AMERICAN JOURNAL, OF PHYSICAL ANTHROPOLOGY 87:349-357 (1992) Corpus Callosum in Sexually Dimorphic and Nondimorphic Primates RALPH L. HOLLOWAY AND PETER HEILBRONER Department of Anthropology, Columbia University, New York, New York 10027 KEY WORDS Brain, Corpus callosum, Sexual dimorphism, Cerebral lateralization, Primates ABSTRACT The midsagittal area and other morphological measures were taken on the corpus callosum of four different species of primate: Macaca mulatta, M. fascicularis, Callithrixjacchus, and Saguinus oedipus. The first two species are strongly dimorphic, whereas the New World forms show little dimorphism with regard to overall body size, canines, and brain weight. Neither total corpus callosal area (TOTALCC), or other parts of the corpus callosum (CC) showed any significant sexual dimorphism in any of the primate species sampled. Only in M . mulatta did a sexual dimorphism appear to be significant. In males of this species, the dorsoventral width of the splenium was larger than in females. In addition, the anterior commissure (ANTCOMM) evinced no sexual dimorphism in the different species. Brain weight was significantly dimorphic in only M. mulatta, and when ratio data were used to correct for brain weight, no significant differences were found in the corpus callosum. This is in contrast to Homo sapiens, where the relative size of the CC has been reported to be larger in females, and particularly so in the posterior, or splenial portion of the CC. Correlation coefficients were calculated for the various variables within each species. In general, most of the callosal measures are significantly inter-correlated, although the exact pattern varies for each species. Thus, unlike Homo sapiens, or pongids such a s Gorilla and Pan, neither New nor Old World monkeys show any striking evidence for sexual dimorphism in the corpus callosum. In recent years, a number of investigators (e.g., de LaCoste-Utamsing and Holloway, 1982; Holloway et al.; 1986, Kertesz et al., 1987; Demeter et al., 1988) have examined the anatomy of the corpus callosum (CC), the large fiber bundle interconnecting the two cerebral hemispheres in the human brain. (For details regarding fiber distributions see Pandya e t al., 1971; and Seltzer and Pandya 1983.) In two of the human studies, significant sex differences have been reported for the midsagittal sectional area of the CC, and its splenial portion, in particular (de LaCoste-Utamsing and Holloway, 1982; and Holloway et al., 1986; Holloway, 1990). (It is the splenial portion that carries the fibers interconnecting the parietal, occipital, and a small portion of the temporal cortex. The @ 1992 WILEY-LISS, INC. rostrum and genu contain fibers mostly related to the frontal cortex.) Although the existence of this dimorphism has recently been contested (e.g., Kertesz et al., 1987, Demeter et al., 1988, Witelson, 1985, Witelson and Kigar, 1987, Byne et al., 1988, Weber and Weis, 1986), sexual dimorphism in this non-reproductive area of the brain remains a potentially intriguing correlate of some of the sex differences in human cerebral lateralization that have been reported (e.g., Harris, 1978; McGlone, 1980; Kimura, 1983; Kimura and Harshman, 1984). All of Received November 12,1990; accepted July 23,1991. Address reprint requests to Dr. Ralph L. Holloway, Dept. of Anthropology, Columbia University New York, NY 10027. 350 R.L. HOLLOWAY AND P. HEILBRONER the above studies on the human corpus callosum have failed to properly take human brain size into consideration, thus ignoring the possibility that relative differences in corpus callosal size will favor the female, as originally suggested by de Lacoste-Uitamsing and Holloway, (1982). This matter has been reviewed elsewhere (Holloway, 1990). In nonhuman primates, which appear to exhibit less lateralized brains than humans (Holloway and de Lacoste-Utamsing, 1982; Heilbroner and Holloway, 1988, 1989; Falk 19861, the possibility of sexual dimorphism in the morphology of the CC has only recently been investigated. de Lacoste and Woodward (1988) have examined the CC in brain samples representing major taxonomic groups within the Primates. Their evidence