Do sympatric associations among dolphins and African apes reflect convergently evolved social adaptations?
The pattern of ecological divergence described above can be found in a wide variety of associations among terrestrial or marine animals. The most striking ecological parallel between the two taxa is the tendency toward a fission-fusion grouping pattern, in which proximate changes in group size and composition appear to reflect proximate availability and distribution of food resources. The key food resource that may account for this grouping structure in primates is ripe fruit, the occurrence of which is ephemeral and patchy. This pattern is seen in only a few primate species, and in very few non primate mammals. In addition to chimpanzees, bonobos (Pan paniscus; Kano, 1992) and spider monkeys (Ateles sp.; Symington, 1987) most prominently exhibit fission-fusion. Such a social system may, among other functions, enable the animals to forage for widely dispersed, frequently changing food sources, and to cope with the complexities of group life that follow from such a fluid foraging pattern. In these respects, many dolphin species (e.g., bottlenose dolphins, killer whales, common dolphins, spinner dolphins) offer a striking parallel to chimpanzees and bonobos. If fission-fusion grouping is a response to a complex foraging environment and has placed intelligence and social complexity at a premium in these two distantly related taxa, how may it have affected sympatric associations in the two groups?
Among African apes, sympatry between chimpanzees and gorillas has only recently been studied in detail. In sympatric association with gorillas, chimpanzees utilize many of the same food resources in a different manner, foraging widely on a daily basis while sympatric gorilla group forage more slowly within a smaller area while remaining in cohesive groups. Since these two species and their ancestors have presumably been broadly sympatric for millions of years, the possibility exists that chimpanzee-gorilla foraging differ at least to avoid direct competition for food and other resources. Such a divergence may have been facilitated by the tendency of chimpanzees to adopt a far-ranging foraging strategy that relies on their ability to remember the locations and timings of ripe fruit patches from seasons and even years past. Even though many more field studies have been conducted on dolphin sympatric ecology than have been done on African apes, we have less detailed ecological information about their associations. However, the socioecology of at least some species shows striking parallels to those of African apes (Marino, 1996, 1998; Reiss et al., 1997).
Although apes and dolphins are only distantly related, the ecological convergence of the way in which food resources are shared when two or more species are in sympatry is striking. The behavioural flexibility associated with switching prey items and foraging patterns, and coordinating these dynamics with a sympatric species, characterizes both primates and dolphins. This behavioural flexibility may contribute to avoidance of interspecific food competition. Further studies focusing on behavioural ecology in each taxon may elucidate the role of social adaptations that allow multiple species of each group to co-exist.