It is a rare moment when one is given the chance to proselytize for a methodological approach, and I am grateful for this opportunity to advocate for a particular way of studying natural phenomena that affords researchers almost unlimited creativity to design, explore, and generate predictions about a system. Almost 25 years ago, as a first-year graduate student, I read a paper that wormed its way into my head and has remained influential to this day. That paper – by Irenaeus te Boekhorst and Paulien Hogeweg, published in 1994 in the journal Behavior – was radical because it offered a countervailing hypothesis to the standard socioecological model proposed to explain the flexible association and subgrouping patterns of chimpanzees. The “standard” model posited that the benefits of cooperation among male kin in defending access to females and territory were key in explaining why male chimpanzees were commonly found in parties with other males, while female chimpanzees were more often solitary because of the costs of feeding competition.

Introduction

Spider monkeys are distributed widely throughout Central and South America and studies have been conducted at a variety of sites across the geographic range of the genus (see Table 1.1 in Campbell, this volume). However, detailed information about group composition and demography of spider monkeys remains largely unavailable. Because their fission–fusion social organization allows researchers to observe only a part of a group at any time, short-term surveys can rarely document overall group size and composition. Only a cumulative data set of party composition based on individual identification and longitudinal research can help determine the full composition of a group. Furthermore, the rarity of births and deaths make other demographic variables such as interbirth intervals only available through long-term investigation. In the 1980s, relevant demographic information from wild populations was available only for seven groups from five sites for three Ateles species. In this chapter, we present an updated summary of existing data on four Ateles species from 18 groups and 13 sites. We analyze both previously published and new data from these sites and compare them in order to re-examine the demographic characteristics of spider monkey groups.

Methods

Demographic data from 18 groups and 13 sites (Table 12.1) were gathered from the literature and augmented with data from a questionnaire sent to spider monkey researchers in 2005.

Introduction

The first comprehensive field study of wild spider monkeys was undertaken in Panama in the early 1930s by C. R. Carpenter. In discussing the diet of Ateles geoffroyi, Carpenter (1935) wrote, “Red spider monkeys have been classed correctly as frugivorous. It is estimated that about 90 percent of their food consists of fruit or nuts” (p. 174). Since that time, field studies of wild Ateles have consistently confirmed Carpenter's early assessment of the highly frugivorous diet of spider monkeys, to the point where they are now treated as a classic example of a frugivorous primate and are often considered to be “ripe fruit specialists” (Cant, 1977; Klein and Klein, 1977; van Roosmalen, 1985; van Roosmalen and Klein, 1988; Cant, 1990; Dew, 2005; Wallace, 2005; Di Fiore and Campbell, 2007). In this chapter, we first review what is known of the diet of wild Ateles, paying particular attention to data from long-term ecological studies. In doing so, we address the physiological and morphological adaptations for frugivory that spider monkeys have evolved, as well as the connections among diet, food resource distribution and foraging behavior that are relevant to understanding the characteristic “fission–fusion” social organization of Ateles.

From there, we move on to discussing the interesting variation seen in the diets of spider monkeys across tropical forest sites, and we address in more detail the diets of two populations of white-bellied spider monkeys (Ateles belzebuth belzebuth) which we and our colleagues and collaborators have studied over multiple years in Colombia and Ecuador.

Introduction

Many researchers have assumed that the risk of predation is a major factor – perhaps the most important factor – favoring sociality in primates (Alexander 1974, Dunbar 1988, Terborgh 1983, Terborgh and Janson 1986, van Schaik 1983). Testing this assumption has proven difficult because actual cases of predation on primates are rarely observed (Cheney and Wrangham 1987, Isbell 1994). None the less, if predation is a significant cause of mortality in contemporary primate populations, we would expect individual animals to be sensitive to the risk of predation and to adjust their behaviors in response to changing perceptions of that risk. We would particularly expect to see adjustments in the realm of foraging behavior – in decisions over where or when or for how long to forage, or over how vigilant to be while foraging – because the trade-off between avoiding predation and acquiring food is presumed to be fundamental for most taxa (Fraser and Huntingford 1986, Lima and Dill 1990). Although this trade-off has been studied extensively in many nonprimates (reviewed in Lima and Dill 1990), it has been investigated directly in only a handful of primate taxa (e.g., desert baboons: Cowlishaw 1997; brown capuchins: van Schaik and van Noordwijk 1989; wedge-capped capuchins: de Ruiter 1986, Robinson 1981; see also other chapters in this volume).

By virtue of their large body size, ateline primates – woolly monkeys, spider monkeys, howler monkeys, and muriquis – are less susceptible to predation than other, smaller neotropical primates, and thus predation risk is presumed to be a less important factor influencing sociality and subsistence behavior in these species.