suggests greater mean total crosssectional callosal area in female than male brains in two of these groups (pongids and strepsirrhines), as well a s greater dorsoventral splenial width in female pongids than in males. In contrast, these workers did not find any sex differences in cercopithecoid or ceboid brain samples. Unfortunately, it is not clear from their sample of 34 primate species (54 brains) just how many males and females were studied within the pongids. Their Table I11 (p. 320) indicates only that the total number of pongids was 15,and thus one cannot be certain how strong or weak the within-species dimorphism was (e.g., for gorilla or chimpanzee). In almost all studies mentioned above, sample sizes have been rather small, and given the controversial aspects of some of these studies, particularly in Homo, replication studies using larger samples and as many different species as possible seem in order. In the present study, we attempt to assess the degree of sexual dimorphism in the cross-sectional area of the CC and anterior commissure (ANTCOMM) in four nonhuman primate species. These species, two each from Old and New World monkey groups respectively, were chosen for the contrasting patterns of anatomical and behavioral sexual dimorphism which they represent. Two of the species (Callithrixjacchus and Saguinus oedipus) exhibit little sexual dimorphism in either reproductive anatomy or in behavior (Herskovitz, 1977); the other two species, Macaca mulatta and M. fascicuZaris, exhibit anatomical sexual dimorphism in body size and canine development, and are also behaviorally sexually dimorphic with respect to infant care roles, mating strate- gies, group defense, and other social parameters (Roonwal and Monot, 1977). In addition, it has been hypothesized by Holloway (1983) that human sexual dimorphism in the corpus callosum may be a species-specific dimorphism related to evolutionary forces acting upon a complemental social behavioral set of strategies, and thus a n evolutionary residuum with less importance today than in the past. In essence, it was hypothesized that the lower degree of asymmetry in the female cerebral cortex would be supported by a morphological pattern that showed a larger splenial portion in the female brain, signifying a n increase in fibers communicating between the two cerebral sides, particularly in the posterior parietal region. Testing this speculation must involve the study of numerous primate and other animal species, as well a s larger samples within Homo and the pongids. This study is intended to add to our growing knowledge regarding the organization of primate brains. MATERIALS AND METHODS The brain materials for Macaca mulatta and M . fascicularis were kindly provided by the New England Regional Primate Center, the Bowman-Gray School of Medicine, N. Carolina, the Oregon Regional Primate Center, Oregon, and the Wisconsin Regional Primate Center as described in Table 1 of Heilbroner and Holloway (1989). The brains af Callithrix jacchus and Saguinus oedipus were received from Oak Ridge Associated Universities, and were removed from freshfrozen crania that had been shipped express from Oak Ridge, Tennessee. All brains examined were immersionfixed specimens from adult or older juvenile monkeys from these four species with no history of neuropathology or cerebrovascular disease. Relatively large brain samples were used in this study: 23 (10 males, 13 females) Macaca mulatta 29 (16 males, 13fema1es)M. fascicularis; 17 (10 males, 7 females) CalZithrix jacchus; 21 (11 males, 10 females) Saguinus oedipus. Each brain was weighed to the nearest gram and then sectioned along the midsagittal plane. The cerebral hemisphere in which the CC and ANTCOMM appeared to be in the best condition was placed on a level platform with the medial surface facing up. Modelling clay was placed under the lateral surface of the hemisphere to level the medial surface. A millimeter ruler was placed along- SEXUAL DIMORPHISM IN PRIMATE CORPUS CALLOSUM side the hemisphere at the height of the medial surface. An Olympus 35 mm camera (with macrolens adapter), mounted on a tripod, fixed at a constant height for all of the brains, was