Between Presence and Absence: Comparative Morphology of the Upper Limbs in the Family Atelidae (Primates) with Emphasis on the Vestigial Thumbs of the Genus Brachyteles Spix, 1823

• Grant number: 24/12921-8
• Support Opportunities: Scholarships in Brazil - Master
• Start date: July 01, 2025
• End date: January 31, 2027
• Field of knowledge: Biological Sciences - Zoology - Taxonomy of Recent Groups
• Principal Investigator: Joyce Rodrigues Do Prado
• Grantee: Isabela Cavalcante Barbosa
• Host Institution: Museu de Zoologia (MZ). Universidade de São Paulo (USP). São Paulo , SP, Brazil
• Associated research grant: 22/12632-0 - An old sample tells a good story: the evolutionary history of mammals from the diagonal of open vegetation of South America told by biological collections, AP.BTA.JP

Abstract

The platyrrhine primates of the Atelidae family (genera Alouatta, Ateles, Brachyteles, and Lagothrix) are known for their arboreal locomotion abilities and exceptionally long prehensile tails. The genus Brachyteles (woolly spider monkeys or muriquis) presents a peculiar characteristic: a vestigial thumb, whose size varies among its populations. Unlike many primates that use their thumbs for grasping, muriquis have reduced and non-functional thumbs, an adaptation reflecting their locomotion style and foraging methods. While some members of Atelidae retain fully developed and functional thumbs (as seen in Alouatta and Lagothrix), the absence of this feature in Ateles and Brachyteles may suggest a convergent evolutionary event in these two genera. However, this thumb reduction also appears in catarrhine primates such as Colobus, indicating that this trait independently arose in both parvorders. Additionally, the presence or absence of the vestigial thumb in Brachyteles has been used as a diagnostic trait to distinguish between northern and southern muriquis populations. This proposal aims to understand the basic anatomical structure of the thumb in South American atelids, and others forelimbs structures, with an emphasis on population variation in Brachyteles. For this purpose, samples from biological collections will be used to generate data through computed tomography (CT scans), X-rays, and volumetric images obtained from virtual repositories. The 3D images generated will be processed using specialized software and made freely available. The traits associated with the thumb will be compared considering the current evolutionary proposals for the family available in the literature to assess their phylogenetic signal and possible morphological convergences among the genera. The results are expected to contribute not only to a broader morphofunctional understanding of the vestigial thumb in primates but also to a better comprehension of the ecomorphological and adaptive aspects of this group. (AU)