Introduction

The chapters in this volume point to four major conclusions. First, infanticide by males in many kinds of animal is most reasonably interpreted as a reproductive strategy, although not every observed case fits neatly all the criteria specified by this hypothesis (van Schaik, Chapter 2; Palombit et al., Chapter 6; Blumstein, Chapter 8; Veiga, Chapter 9). As further evidence accumulates, the patterns appear to reinforce the sexual selection hypothesis (e.g., Borries et al. 1999b) rather than non-adaptive interpretations (see Sommer, Chapter 1). Second, there are predictable correlates of infanticide risk among primates and other mammal species, including life history factors such as long infant dependency relative to gestation, large litter size, altriciality and social factors such as the loss of protectors, in particular the rate of breeding male replacement (van Schaik, Chapters 2 and 3; Borries & Koenig, Chapter 5; Blumstein, Chapter 8; van Noordwijk & van Schaik, Chapter 14; Nunn & van Schaik, Chapter 16). Third, females in species with infanticide by males are not just passive recipients of male aggression, but have developed a broad array of behavioral and physiological strategies to reduce infanticide risk. These traits are a major focus of this volume and will be developed more fully later.

A fourth conclusion to emerge is that females may also kill infants, in some species more frequently than do males (Blumstein, Chapter 8; Veiga, Chapter 9; Digby, Chapter 17; Voland & Stephan, Chapter 18).

Introduction

Food acquisition and predator avoidance are two major factors proposed to promote social grouping in animals (Krebs & Davies 1987). In the primate literature, these two factors have been presented as alternative hypotheses (Wrangham 1980; van Schaik 1983). Defense against potential infanticide by new males also favors social grouping in primates: females form permanent associations with protective males, including their dependent infants' fathers (Wrangham 1979; van Schaik & Dunbar 1990; Smuts & Smuts 1993; van Schaik & Kappeler 1993, 1997; Sterck et al., 1997). The present volume originated from the recognition of the importance of infanticide avoidance in shaping primate social organization.

Factors favoring the formation of permanent social groups of females and males are not necessarily the same factors influencing the size of the groups. For example, many believe food competition to be the principal factor limiting primate group size (Janson 1988; van Schaik 1989; Isbell 1991; Janson & Goldsmith 1995). Upper group-size limits may be mediated by the maximum daily travel distance individuals can sustain as they forage in groups (Wrangham et al. 1993). In this chapter, we present evidence from red howler monkeys (Alouatta seniculus) that infanticide, too, may play a role in limiting group size in primates (see also Steenbeek, Chapter 7). When infanticide rates increase with the number of reproductive females, females may opt for dispersal, thus keeping total group size small. We propose that indirect evidence previously suggesting the role of food competition in limiting group size might actually reflect infanticide.

INTRODUCTION

The concept of carrying capacity is fundamental in determining a species' density and biomass, which enter into the equations used in ecological theory to predict species diversity and community structure (MacArthur, 1972). Therefore, you might think that a great deal was known about how to determine the carrying capacity of species in nature; however, this is not the case. In the vast majority of species, if the carrying capacity is measured at all, it is estimated as the population density of individuals associated with a zero growth rate (Dennis & Taper, 1994). The more fundamental question of what resources determine that population level is often left unanswered.

Debate has raged over the past two decades as to the extent or even existence of resource limitation in primate populations. Early observations (Struhsaker, 1969) suggested that many fruit-eating primates faced an overabundance of resources in the trees that they used. Later study quantitatively confirmed that some trees produced more fruit than a group of monkeys could consume in one sitting (Janson, 1988b), but also showed that many trees did not produce enough fruit to satiate even a single animal. In one of the few attempts to relate primate population densities to resources directly, Coehlo et al. (1976) compared the biomass and consumption rates of howler and spider monkeys in Guatemala to the availability of the major food resources that they used. They found that over an annual cycle total food production vastly exceeded the requirements of the population density in that site. However, in Peru, Janson (1984) found that the productivity per unit area of food species used by capuchin monkeys varied enough so that in the dry season insufficient fruit was produced to sustain the population. At that time, these fruit-eating monkeys were seen to switch to alternate plant products.