positioned directly above the hemisphere and ruler. The camera back was leveled, and the camera was positioned so that the body of the CC appeared in the center of the focusing window in the viewfinder. A color slide photograph was then taken. The slides were projected onto 8” x 11” paper with a darkroom enlarger. The greatest enlargement possible ( 3 ~ - 1 0x), (depending on brain size) that permitted clear tracing of the CC outline was used. Two marks were made on the projection paper indicating one centimeter on the ruler in the photograph to denote scale. The perimeters of the CC and ANTCOMM were then traced onto the paper by hand from each slide. As the corpus callosum is a single continuous structure, it is necessary to divide the CC into somewhat arbitrary divisions that permit measurement of its various sections, such as the rostrum, genu, body, and splenium. These divisions and the rationale for using them have been spelled out in detail elsewhere (e.g., de Lacoste-Utamsing and Holloway, 1982). I n brief, following Figure 1, we divide the corpus callosum into fifths along a n anterior-posterior axis from rostrum through splenium. The posterior fifth thus approximates the whole of the splenial portion of the CC. From previous studies (de Lacoste-Utamsing and Holloway, 19821, we ANT CC 351 have not found any differences in the anterior % of the CC, thus we lump together the rostrum, genu, and a part of the body of the CC into the first Y2, or ANTCC (see Figure 1). On each tracing, the CC is divided as follows: first, a straight line was drawn connecting the most caudal and rostral points on the border of the CC. The midpoint, the most caudal fifth (POST5AREA), and the next-tomost caudal fifth of this line were determined with vernier calipers and marked. Line segments perpendicular to the line were drawn from these three points to subdivide the CC into anterior and posterior halves (ANTCC and POSTCC), and to indicate the most posterior fifth (POST5AREA or splenium) and next-to-most posterior fifth (NEXTAREA). We include this region, a s it might contain some of the most anterior part of the splenium along with the posterior part of the body. The area of each of these subdivisions (see Figure 1) and the area of the ANTCOMM were then determined using a Tamaya Planix-2 rolling digital planimeter. Each planimetric measurement was repeated three times and the results averaged for each specimen, using the mean value for our statistical calculations. In addition, splenial width (greatest dorsoventral distance across the splenium perpendicular to the main axis of this region of the CC) was measured using vernier calipers. The SPSSX (Statistical Package for the Social Sciences) was used for the statistical analyses of these data. Student t-tests and POST CC POST 5 Fig. 1. A schematic diagram of primate corpus callosum which is the major fibrous interconnection between the two cerebral hemispheres. The splenial portion is the posterior section toward the right; the g e m and rostrum are anterior and to the left. The dashed lines represent the approximate transect, at right angles to the axis of the splenium, along which dorsoventral splenial width is measured. The anterior ccmmissure (ANTCOMM) is just slightly posterior to the pointed end of the rostral part of the corpus callosum, and given its very small size, is not drawn on this diagram. The divisions shown in this diagram are as described in the Methods sections. 352 R.L. HOLLOWAY AND P. HEILBRONER ANOVA subroutines were used in the analyses of the sex differences. For ANOVA, F-ratios for brain weight (Fbr) and sex (Fsx) were calculated to determine the degree of sexual dimorphism resulting from allometric and nonallometric factors. In these calculations, the callosal variable was treated a s the dependent variable, brain weight as a eovariate, and sex as the main factor. Because there have been indications from previous publications of a relative sexual dimorphism (i.e., relative to brain size in some CC measurements), ratio data were also tested using t-tests (Ratiol-Ratio5, see below), and ANOVA was also used on Ratio4, as it did not contain brain weight a s a variable. Ratiol