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Reproductive Decisions: An Economic Analysis of Gelada Baboon Social Strategies

Reproductive Decisions: An Economic Analysis of Gelada Baboon Social Strategies

Copyright Date: 1984
Pages: 274
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  • Book Info
    Reproductive Decisions: An Economic Analysis of Gelada Baboon Social Strategies
    Book Description:

    Robin Dunbar uses economic models to explore the social behavior of the gelada baboon (Theropithecus gelada), a unique species, whose social system is one of the most complex among the primates. His work illustrates the value of an approach that views social behavior as being ultimately concerned with reproduction and with the maximizing of an individual's contribution to its species' gene pool.

    Originally published in 1985.

    ThePrinceton Legacy Libraryuses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These paperback editions preserve the original texts of these important books while presenting them in durable paperback editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

    eISBN: 978-1-4008-5384-7
    Subjects: Zoology

Table of Contents

  1. Front Matter
    (pp. i-iv)
  2. Table of Contents
    (pp. v-viii)
  3. Acknowledgments
    (pp. ix-2)
  4. 1 Introduction
    (pp. 3-15)

    As our knowledge of the behavior of particular species has increased with time, it has become apparent that the traditional ethological notion of “species-specific behavior” is often inappropriate for social behavior. More than anything else, field workers have come to appreciate that the degree of variability in the behavior of natural populations of animals is quite extraordinary. Concomitantly, we have seen a shift in emphasis over the past two decades from the early ethologists’ view of animals responding more or less automatically to stimuli to one where animals are seen as decision-makers engaged in a process of evaluating strategic options....

  5. 2 Structure of Gelada Populations
    (pp. 16-23)

    The gelada social system is a complex arrangement of hierarchically organized social groupings, each of which corresponds to a different functional unit. These groupings are analogous to those of the hamadryas baboon,Papio hamadryas(see Sigg et al. 1982), these two species apparently being unique among the primates in the degree of organizational complexity that they have evolved.

    In this chapter, I describe the social units that make up the gelada system. The species’ ecological niche and population dynamics will be outlined in Chapters 3 and 4, while the structure of social relationships between the individual animals will be discussed...

  6. 3 Ecological Constraints
    (pp. 24-31)

    In this chapter, I provide an overview of the more important aspects of gelada ecology. I draw on comparative data to show how the species’ dietary specializations influence time budgets, population density, and ranging patterns. This specialization has important consequences for the animals’ reproductive strategies since it limits and constrains their day-to-day behavior. Detailed accounts of the species’ ecology can be found in Dunbar (1977a), Iwamoto (1979), Kawai and Iwamoto (1979), and Iwamoto and Dunbar (1983). These sources should be consulted for the bases on which causal relationships were deduced from comparative data.

    Gelada rarely eat anything other than grass...

  7. 4 Demographic Processes
    (pp. 32-38)

    This chapter summarizes the main demographic processes and life-history variables and provides the demographic information on which most of the analyses of reproductive strategies depend. The results presented here are based on analyses given by Dunbar and Dunbar (1975), Ohsawa (1979), Dunbar (1980a), and Ohsawa and Dunbar (1984). These papers should be consulted for details of the deduction of causal inferences.

    Figure 11 shows estimated survivorship curves for males and females of the Sankaber population. The curves follow the pattern typical of most primate species: mortality among immatures is highest among infants and plateaus out during the juvenile period before...

  8. 5 Social Structure of Reproductive Units
    (pp. 39-50)

    Gelada reproductive units are closed social microcosms (see Dunbar and Dunbar 1975, Mori 1979b). Consequently, an individual’s social and reproductive strategies are constrained by the behavior of the other members of its unit. Understanding the principles that underlie social relationships between the adult members of a unit is thus critical for any analysis of the reproductive strategies of both males and females. In this chapter, I draw on analyses given by Dunbar (1979b, 1980b, 1982c, 1983b,c,d) and also present some new data.

    Analysis of the structure of relationships between reproductive females shows that females tend to form dyads whose members...

  9. 6 Constraints on Female Reproduction
    (pp. 51-55)

    In this chapter, I consider some important reproductive consequences of the social relationships among females that were described in Chapter 5. These consequences provide the reproductive context within which females make their strategic decisions. It is thus in the constraints imposed by these consequences that explanations for the behavior of the females are likely to be found.

    As far as we know, gelada do not differ significantly from other Old World monkeys in the general features of female reproductive physiology (though it is conceivable that there are quantitative differences in hormone titres). They do, however, exhibit a number of unique...

  10. 7 The Female’s Socio–Reproductive Strategies
    (pp. 56-76)

    In order to understand what the females are trying to do, we need to know what the null condition is around which their strategies of reproduction are built (i.e. the constraint-free strategy). In the present case, this is the state that would result if females relied only on their intrinsic natural fighting abilities. Other things being equal, a female would gradually rise in rank as she matured, reaching maximum rank during her prime (and in smaller units where few females are of similar age, this maximum will usually be rank 1), to decline again as she grew older. By and...

