Reproductive Decisions: An Economic Analysis of Gelada Baboon Social Strategies

Reproductive Decisions: An Economic Analysis of Gelada Baboon Social Strategies

by Robin Dunbar


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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.

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Product Details

ISBN-13: 9780691639949
Publisher: Princeton University Press
Publication date: 04/19/2016
Series: Monographs in Behavior and Ecology , #1078
Pages: 276
Product dimensions: 5.20(w) x 8.20(h) x 0.90(d)

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Reproductive Decisions

An Economic Analysis of Gelada Baboon Social Strategies

By R.I.M. Dunbar


Copyright © 1984 Princeton University Press
All rights reserved.
ISBN: 978-0-691-08361-2



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.

This book is concerned with decision-making by animals. In it, I analyze the social behavior of gelada baboons (Theropithecus gelada) as a complex set of alternative strategies among which an individual has to choose. I ask the question: given that the gelada have the social system and ecological niche they do, how does an individual animal set about maximizing its personal reproductive output?

I also have an ulterior motive in that I use the gelada to illustrate a methodological approach to the study of social systems that is now beginning to yield increasing dividends. This approach takes the view that social behavior is concerned ultimately with reproduction and can most usefully be interpreted in terms of strategic decision-making aimed at maximizing an individual's contribution to its species' gene pool (see for example Daly and Wilson 1983). Of course, not everything an animal does during social interactions is immediately concerned with reproduction. Most behavior is concerned directly with objectives that are logically more proximate. For this reason, it is particularly important to distinguish between proximate and ultimate explanations of behavior. These can, perhaps, best be viewed as g series of increasingly direct influences on an individual's reproductive prospects and, at one further remove, on its contribution to the species' gene pool (see Dunbar 1983a). Reproduction, in a word, is the central problem in the life sciences, for it is the issue around which all other aspects of biology hinge.

I shall argue that such a view is becoming increasingly necessary if we are ever to understand social behavior completely. This is because it is essential to know not just what an animal does, but also what it is "trying" to do in order to understand why, in the end, it does what it does (see also Seyfarth 1980). This inevitably demands a much more sophisticated approach to the problems of social behavior: indeed, the success of such a program is dependent on the existence of a body of theory capable of making detailed predictions about behavior. That theoretical framework now exists in what has come to be known as sociobiology, while the analytical techniques have long been available in economics and operations research (McFarland and Houston 1982).

The perspective I adopt is strictly sociobiological, though I hasten to add that I do not espouse narrow "socio-genetical" arguments: the decisions made by the animals are too complex and too deeply nested in a hierarchical network to constitute a case of simple genetic determinism. Nonetheless, a broad sociobiological viewpoint is heuristically valuable in that it provides a powerful Darwinian explanatory basis for observed behavior. I take it as axiomatic that an animal's genetic inheritance obliges it to strive to maximize its contribution to its species' gene pool, but that the actual choice of means to achieve that end is a consequence of the evaluation of the relative costs and benefits of different strategies.

In this respect, I shall make frequent use of the language of conscious decision-making in defiance of Lloyd Morgan's proscription of anthropomorphisms. I do so partly because this is much the easiest way to discuss the animals' behavior, but also partly because fifteen years of field work have made it abundantly clear to me that strategy evaluation is precisely what the animals are doing (see also Kummer 1978, 1982).

Theory of Reproductive Strategies

Evolution occurs as a result of a number of processes that influence a species' gene pool. One of the most important of these is the production of offspring, since it is through offspring that an animal usually makes its main genetic contribution. Naive Darwinian considerations lead us to expect that animals will seek to maximize, by one means or another, the number of offspring they produce. As with most biological phenomena, however, sheer maximization is often counter-productive: the more offspring an individual produces, the less parental care can be given to each and the more likely it is that a high proportion of the young will fail to reach maturity. Lack (1966) gives examples from birds showing that the number of fledgling young peaks at intermediate clutch sizes. In reproductive terms, more does not always mean greater success in the long run.

Both the strategies that an animal can pursue and their relative efficiencies (in terms of generating mature offspring) are determined by the interaction of ecological, demographic, and social factors (Dunbar 1982a). The components of this system place conflicting demands on an animal's limited time and energy budgets and its morphological characteristics (Goss-Custard et al. 1972, S. Altmann 1974). Consequently, the optimal solution to one problem will often be incompatible with the preferred solutions to other problems. The resulting conflicts of interest will force the animal to re-evaluate its priorities and so to compromise on its original objectives.