is total CC area divided by brain weight raised to the exponent of .66, a s this approaches a n areal dimension more commensurate with CC area. Ratio2 is the splenial area (POST5AREA) divided by the .66 power of brain weight. Ratio3 is splenial width divided by the .33 power of brain weight (commensurate with a linear dimen- sion). Ratio4 is the POSTSAREA divided by TOTALCC area, and Ratio5 is RATIO4 divided by brain weight raised to the .66 power. In fact, simply using brain weight provides essentially the same statistical patterns. We are using exponential values of brain weight to keep mensurational consistency. Finally, correlation analyses were run for each of the species to show the relationship between the measurements of the corpus callosum and brain weight. RESULTS Few sex differences in the cross-sectional area of these callosal subdivisions were revealed (see Tables 1 4 ) . The differences found appear in both species of Macaca and show greater callosal areas in males, reflecting brain weight a s shown in the ANOVA analyses. In Macaca mulatta, however, splenial width showed a significant size difference in terms of sex, favoring males. The posterior fifth, or splenial area, did not show TABLE 1. Macaca fascicularis' Region TotalCC AntCC PostCC Next 115 Sex N Area2 s.d. t P Fsx P Fbr P M F M F M F 16 13 16 13 17 13 17 13 17 13 17 13 15 13 17 ,655 ,677 ,390 ,378 ,298 ,294 ,095 ,102 .149 ,158 ,349 ,322 ,035 ,041 59.24 84.30 ,044 ,049 ,011 .011 ,091 ,086 .239 ,219 1.636 1.530 ,107 .115 .115 -083 .057 .053 .025 .018 -032 -028 .051 .048 .009 .012 6.17 5.36 .006 -007 .002 .002 -013 .012 .026 .022 .223 .I26 -544 ,591 2.415 ,133 5.275 ,031 ,196 ,846 .050 ,827 ,903 ,361 ,218 ,828 ,427 ,526 6.576 ,017 .458 ,839 .180 5.962 327 ,844 ,406 ,008 ,930 7.287 ,012 1.506 ,143 ,798 ,390 3.669 .067 ,213 4.621 ,042 1.714 ,703 4.146 ,053 ,764 ,253 M F Post 115 M F Sp1enia 1 M width Antcomm F Brain weight Ratiol Ratio2 M F M F M F M F Ratio3 M F Ratio4 M Ratio5 F M F 11 16 11 17 11 17 11 16 11 16 11 -.752 -1.276 1.102 -1.614 ,280 ,119 ,062 .95 1.053 ,302 2.263 .032 1.417 ,169 'Results from t-tests andANOVA(ana1ysis 0fvariance)for the regions studied.The t-value is based on apooled variances. Fsx and Fhr are F-ratio values from ANOVA for sex and brain weight respectively. Values significant at less than p = .05 are italicized. "Area is in mm2. 353 SEXUAL DIMORPHISM IN PRIMATE CORPUS CALLOSUM TABLE 2. Macaca mulattal Region TotalCC AntCC PostCC Next 1/5 Post 1/5 Splenial width Antcomm Brain weight Ratio1 Ratio2 Ratio3 Ratio4 Ratio5 Sex N Area2 s.d. M F M F M F M F M F M F M F M F M F M F M F M F M F 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 10 13 ,959 ,942 ,523 .530 .444 .413 .142 .133 ,240 .220 ,417 .367 ,075 ,047 91.15 84.30 ,049 ,050 ,012 ,012 .094 ,085 .253 .235 1.304 1.260 ,136 ,154 ,097 ,090 .05 1 .084 .024 ,030 ,022 ,038 ,038 ,060 ,115 ,014 7.29 5.36 .006 ,007 ,001 ,002 .009 ,013 ,028 ,020 ,206 .117 P Fsx P Fbr P ,278 .78 ,639 ,442 5.081 .036 p.190 .85 1.602 ,220 4.114 ,056 1.009 .32 .048 .831 3.509 .076 .734 .47 ,021 .887 3.992 ,059 1.457 .16 ,771 .400 2.187 255 2.261 .03 2.93 ,102 2.384 ,138 ,876 .39 .001 ,975 3.158 ,090 ,010 1.386 ,253 t 2.65 .01 -.461 .65 ,766 .45 1.92 .07 1.82 .08 .66 .52 8.12 'Results from t-tests and ANOVA (analysisof variance) for the regions studied. The t-valueis based on a pooled variances. Fsx and Fbr are F-ratio values from ANOVA for sex and brain weight respectively. Values significant at less than p = .05 are italicized. 2Area is in mm2. significant sex differences in any of the species, Brain weight was significantly higher in male M . mulatta than i n females of this species. Using ratio data, i.e., correcting for brain weight, these data did not provide evidence for significant sex differences, except in the case of Ratio4, the relative amount of splenial portion comprising totalCC area. Here, ANOVA shows the only sex effect that is significant in M . mulatta, The p value for M . fascicularis is .053, also suggesting some dimorphism. In both cases, however, the male values are higher, a finding the opposite of that from the human data (see Discussion). Neither of the New World monkey