the other parent is to continue caring for the offspring. Since the parents' interests were similar, but not identical, Maynard Smith's idea was to search for a pair of strategies, one for the male and one for the female, which, when performed together, produced an 'evolutionarily stable strategy,' or ESS. Such a strategy would occur when it would not pay a male to depart from his strategy as long as the female continued to follow hers, and it would not pay the female to depart from her strategy as long as the male continued to follow his. The elegance of this approach was that all the pairwise options of: 1) both male and female care; 2) female care, male desert; 3) male care, female desert; and 4) both male and female desert, can be pitted against each other in a thought experiment and compared. Thus, the model offered the hope of determining the conditions that favored monogamy, polygyny, polyandry, and promiscuity, the four basic mating systems. Whether there is to be parental care and, if so, which parent is to provide it depends on the relative effectiveness of uni- or biparental care, the likelihood of a deserting partner finding an additional mate, and the extent to which a female's ability to provide care reduces her ability to produce future offspring. By estimating these parameters, the favored mating system can be predicted.
The model has many powerful features. First, it emphasizes sexual asymmetries, and focuses attention on a limited number of measurable variables that ultimately are associated with reproductive energetics. Second, it demonstrates that when selection favors only uniparental care, whether it is provided by the male or the female often depends on historical conditions and how they set the parameter values. This should provide powerful support to the comparative approach. The only problem with the model is its misuse by some in narrowing, rather than expanding the scope of the problem. Focusing on mating systems and their environmental determinants only provides a first step toward understanding a social system where social relationships other than those between males and females must be explained. Although a limited set of variables can adequately account for the evolution of each broad class of mating system, the problem will grow as explanations on a finer scale are required. The present reduction of dimension only becomes a problem if field workers limit their observations of natural systems to just this simple set of variables. If this were to happen, valuable insights would be lost.
While the social theorists were elaborating general rules for the evolution of the major mating systems, ecologists were expanding Orians' principle to account for the complex systems found in species other than blackbirds. Emlen and Oring (1977) proposed that the potential for polygyny depended on both the spatial and temporal distribution of members of the limiting sex. For example, an even, scattered distribution of females makes it unlikely that one male will control more females than others. But when females live in groups, or as Orians suggested, the resources they need occur in patches, then some males will have the opportunity ultimately to control large numbers of females. Under these conditions the potential for polygyny is high. Whether it is realized depends on other factors, such as the degree to which females synchronize their reproductive behavior. If all females are receptive at exactly the same time, then males have little chance of mating with more than one female, and the polygyny potential is lost.
For the first time, a general theory of mating systems was emerging, but what it lacked was an understanding of how ecology shaped female reproductive interests. For Emlen and Oring, female distributions were given. But reliance on this assumption was eliminated by studies on antelope (Jarman, 1974) and bats (Bradbury and Vehrencamp, 1977a, b). Both studies showed that female distributions were finely tuned to the environment and depended on the needs of females to seek food and safety from predators. For antelope females, Jarman argued that diet was determined by body size; small females needed high-quality forage, whereas large females needed large quantities of low-quality food. And he showed that because high-quality items were more widely dispersed than low-quality items, smaller females were forced to live more solitary lives than larger females. As a consequence, he concluded that monogamy was the typical mating system of the smaller antelope, whereas polygyny, in its various guises, was to be the norm for the larger ones.
As Jarman's (1974) study shows, a framework is now in place for examining how ecology shapes certain intra- and intersexual relationships. Of course the framework will be strengthened as the ESS approach is applied to more types of competitive and cooperative interactions among social animals. But even in its simplest form, the model expects first, that female behavior — and this includes their social relationships — will be adapted primarily to meeting demands imposed by the physical environment. This is because the reproductive rate of females will normally be raised, or lowered, more predictably by their success in meeting these demands than by their success in other endeavors such as finding, or choosing among, mates. Second, it expects that male strategies are adapted primarily to competing for mating opportunities, because male fitness is more closely tied to mating success than to the acquisition of other resources. Taken together, they imply that to understand the ecological basis of social organization, it is imperative to understand separately the ecological and, as the ESS approach shows, social pressures operating on each sex.
Although only a beginning, the model contains some very basic ideas whose usefulness must be evaluated. With the recent proliferation of long-term field studies, there is sufficient data to apply these ideas to many different species with different ecologies and different phylogenetic histories. The collection of essays in this volume represents such an application, and one that succeeds in refining the old ideas and defining new problems.