Following upon the chapters in the first section of this volume, which provided geographically restricted overviews of a single biogeographical region, the chapters in this section have provided broad comparisons between the primates of different regions. Fleagle & Reed (chapter 6) review the fossil record of primate evolution over the past 60 million years and examine the relationship between quantitative measures of ecological distance and phylogenetic divergence times within and between communities in the four major geographical regions. Their results indicate that the rate of ecological divergence between pairs of taxa is constant for primates of all regions, but that the strength of the correlation varies depending upon the biogeographical history of the region. They find a major distinction between the extant faunas of Africa and Asia, which contain elements from many different radiations from the past 60 million years and show a high correlation between divergence time and ecological distance, and the faunas of South America and Madagascar that are the result of more temporally restricted explosive radiations.

In contrast to most of the chapters in this volume which compare communities in terms of the component species, Reed's chapter 7 compares modern communities in the four geographical areas from the perspective of how much primate biomass is supported by different types of resources. Comparing the body masses of primates in different regions, she finds Asian communities stand out in having a very restricted size range compared with primates of the other regions.

Ganzhorn (chapter 8) compares patterns of body mass among the primate communities of different biogeographical regions to examine the extent to which there is evidence for competition among primate species within communities.

The chapters in this section have provided a series of geographically focused reviews of the diversity of ecological communities in the four major biogeographical regions currently inhabited by non-human primates – Africa, Asia, Madagascar, and the neotropics of South and Central America. The comparative data that are available for these different regions are by no means uniform. As a result, these chapters not only offer an opportunity to compare the similarities and differences in the primate communities of these different regions, but also highlight the gaps in our current knowledge. Despite differences in the scope and focus of the different chapters, several general themes emerge in the intraregional studies found in this section.

In all of the chapters, the authors emphasized that there are very few primate communities in the world today that have not been affected in some way by human activity (see also Tutin & White, chapter 13, Peres, chapter 15 and Struhsaker, chapter 17, this volume). In addition, it is important to keep in mind that many of the sites where primates have been most thoroughly studied have often been chosen precisely because they have extremely high numbers of species and often easy access from roads. As a result all comparisons either within regions or between them need to make some attempt to take these factors into consideration.

The chapters in the preceding section have reviewed evidence that the present structure of primate communities is affected by resource abundance (Janson & Chapman, chapter 14, Peres, chapter 15), food quality and digestive strategies (Janson & Chapman), seasonality (Janson & Chapman; Tutin & White, chapter 13), disease (Tutin & White), contemporary hunting (Peres), recent land-use history (Tutin & White), climatic change in the recent past and the Pleistocene (Tutin & White; Eeley & Lawes, chapter 12), and regional species source pools (Eeley & Lawes; Peres). Despite the diversity of these themes, they fall into two broad contrasts which we shall use to review the preceding results: (1) local vs. regional explanations of primate community structure; and (2) equilibrial vs. nonequilibrial views of community structure.

LOCAL VS. REGIONAL DETERMINANTS OF PRIMATE COMMUNITY STRUCTURE

There is a tension between explanations that rely on local versus regional ecological factors to explain local site diversity. Local factors include resource abundance and quality, seasonality, competition and contemporary hunting. Regional analysis focuses on regional source pools of species – a local site cannot have a primate species that is not in the regional source pool. This distinction is essentially parallel to that between alpha-and gamma-diversity in community ecology. Alpha-diversity reflects the number of coexisting species in a single site, whereas gamma-diversity includes additional species that may replace each other in separate areas of similar habitat only because of geographical isolation. Proponents of local factors find support in the fact that primate population densities predictably increase in areas of greater soil fertility (Peres), high food quality (Janson & Chapman), and low hunting pressure (Peres).