Vertical clinging and leaping induced evolutionary rate shifts in postcranial evolution of tamarins and marmosets (Primates, Callitrichidae)

Background Callitrichids comprise a diverse group of platyrrhine monkeys that are present across South and Central America. Their secondarily evolved small size and pointed claws allow them to cling to vertical trunks of a large diameter. Within callitrichids, lineages with a high affinity for vertical supports often engage in trunk-to-trunk leaping. This vertical clinging and leaping (VCL) differs from horizontal leaping (HL) in terms of the functional demands imposed on the musculoskeletal system, all the more so as HL often occurs on small compliant terminal branches. We used quantified shape descriptors (3D geometric morphometrics) and phylogenetically-informed analyses to investigate the evolution of the shape and size of the humerus and femur, and how this variation reflects locomotor behavior within Callitrichidae. Results The humerus of VCL-associated species has a narrower trochlea compared with HL species. It is hypothesized that this contributes to greater elbow mobility. The wider trochlea in HL species appears to correspondingly provide greater stability to the elbow joint. The femur in VCL species has a smaller head and laterally-oriented distal condyles, possibly to reduce stresses during clinging. Similarly, the expanded lesser trochanters visible in VCL species provide a greater lever for the leg retractors and are thus also interpreted as an adaptation to clinging. Evolutionary rate shifts to faster shape and size changes of humerus and femur occurred in the Leontocebus clade when a shift to slower rates occurred in the Saguinus clade. Conclusions Based on the study of evolutionary rate shifts, the transition to VCL behavior within callitrichids (specifically the Leontocebus clade) appears to have been an opportunity for radiation, rather than a specialization that imposed constraints on morphological diversity. The study of the evolution of callitrichids suffers from a lack of comparative analyses of limb mechanics during trunk-to-trunk leaping, and future work in this direction would be of great interest.

Automated summary

The study found that callitrichids with vertical clinging and leaping behavior have narrower trochlea in the humerus, possibly contributing to greater elbow mobility, while those with horizontal leaping behavior have wider trochlea for greater elbow joint stability. The femur in vertical clinging and leaping species has a smaller head and laterally-oriented distal condyles to reduce stresses during clinging, and the expanded lesser trochanters provide a greater lever for leg retractors, interpreted as an adaptation to clinging. Evolutionary rate shifts to faster shape and size changes of the humerus and femur occurred in the Leontocebus clade while slower rates occurred in the Saguinus clade.