We deliberately limited the scope of the analysis to birds and mammals, the main reason being that the complexity and individualistic nature of the social relationships exhibited by these groups poses the greatest challenge for the existing framework. The birds and mammals also exhibit the full range of standard mating systems, and comprise a sufficiently large number of different taxonomic groups. This has allowed us to maintain diversity, without spreading ourselves taxonomically so thin that no generalizations could be made. Moreover, for the first time, the long-term studies necessary for interpreting changes in social relationships of such long-lived animals are numerous enough to be evaluated.
The book is divided into two parts — Monogamous Variations and Polygynous Patterns. As is the case in the world at large, most of the avian studies are found in the former section and most of the mammalian studies are found in the latter. But as these studies show, similarities in the ecological bases underlying the evolution of the social systems of each type is surprisingly low.
The book begins with studies on the monagamous theme, in particular with Oring's study of facultative polyandry in spotted sandpipers. Of all the mating systems, polyandry is perhaps the least well understood. Few correlations with habitat type have been found, and when contrasted with monogamous systems, polyandry emerges from most studies as favoring females, but not males (Verhencamp and Bradbury, 1984). As our general framework suggests, such an analysis, if true, would certainly not be evolutionarily stable. Oring's analysis suggests that, at least for sandpipers, ecology plays an important role, and that both males and females benefit. According to Oring, sandpipers nest in areas where nest predation is extremely high, except on some islands. In these isolated places, the density of birds rises and because of intense female competition, some are excluded. Conversely, some secondary males are included. For males and females that breed the benefits are higher than they would be by breeding monogamously in less safe areas. Given that most polyandrous birds nest in open and defenseless areas, predation might be a critical variable for these other species as well.
Predation is clearly the major selective agent in the moorhen, a bird that usually mates monogamously, but occasionally exhibits polyandry. Petrie, in Chapter 3, has been able to show that nest failure resulting from predation places pressures on males to compete for territories comprising dense stands of aquatic vegetation. Yet females do not seem to choose mates on the basis of territory quality. Instead, they seem to choose males that are disproportionately fat. Petrie resolves this paradox by demonstrating that males perform most of the nest-guarding functions while females feed. With such a high likelihood of nest failure, it seems to be critical that a female stay well provisioned so she can lay replacement eggs, and that a male start the season with enormous fat reserves so he can incubate eggs laid late in the season. As in Oring's sandpipers, the threat of predation seems to establish a situation in which female behavior controls that of males. But in the moorhen the control is only partial, as males are simultaneously driven to compete for the safest territories. Nevertheless, in this monogamous society the division of posthatching parental labor is far from equal.
Excerpted from Ecological Aspects of Social Evolution by Daniel I. Rubenstein, Richard W. Wrangham. Copyright © 1986 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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Table of Contents
- FrontMatter, pg. i
- Contents, pg. v
- Preface, pg. vii
- Contributors, pg. ix
- 1. Socioecology: Origins and Trends, pg. 1
- 2. Polyandry in Spotted Sandpipers: The Impact of Environment and Experience, pg. 21
- 3. Reproductive Strategies of Male and Female Moorhens (Gallinula chloropus), pg. 43
- 4. Ecology of Cooperation in Canids, pg. 64
- 5. Sexual Asymmetries in the Life History of the Florida Scrub Jay, pg. 87
- 6. Hornbill Social Dispersion: Variations on a Monogamous Theme, pg. 108
- 7. Ecology and Social Evolution in the Mongooses, pg. 131
- 8. Ecological Factors Influencing the Social Systems of Migratory Dabbling Ducks, pg. 153
- 9. The Evolution of Social Behavior and Mating Systems in the Blackbirds (Icterinae), pg. 175
- 10. Ecological and Social Determinants of Cercopithecine Mating Patterns, pg. 201
- 11. Resource Distribution, Social Competition, and Mating Patterns in Human Societies, pg. 217
- 12. The Evolution of Mating Strategies in Male Antelopes, pg. 244
- 13. Ecology and Sociality in Horses and Zebras, pg. 282
- 14. Marmot Polygyny Revisited: Determinants of Male and Female Reproductive Strategies, pg. 303
- 15. The Social Ecology of Gelada Baboons, pg. 332
- 16. Ecology and Social Relationships in Two Species of Chimpanzee, pg. 352
- 17. Male and Female Mating Strategies on Sage Grouse Leks, pg. 379
- 18. Grouping, Associations, and Reproductive Strategies in Eastern Grey Kangaroos, pg. 399
- 19. The Ecology of Sociality in Felids, pg. 429
- 20. Social Evolution in Birds and Mammals, pg. 452
- Acknowledgments, pg. 471
- Literature Cited, pg. 475
- Author Index, pg. 529