Dolphins and African apes are two distantly related mammalian taxa that have produced socially complex, large-brained nonhuman animals. Comparisons between the two taxa have burgeoned in recent years, as field data on the various species in each group have accumulated. These comparisons have tended to stress parallels between two African apes (Pan troglodytes and P. paniscus) and dolphins (order Cetacea, suborder Odontocetes, family Delphinidae) in brain and social cognition (Herman, 1980; Marino, 1998; McCowan et al., 2000; De Waal and Tyack, 2003; Johnson and Herzing, 2006, Lefevbre et al., 2006), evidence for cultural traditions (Smolker, Richards, Connor, Mann, and Berggren, 1997; Whiten, et al., 1999; Rendell and White-head, 2001, Whitehead et al., 2006), and the influence of ecology on social structure (Connor, Read, and Wrangham, 2000b; Connor, Heithaus, and Barre, 2000c; Anderson et al., 2005). Similarities in the pattern of sympatric associations between the two taxa have not been described, even though chimpanzees, bonobos and some well-studied dolphin species (e.g., bottlenose dolphins Tursiops truncatus, spinner dolphins, Stenella longirostris) share many characteristics of their socioecologies: fission-fusion polygamy, male coalitions, strong mother-son bonds, and dispersed foraging for high-quality, patchy food resources (Table 1).
Dolphins and African apes both exhibit sympatric species associations. Chimpanzees and gorillas (Gorilla gorilla) are broadly sympatric across equatorial Africa, and several field studies have examined the way in which these two apes exhibit ecological overlap and potentially partition resources and habitat use (Jones and Sabater-Pi, 1971; Tutin, 1996; Stanford and Nkurunungi, 2003; Yamagiwa and Basabose, 2006). Although the earliest studies of each ape species seemed to show stark interspecific contrasts (e.g., Goodall, 1986; Fossey and Harcourt, 1977), more recent work has blurred some of these distinctions by providing much evidence of intraspecific variability between both species, especially gorillas (Doran and McNeilage, 1998). Detailed field studies of dolphins have lagged behind those of African apes, largely due to the logistical difficulties of observation in a marine habitat (Bearzi, 2003). The existing literature contains numerous accounts on the biology and ecology of different species of dolphins worldwide, but only a few sympatric populations of small odontocetes have been well investigated in the field (Table 2a).
In this paper we compare the sympatric ecology of dolphins with that of African apes. By sympatry, we mean the co-occurrence of two or more ape or dolphin species in the same immediate habitat, which might be called direct sympatry, where broad sympatry simply means two or more species occur over the same wider geographic area (Futuyma, 1997). The implication of direct sympatry is that the species in question must subsist on the same resource base. According to the ecological principle of competition and mutual exclusion, without some form of resource partitioning one species or the other would eventually be driven to local extinction. By taxa we mean forms that are genetically and morphologically distinct. Among dolphin species there is some controversy as to weather some species consisting of multiple forms should be regarded as distinct taxa (e.g., transient and resident orcas, Orcinus orca, were included in this review because the two forms are genetically and morphologically distinct).
We limit the comparison to the African apes because their societies - chimpanzees and bonobos in particular - display social complexity lacking in the social systems of the lesser apes (Reichard, 2003) and orang-utans (Pongo pygmaeus) (Delgado and van Schaik, 2000). Orang-utan sociality has been well demonstrated to be highly restricted by availability of fruit (van Schaik, 2004). The more distantly related gibbons are now known not to be entirely monogamous reproductively, but most of the taxa are socially pair-bonded (Sommer and Reichard, 2000). It is for these reasons that chimpanzees and bonobos in particular have been the focus for using great ape social ecology to reconstruct the possible behaviour of early hominids (Finch and Stanford, 2004; Stanford, 2006). We hope to elucidate parallels in the ways in which the grouping patterns and feeding strategies of large-brained, socially complex mammals may be an evolved response to their environment.
Habitats as dissimilar as tropical forests and oceans appear to have provided some similar ecological context in which natural selection shaped parallel behavioural adaptations. The co-occurrence of two or more closely related species is thought to have been an important ecological influence in hominin evolution (Coppens, 1994) as it has been in many other mammalian taxa. Behavioural, dietary and physiological habitat specializations may have evolved in response to the presence of potential competing species. Reaching a greater understanding of similarities and differences between sympatric dolphin associations and those of African apes may provide a new perspective on the evolutionary context of social complexity and intelligence.
Table 2b. Field studies of sympatric associations of African apes. All studies have been conducted in Africa on chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla).
The main goal of this paper is, therefore, to compare similar patterns of multi-species associations in dolphins and African apes. We try to identify important ecological variables, and suggest that there may be specific features of both dolphin and ape socio-ecologies that select for the large brains and social complexity that characterize both taxa.