species examined here showed any significant dimorphism in primary measures (including brain weight) or in the ratios. DISCUSSION Very little evidence for sexual dimorphism in the morphology of the CC in four monkey species was found in the samples presented here. These results are consistent with those of de Lacoste and Woodward (1988),who did not find dimorphism in the cross-sectional areas of the CC in New and Old World monkey brain samples. The TOTALCC area ofM. fascicularis was higher in females, but not significantly so. In contrast, the M . mulatta male sample had a larger TOTALCC area. SPLENIAL WIDTH was higher in males than in females in both macaque species, a finding not reported above and opposite to those reported by de Lacoste-Utamsing and Holloway (1982) and Holloway et al. (1986) for Homo. In the former case, i.e., TOTALCC area, the ANOVA analysis shows that the dimorphism is best explained by dimorphism in brain weight. However, Ratio4 (POSTERIOR % divided by TOTALCC) is a true sex effect, without strong allometric dependence, which in these samples, favors the male of the macaque species. We frankly do not understand why this reversal occurs. There does not appear to be any behavioral data to suggest that visuospatial task performance shows significantly higher scores in male macaques than in females. While be- 354 R.L. HOLLOWAY AND P. HEILBRONER TABLE 3. Callithrix jacchus Region TotalCC AntCC PostCC Next 1/5 Post 1/5 Splenial width Antcomm Brain weight Ratio1 Ratio2 Ratio3 Ratio4 Ratio5 Sex M F M F M F M F M F M F M F M F M F M F M F M F M F N 10 7 10 7 10 7 10 7 10 7 6 5 10 7 10 7 10 7 10 7 6 5 10 7 10 7 Area ,167 ,161 ,081 ,079 ,087 ,082 ,030 ,028 ,046 ,045 ,043 ,011 ,165 ,184 7.85 7.61 ,043 ,042 ,012 .012 ,022 ,006 ,280 ,280 7.19 7.34 t ,030 ,018 ,020 .012 .012 .010 ,007 ,003 .006 ,006 ,072 ,497 .63 ,157 ,702 ,208 ,660 .136 .89 .018 396 ,000 ,999 .927 .37 .491 ,502 1.350 ,265 .606 .55 ,141 ,717 1.358 ,263 ,447 .66 .061 A11 .961 ,354 ,989 .34 4.654 ,063 ,642 ,454 .43 1.734 ,209 3.556 .080 ,026 377 ,472 ,510 P Fsx Fbr s.d. P P ,002 .059 ,025 ,511 ,689 ,007 ,005 ,001 .002 ,038 ,001 ,037 ,037 1.003 377 -309 ,820 .42 ,173 .86 ,103 .92 .972 .36 p.015 .99 p.315 .75 'Results from t-testsand ANOVA (analysis of variance) for the regions studied. The t-value is based on a pooled variances. Fsx and Fbr are F-ratio values from ANOVA for sex and brain weight respectively. Values significant a t less than p = .05 are italicized. havioral reasons cannot be ruled out (as many have yet to be thoroughly studied), we prefer to regard this unique finding as a n example of statistical artifact, until larger samples are described. While sexual dimorphism appears in humans, and de Lacoste and Woodward (1988) suggest that it is present in pongids and prosimians, we strongly urge that larger samples will be necessary to demonstrate whether or not larger female CC's are a shared character state in haplorrhine primates. The correlational Tables 5-8 indicate many interesting possible relationships. We note, in particular, the higher correlations between variables in the larger sample for M. fusciculuris than in M . mulattu. Yet neither the splenial wideth nor the splenial area correlates significantly with brain weight, while showing strong and significant correlations with TOTALCC area. The anterior commissure only has significant correlations with TOTALCC area and the anterior half of the CC, ANTCC. In effect, each species shows a unique set of correlation coefficients, with negative correlations between splenial width, brain size, and TOTALCC area being more common in Callithrix and Sa.guinus. We frankly do not know whether these unique patterns are truly species-specific indicators of possible neural reorganizational differences in the corpus callosum, or simply statistical artifacts due to sampling. Only larger sample sizes including more species will clear up this question. The anterior commissure interconnects contralateral temporal polar cortices (McCullough and Garol, 1941; Ebner and Myers, 1965; Cippolini and Pandya, 1985). This cortical area is generally thought to have a close affiliation with the limbic system. No sex