Gregariousness increases brain size in ungulates

Abstract

The brain’s main function is to organise the physiological and behavioural responses to environmental and social challenges in order to keep the organism alive. Here, we studied the effects that gregariousness (as a measurement of sociality), dietary habits, gestation length and sex have on brain size of extant ungulates. The analysis controlled for the effects of phylogeny and for random variability implicit in the data set. We tested the following groups of hypotheses: (1) Social brain hypothesis—gregarious species are more likely to have larger brains than non-gregarious species because the former are subjected to demanding and complex social interactions; (2) Ecological hypothesis—dietary habits impose challenging cognitive tasks associated with finding and manipulating food (foraging strategy); (3) Developmental hypotheses (a) energy strategy: selection for larger brains operates, primarily, on maternal metabolic turnover (i.e. gestation length) in relation to food quality because the majority of the brain’s growth takes place in utero, and finally (b) sex hypothesis: females are expected to have larger brains than males, relative to body size, because of the differential growth rates of the soma and brain between the sexes. We found that, after adjusting for body mass, gregariousness and gestation length explained most of the variation in brain mass across the ungulate species studied. Larger species had larger brains; gregarious species and those with longer gestation lengths, relative to body mass, had larger brains than non-gregarious species and those with shorter gestation lengths. The effect of diet was negligible and subrogated by gestation length, and sex had no significant effect on brain size. The ultimate cause that could have triggered the co-evolution between gestation length and brain size remains unclear.

Appendix I

Phylogenetic relationships between species, brain mass, body mass, gestation length, diet and gregariousness of the species used in this study. Body mass and brain mass have been averaged within species by pooling sex and individual information in order to present a summarized table of the 422 records used in the analyses (the complete data set can be obtained from the authors on request). Diet was represented by the percentages of grass, browse and fruit in the animals' diet in field conditions. The percentages of grass, browse and fruit are averaged values from references in the literature (see Methods). a. percentages of grass and browse in diet estimated by qualitative information from Nowak (1999). Gregariousness, 1. gregarious species; 0. non-gregarious species, information from, a. (Nowak 1999); b. (Brashares et al. 2000); c. (Goldspink et al. 2002). Gestation ref., sources used to estimate the gestation length: 1. Nowak 1999; 2. http://animaldiversity.ummz.umich.edu/; 3. http://www.ultimateungulate.com; 4. http://medicine.ucsd.edu/; 5. http://www.djuma.com; 6. http://www.americazoo.com. Source ref., information on brain masses come from the following references: 1. Graham A.J. Worthy (pers. comm.); 2. (Anderson et al. 1974); 3. Toni Milewski (pers. comm.); 4. (Herre and Thiede 1965); 5. (Honda et al. 1987); 6. (Hrdlicka 1905); 8. (Oboussier 1974a, b); 9. (Quiring 1938); 10. (Ronnefeld 1970);11. (Sigmund 1981);12. (von Tyska 1966); 13. (Wemmer and Wilson 1987); 14. D. Willianson (pers. comm.); 15. IJG's data base (Museum of Comparative Zoology, Cambridge University, Cambridge, U.K.); 16. (Crile and Quiring 1940); 17. (Spector 1956); 18. (Altmann and Dittmer 1972); 19. (Sacher and Staffeldt 1974); 20. (Quiring 1938); 21. (von Bonin 1937).

Fig. 2