Primate Life Histories and Socioecology. Peter M. Kappeler and Michael E. Pereira, eds. University of Chicago Press, Chicago, IL, 2003. 395 pp., illus. $30.00 (ISBN 022642634 paper).
Life-history analysis involves a host of variables that shape the distribution of reproductive effort across the life span, as well as the length of the life span itself, whereas socioecology includes the study of the different behaviors responsible for social organization in relation to ecological systems. Primate Life Histories and Socioecology, a multiauthored volume edited by Peter M. Kapeller (Department of Behavior and Ecology, German Primate Center, Göttingen) and Michael E. Pereira (research associate at the Lincoln Park Zoo in Chicago), brings these fields together in an effort to evaluate the influences of diverse life-history parameters on the variety of behaviors underlying social systems, and vice versa.
Thus, the book addresses relationships among such variables as the rate of pubertal development, weaning age, interbirth intervals, growth rates, and parental care and life span, as well as variables associated with social behavior, such as group size, infant care strategies, brain size, and various ecological variables. If the book can be said to have a problem, it may already be apparent: It can sometimes bombard a reader with so many different interrelated issues and data that it leaves the head spinning! A reader may sometimes lose sight of the big picture and the overriding themes. Fortunately, the focus of the individual chapters is interesting, and when one does step back, the common themes are intriguing. Consequently, this edited volume is both rewarding and fun, and is well worth the effort that goes into reading it.
The book starts by highlighting how the study of life histories has progressed over the years from a focus on models of relationships between ecology and overall fast (r) versus slow (K) reproductive strategies to the examination of complex patterns of relationships among the various components of life history that make a species live in the fast lane or in the slow one. Selection has operated on particular components in relation to one another and has done this differently across species and taxonomic groups, such that they have moved along the r–K continuum via adjustments of different suites of life-history traits. The first part of the book highlights these interrelationships among traits, how they evolve and develop relative to one another, how they are linked and unlinked, and how these linkages differ across taxonomic groups.
One example of the kinds of relationships identified here is an association between large group size, low body mass at birth, and high body mass at weaning among the different orders of primates (chapter 3, by Phyllis C. Lee and Kappeler). Links between social group size and several life-history parameters are carefully evaluated in chapter 5. The analysis leads the author, Charles H. Janson, to reject the commonly held view that the size of a social group reflects a compromise between predator benefits and foraging costs. This is because large primates have fewer predators than small ones do, and there is actually a positive relationship between group size and body size, which clearly runs counter to the predictions of the model. Janson suggests that, instead, larger primates gain greater benefits from a reduction in predation risk than do smaller ones because the impact of predation is felt across the life span, which is longer for large primates than for small ones. This is a nice example of how consideration of life-history traits can lead to a better understanding of sociality.
The second portion of the book focuses on how changes in developmental patterns appear to have led to changes in basic life-history parameters and sociality. The authors point out that “divergent modes of development result from dissociation of phenotypic elements” (p. 146), which can lead in turn to new suites of adaptive behaviors. Pereira and Steven R. Leigh (chapter 7) point out some of the ways that structures of primate societies have been influenced by slow rates of both developmental and reproductive processes, and they suggest that the resulting challenges to the immature primate “may constitute the single most important factor ultimately structuring primate societies” (p. 149). They also show how growth schedules of particular features of an animal's morphology can become dissociated in ways that reflect the social system. For example, the canine teeth of female mangabeys develop far more rapidly than do those of the larger baboons, which leads to considerably more effective weapons in the females of the smaller species. This presumably serves the female mangabeys well, as they must independently compete to achieve their social status in the group, whereas the baboons can rely on help from their maternal kin.
Developmental issues related to human life histories and their influence on the evolution of our brains are discussed in a fascinating chapter by Kristen Hawkes, J. F. O'Connell, and Nicholas G. Blurton Jones (chapter 9). These authors use data from the fossil record as well as living primates to argue that the coevolution of several life-history traits represents a major adaptive shift that set us on an evolutionary trajectory quite different from those of other primates. The traits in question, typically dissociated in other primates, are high fertility, altricial offspring, early weaning relative to independence, late maturity, and extreme longevity. The authors suggest that the key adaptation that enabled these features to be linked is “grandmothering,” the disposition of females to help care for their grandchildren. The model represents one of many examples of the tantalizing speculations offered by many of the authors.
The chapter on grandmothering leads into the third and last part of the book, which focuses on the evolution of primate brains in relation to life histories. Chapters 10 and 11 examine the issues systematically and arrive at somewhat different conclusions. In chapter 10, Robert O. Deaner, Robert A. Barton, and Carel P. van Schaik evaluate which of the correlations between life-history parameters and brain size reflect direct functional linkages rather than processes that simultaneously affect both variables. Their analysis leads to the conclusion that in primates, but not in some other orders of mammals, brain size is linked most tightly to life span. One explanation for this relationship that the authors favor is the “delayed benefits hypothesis,” which suggests that the learning supported by large brains can provide greater benefits to long-lived than to short-lived primates. The next chapter, by Caroline Ross, highlights associations between primate brain size, juvenile growth rates, and the length of the juvenile period. Life span does not emerge as an important variable in this analysis. The reasons for the differing conclusions from chapters 10 and 11 are not clear.
In the final chapter on brain size, Robin I. M. Dunbar refines the analysis in several ways. He focuses on the neocortex, demonstrates its relationship to group size within both simians and hominids, and shows that for a given group size the neocortex is larger in hominids than in simians. The latter finding presents a challenge to hypotheses suggesting that “the evolution of brain size within the primate order was driven by the cognitive demands of living in complexly organized social systems” (p. 298). Dunbar suggests that the paradox may be resolved by considering advanced social functions, such as those that depend on a “theory of mind,” that may be supported by hominid, but not simian, brains. The chapter ends with the speculation that these may have been associated with the dispersed distribution of members of a social group in conjunction with highly territorial behavior among hominids. Interestingly, these patterns are also characteristics of the social system of spotted hyenas, whose brain-to-body-weight ratios are relatively high compared with those of many other carnivores.
As I hope the above descriptions convey, the many chapters in the book are fascinating with respect to the issues raised, the data presented, and the speculative interpretations of those data. The book brings together many lines of work to highlight how they all touch on the ways that life-history traits and primate social behavior are linked. It presents a solid argument for why further examination of such relationships is critical if we are to truly understand the evolution of primate brains and the social behavior that emerges from them.