Modularity and Integration in the Hominoid Scapula NATHAN YOUNG n Harvard University, Department of Anthropology, Cambridge, Massachusetts ABSTRACT In this paper, several hypotheses of morphological integration within the hominoid (ape) scapula are tested. In particular, whether the scapula represents a set of developmental tissues sharing tight correlations between constituent parts (i.e., highly integrated) or is more modularly organized (i.e., covariation is greater within regions than between) is tested. Whether the patterns of integration in the scapula have changed over phylogenetic time or in response to selective forces is also examined. Results from two different analyses (matrix correlations and edge deviance) indicate traits comprising the blade and acromion, and to a weaker degree the glenoid, correlate highly with each other. The coracoid exhibits more independence from other parts of the scapula, perhaps reflecting its distinct evolutionary developmental history. Overall, similarity in species-specific patterns of correlation was high between all taxa. Correlation matrix similarity was significantly correlated with functional similarity and morphological distance, but not with phylogenetic distance. These results are congruent with other studies of integration that suggest correlation patterns remain stable over evolutionary time. There are changes associated with phylogeny, but the tight link between functional similarity and phylogenetic distance at this level of comparison presents possible challenges to interpretation. Overall similarities in the pattern of integration in all taxa might be better interpreted as relative strengthening or weakening of trait correlations rather than broadscale changes in the pattern of relationship between developmental regions. Larger sample sizes with greater taxonomic/functional breadth, and finer scale analyses of patterns of correlation are needed to test these hypotheses further. J. Exp. Zool. (Mol. Dev. Evol.) 302B:226–240, 2004. r 2004 Wiley-Liss, Inc. INTRODUCTION The study of morphological integration in primates has a rich history, largely due to the efforts of Cheverud (’82, ’88, ’89, ’95, ’96) and colleagues (Cheverud et al., ’89; Ackermann and Cheverud, 2000; Marroig and Cheverud, 2001; Ackermann, 2002). More recently, others have taken up integration as a useful line of morpho- logical analysis in primates and particularly hu- mans [e.g., Strait (2001): use of cranial integration analyses to define morphological complexes and phylogenetic characters; Lieberman et al. (2000): analysis of cranial base integration; Gonza ´les-Jose ´ et al. (2004): patterns of cranial integration in a broad sample of modern humans]. A common focus of primates analyses has been the degree of integration between the face, cranial base and neurocranium. There are both practical reasons for this focus (an abundance of cranial material both extant and fossil; a long history of study), as well as scientific reasons (the cranium involves multiple developmentally distinct tissues with different patterns of growth; cranial evolution is of central importance in human evolution, etc.). On the other hand, studies of integration in the primate postcranial skeleton, and particularly the appendicular skeleton, have been much less frequent. However, the primate postcranium has great potential due to a wealth of comparative skeletal material, well-characterized developmen- tal genetics of the trunk and limbs, and the potential to corroborate and/or test findings with model species (Chiu and Hamrick, 2002; Hallgrı ´msson et al., 2002). This paper seeks to fill in some of this gap by testing several hypotheses of morphological inte- gration within one element of the primate post- cranium: the hominoid scapula. There are a number of reasons to study this structure in this group. At a general level, understanding how the scapula is integrated is a potentially interesting comparison to largely cranial studies of integra- tion. Like the cranium, the scapula is a single n Correspondence to: Harvard University, Department of Anthro- pology, 11 Divinity Avenue, Cambridge, MA 02138. E-mail: nyoung@fas.harvard.edu Received 16 October 2003; Accepted 29 March 2004 Published online in Wiley Interscience (www.interscience.wiley. com). DOI: 10.1002/jez.b.21003 r 2004 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY (MOL DEV EVOL) 302B:226–240 (2004)