The way a monkey descends a tree – cautiously, head-up, almost backing down a ladder – differs markedly from the headfirst approach of squirrels and many other mammals. This seemingly subtle distinction is now offering researchers new insights into the evolutionary history of primate posture and movement.
A new study, published in the journal eLife, presents a comparative analysis of tree-dwelling mammals, examining how 21 arboreal species navigate vertical surfaces like tree trunks and vines. Researchers conducted the first broad study to analyze both upward and downward climbing across a diverse range of mammal groups, from primates to rodents and marsupials.
The research reveals a pattern shaped by evolutionary history and anatomy, not solely by body size. “While not all arboreal mammals traverse narrow terminal branches, they all rely on vertical supports to reach tree canopies,” explained Séverine Toussaint, lead author of the study and researcher at the Center for Research on Paleontology in Paris. “Their ability to safely descend sloping and vertical supports remains important, yet largely understudied.”
Researchers identified three primary descent styles: head-first, side-on, and tail-first. Head-first movement involved keeping the body parallel to the trunk with the face downward. Tail-first resembled climbing, with the head pointing upward. Side descents involved rotating the body sideways against the support.
Significant differences emerged between primates and other mammals. Most non-primates consistently descended headfirst, regardless of the surface size. Rodents, marsupials, and treeshrews followed this pattern. Primates, however, exhibited greater variety. Lemurs frequently used tail-first descents, while many New World monkeys favored sideways descents, particularly on narrower supports. “Primates generally used tail-first and side descents on small vertical supports, suggesting that narrower supports impose constraints that call for more upright postures,” said co-author Dionisios Youlatos of Aristotle University of Thessaloniki.
Smaller primates, such as lorises and tamarins, tended to behave more like non-primates, primarily relying on head-first descent.
The study, which involved analyzing 57 animals and recording over 2,790 ascents and descents using high-speed video, also found that animals adjusted their movement mechanics when descending compared to climbing. Head-first descents were 30 to 43 percent slower than ascents, and animals increased the time their limbs remained in contact with the surface, likely to enhance stability. Many species also adopted more asymmetrical gaits during descents, utilizing movements similar to bounding or galloping.
Primates employing tail-first or side-on strategies maintained speeds comparable to climbing, suggesting these postures may help preserve efficiency while minimizing risk.
The research also examined the influence of body proportions on descent behavior. Factors such as limb length, tail length, body mass, and relative head size were linked to posture choice. Animals with longer limbs and tails, larger bodies, and heavier heads were more inclined to descend tail-first. Shorter-limbed species with more balanced fore- and hindlimb lengths tended to descend headfirst.
The study suggests that head size may play a more significant role than previously understood. Primates generally have larger brains relative to body size than other mammals, increasing head mass and shifting the body’s center of gravity. This shift could make head-first descent less stable, potentially encouraging more upright strategies. The researchers noted this is the first study to examine the potential impact of relative head mass on arboreal movement.
By applying relationships between anatomy and movement in living species, the team developed a model to estimate how extinct mammals may have climbed. They applied this model to 13 fossil species related to early primates. Most were predicted to descend primarily headfirst. However, two early primates, Darwinius masillae and Europolemur kelleri, showed indications of more varied strategies, potentially reflecting longer limbs and tails.
“Considering that early euarchontoglires were probably small to very small… it is plausible that they used mostly head-first descents,” said co-author John Nyakatura of Humboldt University of Berlin. “As euprimates evolved better grasping abilities… they likely began to adopt side and upright vertical descent postures.”
The authors acknowledge limitations, including the fact that not all animals readily used the experimental supports, which may have influenced behavior measurements. The study also focused on small- to medium-sized species, leaving questions about larger primates and carnivores unanswered.