differences of any statistical significance appear in these data. We draw attention to the very small area of this structure, however, and the greater consequent possibility of 355 SEXUAL DIMORPHISM IN PRIMATE CORPUS CALLOSUM TABLE 4. Saguinus oedipus1 Region TotalCC AntCC PostCC Next 1/5 Post 1/5 Splenial width Antcomm Brain weight Ratio1 Ratio2 Ratio3 Ratio4 Ratio5 Sex N Area s.d. t P Fbr P M F M F M F M F M F M F M F M F M F M F M F M F M F 11 10 11 ,164 ,164 .084 .086 .081 .079 .043 ,043 ,043 ,046 ,177 ,181 ,012 ,014 9.48 9.93 ,037 .036 ,010 .010 ,012 ,017 .011 .012 ,006 ,008 ,006 .039 .97 ,000 .845 ,012 ,915 p.411 .69 .238 637 .040 .845 ,626 .54 ,470 .508 ,018 ,896 -1.007 .33 1.833 -192 ,729 ,413 -1.365 .19 1.474 .241 ,318 ,586 -.465 .65 ,183 -678 ,021 .887 -284 .39 .435 .527 ,997 .345 -1.427 ,170 1.887 ,186 .789 ,395 10 11 10 11 10 11 10 11 10 11 10 11 10 11 10 11 10 Fsx .055 ,006 .005 ,020 ,020 .003 ,004 ,687 ,753 ,003 ,004 ,001 ,001 ,010 ,010 ,029 .028 ,647 ,758 ,084 11 10 11 10 11 10 P ,085 ,263 .284 5.976 6.256 ,737 ,470 -.I47 ,464 -.154 379 -1.617 .122 -.945 .356 'Results from t-tests and ANOVA (analysis of variance)for the regions studied.The t-valueis based on a pooled variances. Fsxand Fbr are F-ratio values from ANOVA for sex and brain weight respectively. Values significant a t less than p = .05 are italicized. In sum, there does not appear to be any correlation between known behavioral and overall anatomical dimorphism with a dimorphic counterpart in the corpora callosa of New or Old World Monkeys. measurement error than with the other structures measured. Anterior commissure measurements on human brains by Demeter et al., (1988),do not indicate any significant human dimorphism. TABLE 5. Correlation (Pearson) matrix for Macaca fascicularis' brain brain totalcc antcc - totalcc .40 (.03) - antcc .16 ~41) .45 (.01) - postcc post5 next antcomm splenial .45 (.01) .90 t.00) .44 .47 (.01) .82 .43 (.02) .80 .24 (.23) .65 .35 (.OO) (.OO) .48 (.01) .92 (.00) .34 (.02) postcc post5 next antcomm - - .91 .I7 (.00) - .20 (.W 61 (.00) .57 (.00) .58 (.00) splenial IPearson Correlations for raw measurements on the corpus callosum for each species with two-sided p values. - .59 (.OO) .46 (.01) .76 (.W .90 (.00) .56 (.0Q .35 ~07) - 356 R.L. HOLLOWAY AND P. HEILBRONER TABLE 6. Correlation (Pearson) matrix for Macaca mulatta' brain brain - totalcc antcc postcc post5 next antcomm .44 .40 (.W .90 .39 (.07) .84 .31 (.15) .I7 .41 (.05) .79 .36 (.08) .43 (.OO) (.OW (.OO) (.OO) .54 .47 (.02) .94 .53 (.W .92 (.OO) (.OW (.W - totalcc - antcc (.W postcc - - post5 .81 (.OW next - .47 (.02) .26 03) .26 (.22) .22 ~32) - antcomm splenial splenial .31 ~ 5 ) .45 (.03) .22 ~31) .65 (.OW .72 (.OO) .47 (.02) .10 (.66) - 'Pearson Correlations for raw measurements on the corpus callosum for each species with two-sided p values. TABLE 7. Correlation (Pearson) matrix for Callithrix jacchusl brain totalcc antcc uostcc Llost5 next antcomm sulenial .43 brain totalcc antcc postcc (.W .02 post5 next antcomm splenial 'Pearson Correlations for raw measurements on the corpus callosum for each species with two-sided p values. TABLE 8. Correlation (Pearson) matrix for Saguinus oedipus' brain totalcc brain totalcc antcc postcc post5 next - -.02 (.91) -.05 (.84) .03 (.W .13 (.58) .51 (.0% .23 (31) .59 p.19 ~41) -.05 - .84 (.OO) antcc postcc - .58 (.OO) .05 (.8U - (.OO) post5 next antcomm - (.82) .09 (.W -.25 (.28) -.06 (.79) - antcomm .25 ~32) -.21 (.43) -.17 C51) -.15 (.57) -.02 (.92) .44 (.08) splenial 'Pearson Correlations for raw measurements on the corpus callosum for each species with two-sided p values. - splenial .03 (38) .56 (.OU .26 (.23 .63 (.OO) .79 (.OO) ,023 (.73) -.03 (.88) - SEXUAL DIMORPHISM IN PRIMATE CORPUS CALLOSUM ACKNOWLEDGMENTS The authors are grateful for partial support from NSF-BNS-84-18921 (R.L.H.) and to the Primate Research centers described in the Methods section. The comments of unknown reviewers were also very helpful to us. LITERATURE CITED Byne W, Bleier R, and Houston L (1988) Variations in human corpus callosum do not predict gender: A study using magnetic resonance imaging. Behav. 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