  11. 8 A Decision Model of Female Reproductive Strategies
    (pp. 77-89)

    The analyses in the previous chapter suggest that a female has six main strategies open to her: (1) to form coalitions with her mother and daughter(s), (2) to form a coalition with a sister, (3) to form a coalition with a less closely related female, (4) to become the harem male’s social partner, (5) to become the new male’s partner when the unit is taken over and (6) to join the follower when her unit undergoes fission. Can we evaluate the relative profitabilities of these various strategies in order to say something about the optimal strategy choice for a female...

  12. 9 The Female’s Tactical Options
    (pp. 90-105)

    Within the general constraints imposed by long-term strategic considerations, females do have some freedom of movement in terms of the tactics they can pursue to offset losses in reproductive rate. There are three groups of tactical options open to females: behavioral tactics during oestrus (aimed at improving the probability of impregnation on each oestrous cycle); reproductive tactics (aimed at increasing the female’s contribution to the species’ gene pool); and tactics involving differential parental investment (aimed at maximizing an offspring’s chances of survival).

    Such tactics are unlikely to be equally attractive to females of all ages, however. Thus, attempts to step...

  13. 10 The Male’s Loyalty Problem
    (pp. 106-123)

    The female’s problems relate strictly to intra-unit competition among females, with matrilineal coalitions forming a crucial basis from which she operates. The male, on the other hand, is effectively on his own and his problems are twofold. First, in order to breed at all, he has to gain control over a harem of females. Secondly, having gained a harem, he has to prevent other males from taking it away from him. His first task is not easy, and his second is compounded by the females’ own strategic designs.

    In Chapter 7, we saw that females are liable to desert their...

  14. 11 Rules and Decisions in Harem Acquisition
    (pp. 124-144)

    In this chapter, I summarize the options open to a male and examine the decisions he makes during the process of acquiring a harem. In the following chapter, I undertake a simulation analysis of harem acquisition strategies aimed at determining whether the alternative strategies that males pursue are equally profitable. Chapter 13 explores the tactics that males use to maximize the length of time they can retain control of their harems. In Chapter 14, I try to integrate the data on male harem acquisition strategies with the ecological and demographic data of Chapters 3 and 4 in order to explore...

  15. 12 An Economic Model of Male Reproductive Strategies
    (pp. 145-164)

    We have seen that a male has two main options in terms of harem acquisition and that these options commit him to lifetime strategies that are quite different. From a theoretical point of view, alternative solutions to the same problem are most likely to arise when the costs of pursuing the constraint-free strategy become so great that an alternative strategy is more profitable (Dunbar 1982a). The pay-offs of the different elements of the strategy-set may or may not be equilibrated, and it is this fact that often pinpoints the nature of the evolutionary explanation for the phenomenon. That males do...

  16. 13 Tactical Options Open to Males
    (pp. 165-181)

    Although a male can expect to do as well by a takeover as by a follower entry, the values of the various parameters are by no means fixed. A male would gain a considerable advantage if he were able to manipulate any of them in his favor. In general, the initial values of most variables are extrinsically determined, being a consequence of various demographic processes over which a male has no control. Nonetheless, there are a variety of ways in which a male might alter the parameter values in his favor. The main tactical options he has available lie in...

  17. 14 Dynamics of Strategy Choice
    (pp. 182-207)

    The analyses of Chapters 12 and 13 refer specifically to the Sankaber population. They have also taken a rather static approach to strategy choice in that they tend to assume that the biological context within which the male is embedded is effectively neutral, or at least constant over time. This, of course, is a gross oversimplification: environmental and demographic variables are in constant flux and influence each other in complex ways (see Chapter 4), and this is bound to have repercussions on the males’ decisions (Dunbar 1979a, Altmann and Altmann 1979).

    Since a male’s strategic decisions are strongly influenced by...

  18. 15 Two Final Problems About Males
    (pp. 208-222)

    Two questions have been left unanswered in the foregoing analyses. First, what becomes of the former harem-holders once they have been deposed? And secondly, is the set of alternative strategies evolutionarily stable? In this chapter, I try to answer these questions.

    In most species in which individual males hold harems that are taken from them by challengers, the defeated harem-holder either retires to an all-male group or moves elsewhere in the hopes of being able to take over another unit. Examples of both strategies are well known, not only in primates (langurs, Sugiyama 1965, Hrdy 1977; hamadryas baboons, Kummer 1968;...

  19. 16 Evolutionary Decisions Under Conflicts of Interest
    (pp. 223-235)

    I have tried to show that the social behavior of gelada baboons can best be understood in terms of reproductive strategies, that these strategies involve decisions on the part of the individual, and that these decisions in turn are made in the context of a variety of constraints imposed by the biological and social environment within which the animal is embedded. I have taken a deliberately broad perspective, arguing that, in order to understand one aspect of an animal’s behavior (namely, its reproductive strategies), it is necessary to understand in detail the structure of the society within which the animal...

  20. Appendices: Outline of Computer Programs
    (pp. 236-244)
  21. References
    (pp. 245-258)
  22. Author Index
    (pp. 259-261)
  23. Subject Index
    (pp. 262-265)
  24. Back Matter
    (pp. 266-266)