The ways in which the various components of each subsystem interact are, in general, well understood. In contrast, the ways in which the subsystems themselves relate to each other have barely been touched on as yet. In particular, the important part played by demographic structure in determining the options available to an animal seems to be less widely appreciated than might be expected (Dunbar 1979a, Altmann and Altmann 1979). Demographic factors determine not only the social and reproductive opportunities available to an animal, but also the level of competition from conspecifics that it will have to face in acquiring whatever resources are relevant.

The system is also subject to density-dependent and frequency-dependent effects. These feedback effects make the "constraint-free strategy" less profitable as more individuals pursue it and are largely responsible for the generation of alternative strategies (Dunbar 1982a and references therein). By constraint-free strategy, I mean the strategy that would (other things being equal and in the absence of any constraints due to increased costs) be the preferred strategy in that particular socio-ecological system because it yields the highest net gain in terms of reproductive output. (In Dunbar 1982a, I refer to this by the less satisfactory terms "primary" or "normal" strategy for the species.) Note that a constraint-free strategy is not the same as an "ideal free strategy": as originally defined by Fretwell (1972), the ideal free distribution is that to which the population evolves (in a non-Darwinian sense) once the frequency-dependent and other constraints are imposed on the constraint-free strategy.

Within the context of the constraints imposed by these factors, animals can choose among a range of strategies. The degree to which the set of strategies is stable in an evolutionary sense depends on the extent to which their profitabilities equilibrate in the long term: that is to say, on the extent to which they yield similar numbers of offspring (or genes) when summed over a lifetime (see Dunbar 1982a, 1983a).

In general, an animal may be expected to pursue those options that are most profitable to it, where profitability may be measured in terms of, for example, expected lifetime reproductive output. Of course, animals do not assess the numbers of offspring that any given strategy is worth: that would require an absurd degree of sophistication even for humans. Rather, they base their decisions on more proximate cues that, over evolutionary time, have come to be correlated with lifetime reproductive output. These proximate cues can take a variety of forms, ranging from overt events (such as the number of matings or mates acquired per unit time) to less easily quantified psychological values (such as general feelings of "contentment" or security). McFarland and Houston (1982) refer to the decision rules based on these cues as "rules of thumb." The correlations between these cues and ultimate profitability (measured in terms of genes contributed to future generations) are rarely one-to-one (Dunbar 1982b). Consequently, we cannot necessarily expect individual animals to make decisions that are evolutionarily optimal, though we can expect them to make decisions that are within a degree of latitude of those optimal decisions on the average.

Reproductive Strategies of Gelada Baboons

My immediate concerns in this book are (1) to describe the range of reproductive strategies pursued by gelada baboons, (2) to identify the proximate factors that give rise to these strategies and, where possible, (3) to evaluate their relative efficiencies.

As is well known, gelada reproductive units contain only one breeding male, and this defines and limits the range of reproductive strategies open to individuals of either sex. Male strategies are mainly related to methods of acquiring control over reproductive units. Females, on the other hand, face a more diff use set of problems, and the range of strategies open to them is in consequence both more closely tied to their social relationships and less easily discerned by the observer. Thus, as is often the case, the problems faced by males and females are quite different, and the optimal solutions they would prefer are commonly in direct conflict with each other's interests. Part of my task here will be to determine how these strategy sets interrelate in order to understand how one sex's options limit the other sex's behavior.

To be able to do this, we need to know a great deal about the animals' background biology, both ecological and social. As far as the gelada are concerned, most of the relevant information has now been published in monographs (Dunbar and Dunbar 1975, Kawai 1979a) and an extensive series of papers. Because few people will be familiar with all these publications, I take the liberty of summarizing the most relevant details from this literature in the first few chapters. In doing so, I have tried to avoid providing a general overview of gelada biology. Instead, I have concentrated on those aspects that bear directly on the animals' reproductive strategies. Without this information, the naive reader is apt to raise all sorts of obviously inappropriate alternative explanations for particular phenomena. Those who require more detailed discussions are referred to the original sources cited in the text. This is especially important with respect to many of the causal statements in these chapters: these will often seem to be based on correlations, when reference to the original sources will reveal that the causal inferences are based on very much more detailed logical and evidential analyses.

Chapters 6 through 15 constitute the meat of the book and present both new data and previously unpublished analyses relating to gelada reproductive strategies. The first four of these chapters deal with female strategies, the remainder with those of the males. The inferential process will generally be very much more explicit here. Finally, in Chapter 16, I reconsider certain theoretical issues in the light of these analyses.

It should be noted that I make no attempt to evaluate the adaptive significance of the gelada social system. Rather, I am concerned with just one component of that system, namely, reproductive strategies within the constraints imposed by a social system that is assumed to have been determined by other factors. The general form of the social system and the species' ecological niche can be considered as constraints within which the individual animals make their decisions, even though in reality it is a two-way process. For present purposes, we can assume an explanation for the system's evolution along the lines suggested by the classical socio-ecological literature (see, for example, Crook and Gartlan 1966, Crook 1970, Denham 1971, Goss-Custard et al. 1972), even though these explanations are almost certainly incorrect.

