Quantifying brain–behavior relations in cetaceans and primates

Abstract

In their recent TREE review, Connor et al.[1xConnor, R.C. et al. Trends Ecol. Evol. 1998; 13: 228–232Abstract | Full Text | Full Text PDF | PubMed | Scopus (116)See all References][1]provide an excellent and thought-provoking comparison of behavioral ecological patterns between toothed whales (odontocetes) and terrestrial mammals, particularly primates. The authors end their review with a provocative call for efforts to quantify the relationship between behavioral ecology and brain size among odontocetes in a similar manner to Dunbar's analyses for primates[2xDunbar, R.I.M. J. Hum. Evol. 1992; 20: 469–493CrossRef | Scopus (618)See all References, 3xDunbar, R.I.M. Evol. Anthropol. 1998; 6: 178–190CrossRefSee all References]. Nevertheless, in doing so they leave the reader with the impression that these quantitative studies have not yet been attempted. There are two studies that do exemplify the very approach Connor et al. advocate. These studies provide quantitative support for the `combination of convergence and novelty'[1xConnor, R.C. et al. Trends Ecol. Evol. 1998; 13: 228–232Abstract | Full Text | Full Text PDF | PubMed | Scopus (116)See all References][1]suggested by observational and qualitative comparisons of brain and behavior between odontocetes and primates.First, there is a significant positive correlation between pod size and encephalization quotient (a measure of relative brain size taking into account brain–body allometry) among 21 odontocete species from all six odontocete families[4xMarino, L. Evol. Anthropol. 1996; 5: 81–85CrossRefSee all References][4]. Therefore, the relationship between one measure of sociality (i.e. social group size and brain size) appears to be similar in primates and odontocetes.Second, although there is a positive correlation between encephalization level and gestation length among primates[5xSee all References][5], in an analysis using the same encephalization values for the 21 odontocete species already mentioned, there is no significant relationship between encephalization and gestation length among odontocetes[6xMarino, L. Mar. Mamm. Sci. 1997; 13: 133–138CrossRefSee all References][6]. Rather, body size accounts for more of the variation in gestation length than encephalization among odontocetes. Therefore, there is quantitative evidence for differences in brain and life history relationships across primates and odontocetes.My point here is not to criticize Connor et al. but to strengthen and extend their emphasis on quantitative analyses of odontocete behavioral ecology by showing that these kinds of studies are already underway. It is, of course, important to continue to further these studies while moving towards formulating and testing hypotheses about the evolution of cetacean brain–behavioral relationships and its implications for general mammalian evolution.