Dolphin social intelligence: complex alliance relationships in bottlenose dolphins and a consideration of selective environments for extreme brain size evolution in mammals

Abstract

Bottlenose dolphins in Shark Bay, Australia, live in a large, unbounded society with a fission-fusion grouping pattern. Potential cognitive demands include the need to develop social strategies involving the recognition of a large number of individuals and their relationships with others. Patterns of alliance affiliation among males may be more complex than are currently known for any non-human, with individuals participating in 2-3 levels of shifting alliances. Males mediate alliance relationships with gentle contact behaviours such as petting, but synchrony also plays an important role in affiliative interactions. In general, selection for social intelligence in the context of shifting alliances will depend on the extent to which there are strategic options and risk. Extreme brain size evolution may have occurred more than once in the toothed whales, reaching peaks in the dolphin family and the sperm whale. All three 'peaks' of large brain size evolution in mammals (odontocetes, humans and elephants) shared a common selective environment: extreme mutual dependence based on external threats from predators or conspecific groups. In this context, social competition, and consequently selection for greater cognitive abilities and large brain size, was intense.

Figure 1

Relationship uncertainty in Shark Bay occurs because of the fission–fusion grouping pattern and is exacerbated by the mosaic of overlapping ranges. Individual B might know A and C well, but minimal range overlap will prevent A and C from knowing each other well.

Figure 2

(a) Within group alliances may be complex when individuals compete for allies, e.g. if A and B compete for C. (b) Alliances against other groups are not usually complex because they are exclusively hostile. They may impact within group alliances by increasing mutual dependence within groups. (c) Humans and dolphins are exceptional to the degree to which they form ‘alliances of alliances’. Here alliance ABC could side with DEF or GHI against the other. Triadic interactions complicate within and between group interactions.