One other point needs to be made explicit. Genetic evolution is a consequence of fitness, a population genetic concept defined in terms of selective advantage (i.e. the rate at which an allele spreads in a population relative to the rates of spread of other alleles at the same locus: see references quoted in Dunbar 1982b). In practice, we are invariably obliged to use more easily quantified measures such as reproductive success, even though the relationship between these measures and fitness itself is not necessarily one-to-one. For practical reasons, I shall in general assume that lifetime reproductive output is a sensitive index of fitness (see also Grafen 1972) unless a particular context forces me to do otherwise.

Most of the data on which this volume is based were obtained from gelada living in the Sankaber area of the Simen Mountains National Park in northern Ethiopia during field studies in 1971–72 and 1974–75. Additional data derive from the Bole Valley some 500 km to the south (based on field work in 1972 and 1974) and from the Gich area of the Simen Mountains (based on our own brief study there in 1971 and the more extensive project carried out by M. Kawai and co-workers in 1973–74). Detailed descriptions of the study areas can be found in Dunbar and Dunbar (1974a, 1975) and Kawai (1979a).

The data themselves derive from three main sources.

The demographic data were obtained from repeated censuses of the study populations. All the members of 11 of the 31 reproductive units of the two main bands in 1971–72 and of 15 of the 17 units in the 1974–75 study were known individually, at least within the context of their particular units. A number of other adults in the remaining units were individually recognizable under any circumstances. Almost every unit in the population (five bands in each study) could be instantly identified, either from its composition or by individually recognizable members. A detailed discussion of demographic methods can be found in Dunbar and Dunbar (1975) and Dunbar (1980a). Terminology and symbols for demographic variables follow standard practice (e.g. Caughley 1977).

The data on the structure of social relationships within units (especially those discussed in Chapter 10) derive from detailed studies of individual units. Scan censuses of non-agonistic interactions were used to determine the overall pattern of social relationships for 11 units in 1971–72 and 14 units in 1974–75. In order to standardize the time base, sampling was carried out on a whole unit as long as there was at least one dyad interacting. This time base is referred to as potential social time. In the gelada, most social activity is confined to the first and last hours of the day, with the period between 1000 hrs and 1700 hrs being devoted more or less continuously to feeding (see Dunbar 1977a, Iwamoto 1979). Most of the data derive from the morning and evening social periods. More detailed sampling of interactions was carried out for 11 of the units during the 1974–75 study: these were used to study dominance relationships, coalition formation, oestrous behavior and the behavioral bases of social relationships. Details of the methods and sample sizes can be found in Dunbar (1980b, 1983c).

The final set of data concerns events associated with the reproductive strategies themselves, especially those of the males. These include events (such as takeover fights and entry by males into units as followers) that occur rarely and at unpredictable intervals. It is impossible to sample these events and the behavior that occurs during them other than by opportunistic recording of all observed occurrences (i.e. by ad libitum sampling, sensu J. Altmann 1974). Usually, once such an event had been detected, the unit was intensively sampled for 2–3 hours each day until things had settled down again. This usually involved scan-sampling of all interacting dyads and ad libitum recording of all interactions involving the main individuals concerned.


Excerpted from Reproductive Decisions by R.I.M. Dunbar. Copyright © 1984 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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Table of Contents

  • FrontMatter, pg. i
  • Contents, pg. v
  • Acknowledgments, pg. ix
  • 1 Introduction, pg. 3
  • 2 Structure of Gelada Populations, pg. 16
  • 3 Ecological Constraints, pg. 24
  • 4 Demographic Processes, pg. 32
  • 5 Social Structure of Reproductive Units, pg. 39
  • 6 Constraints on Female Reproduction, pg. 51
  • 7 The Female’s Socio- Reproductive Strategies, pg. 56
  • 8 A Decision Model of Female Reproductive Strategies, pg. 77
  • 9 The Female’s Tactical Options, pg. 90
  • 10 The Male’s Loyalty Problem, pg. 106
  • 11 Rules and Decisions in Harem Acquisition, pg. 124
  • 12 An Economic Model of Male Reproductive Strategies, pg. 145
  • 13 Tactical Options Open to Males, pg. 165
  • 14 Dynamics of Strategy Choice, pg. 182
  • 15 Two Final Problems About Males, pg. 208
  • 16 Evolutionary Decisions Under Conflicts of Interest, pg. 223
  • References, pg. 245
  • Author Index, pg. 259
  • Subject Index, pg. 262

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