A cotton-top tamarin. Image credit: Toussaint et al. (CCBY 4.0)
For many mammals living in forests, moving in trees is a daily necessity for survival. Arboreal environments offer habitats of varying sizes, orientations and flexibility, requiring animals to adopt specialized ways to move.
One particularly challenging behavior is climbing both up and down vertical supports such as tree trunks or lianas. Vertical climbing has played a significant role in mammalian evolution, especially in primates, whose ancestors developed grasping hands and feet suited for life in trees. While upward climbing has been extensively studied, descending vertically is equally crucial and potentially riskier, yet remains poorly understood.
Animals descending supports must manage their balance, speed, and body posture to prevent falling. These strategies may depend on measurable factors such as body mass, limb proportions, and grasping structures, which influence the distribution of forces along the body. Understanding how living mammals descend vertically can offer a functional framework for interpreting fossil anatomy and reconstructing the behavior of early primates and their relatives.
Toussaint, Youlatos and Nyakatura investigated whether arboreal mammals of small body mass employ consistent strategies when descending supports, and how these relate to body size and limb proportions. The researchers also examined whether these relationships make it possible to infer climbing abilities in fossils of early primates and their kin. Addressing these questions is vital because vertical movement is a high-risk yet common behavior in arboreal mammals that has shaped their ecological adaptations and morphology.
The results showed that vertical descent is not determined solely by body size but reflects distinct evolutionary strategies among arboreal mammals. Using high-speed video analysis of 1,390 descents and 1,400 ascents across 57 individuals from 21 species (including primates, rodents, carnivorans, marsupials and tree shrews), Toussaint et al. identified three descent postures: head-first, sideways and tail-first.
Non-primate mammals predominantly descended head-first, whereas primates displayed unique sideways and tail-first strategies, with variations in speed and performance. These strategies were linked to morphology, including limb proportions, tail length, and relative head size. Comparing contemporary species with fossils allowed them to reconstruct vertical descent strategies in 13 early primates and their relatives, revealing a major evolutionary shift towards upright descent postures (keeping their heads upward) throughout their history.
The findings of Toussaint et al. can benefit primatologists, evolutionary biologists and paleontologists by enhancing our understanding of how animals adapt to their environments and how fossil species can be interpreted through modern analogues. Beyond academic research, this knowledge can improve animal welfare in captivity by promoting enclosures with diverse arboreal supports of varying sizes and orientations for animals to utilize. Finally, these insights could inform the design of bio-inspired robots by identifying stable strategies for moving on vertical structures, relevant for search-and-rescue or inspection missions.
