Heritability of Brain Size and Surface Features in Rhesus Macaques (Macaca mulatta) J. M. Cheverud, D. Falk, M. Vannier, L. Konigsberg, R. C. Helmkamp, and C. Hildebolt The extent of heritability for overall brain size and regional cortical surface features such as sulcus lengths is important for demonstrating a genetic component to the observed phenotypic differences among individuals and for evaluating the potential for evolutionary change in response to selection. Although the genetics of brain size has been extensively considered, the detailed morphology of the cortical surface has not previously been subjected to genetic analysis. We estimated the heritability of brain size and cortical sulcus lengths using 438 endocranial casts taken from skeletons of rhesus macaques (Macaca mulatta) from the Cayo Santiago population. Estimates were obtained both by mother-offspring regression and symmetric-dif- ferences-squared (SDS) methods. Brain size, measured as cranial capacity, was highly and significantly heritable in this population, confirming results of previous studies with laboratory mice. Overall, cortical sulcus lengths were also heritable, with 35% of the sulci significantly heritable at the 5% level in the mother-offspring analysis. The average mother-offspring heritability estimate, 0.31, was the same as the average heritability obtained previously from a series of 56 cranial metric char- acters. The SDS analyses generally corresponded to the findings based on mother- offspring regressions, although the significance test appeared more conservative. Both gross and detailed morphology of the brain are heritable. From the Departments of Anthropology and Cell Bi- ology and Anatomy, Northwestern University, Evans- ton, Illinois (Cheverud and Konigsberg); the Depart- ment of Anthropology, State University of New York at Albany (Falk); the Mallinkradt Institute of Radiol- ogy, Washington University School of Medicine, St Louis, Missouri (Vannier and Hildebolt); and the De- partment of Sociology and Anthropology, Purdue Uni- versity, West Lafayette, Indiana (Helmkamp). Dr. Cheverud is now at the Department of Anatomy and Neurobiology, Washington University School of Med- icine. This research was supported by PHS grant 7 RO1 NS24904. The authors thank the University of Puerto Rico for free access to the Cayo Santiago skeletal col- lection. They also thank Allen Moore and Cashell Jac- quish for their help with the analysis. Address reprint requests to Dr. Cheverud, Department of Anatomy and Neurobiology, Washington University School of Med- icine, Box 8108,660 S. Eudid Ave, SL Louis, MO 63110. Journal of Heredity 1990:81:51-57; 0022-1503/90/12.00 Heritability for brain size in laboratory ro- dents has been estimated frequent- Iy2^9j7j8.42-+i ow j n g to interest in the evo- lution of brain size itself and brain-body scaling in mammals. 2 - 56 These studies re- ported a relatively high heritability for brain size, usually 0.60-0.70, although Leamy 38 is an exception. Leamy estimated a heritability of only about 0.20 for adult brain size using father-offspring regres- sions and suggested that this low estimate may be due to the relatively recent foun- dation of his randomly bred CV1 strain (52 generations of random mating after deri- vation from inbred strain 101) and/or the inclusion of significant prenatal maternal effects in estimates obtained through sib analyses by previous workers but not con- founded with heritability in Leamy's 38 analysis. Even so, the selection experi- ment of Roderick et al. 43 yielded a realized heritability of 0.64, supporting Leamy's 38 suggestion that his estimate is low because of the special character of his population. This realized heritability is perhaps the best guide to the level of heritable varia- tion for evolutionary analyses. Thus, brain size is usually portrayed as a relatively highly heritable feature in mammals. To consider brain evolution solely with regard to size is undoubtedly an oversim- plification. 2iJ2.34.36.4i j p e focus on brain size in previous analyses has been due largely to its relative ease of measurement com- pared with more detailed aspects of brain morphology, not to its status as the single neural morphological feature of interest. In particular, Geschwind and Galaburda 25 " 27 suggested a major role for genes in local neural development and lateralization. We extend previous analyses in two important ways. First, in addition to brain size, we consider the detailed morphology of the brain's cortical surface to determine whether differences in regional brain mor- phologies are inherited. Second, we esti- mate brain size heritability in a free-rang- ing primate colony (rhesus macaques, Macaca mulatto) rather than in a labora- tory rodent population. Specifically, we test the hypothesis that brain size and cortical sulcus lengths are inherited in the free- ranging rhesus macaques from Cayo San- tiago, Puerto Rico. 61