Killer whales have group-specific dialects, sperm whales babysit one another's young and bottlenose dolphins cooperate with other species. These social skills are all closely linked with the aquatic mammals' brain sizes, according to a recent study in Nature Ecology & Evolution.
Scientists first proposed a relation between social living and brain expansion, or encephalization, nearly three decades ago, when they observed that primate species with larger brains typically lived in bigger groups. This theory was later broadened to associate brain size with other social characteristics, such as resolving conflicts and allocating food.
Michael Muthukrishna, an economic psychologist at the London School of Economics, and his colleagues went searching for a similar link between big brains and sociality in cetaceans—the mammalian order that includes whales, dolphins and porpoises. They compiled a comprehensive data set of cetacean brain and body mass, group size and social characteristics. The team's analyses, which covered 90 species, revealed that brain size was best predicted by a score based on various social behaviors such as cooperation with other species, group hunting and complex vocalizations. Bigger brains were also linked to other factors, including dietary richness and geographical range.
The authors say these results are consistent with theories that cetaceans developed large brains to deal with the challenges of living in information-rich social environments. Yet Robert Barton, an evolutionary biologist at Durham University in England, who did not take part in the work, cautions against drawing conclusions about causation from correlation. He also asserts that it is important to examine specific regions of the brain because they might evolve differently. For example, his own research team has found that nocturnal primates' brains develop larger olfactory structures—regions associated with smell—than those found in species active during the day.
Muthukrishna notes that his study's main limitation is the lack of available research on many cetacean species. Discovering more about whales and dolphins could reveal that other factors—such as life span and the duration of the juvenile phase—might also be involved in brain size, he adds.
Understanding how cetaceans developed such big brains could ultimately help us piece together humanity's own evolutionary history. Because these animals occupy a completely different environment than people do, Muthukrishna says, “they provide us with a useful control group for testing hypotheses about human evolution.”