A monkey descending a tree trunk often keeps its head up, moving almost like a cautious climber backing down a ladder. Squirrels and many other mammals, by contrast, tend to go headfirst. That difference turns out to carry clues about how primates evolved their distinctive upright postures. A new comparative analysis of tree-dwelling mammals, published in eLife, examined how animals move down vertical supports such as trunks and vines. The research compared 21 arboreal species, from primates to rodents and marsupials, making it the first broad study to analyze both upward and downward climbing across many mammal groups. The results point to a pattern shaped not just by body size, but by evolutionary history and anatomy. “While not all arboreal mammals traverse narrow terminal branches, they all rely on vertical supports to reach tree canopies,” said lead author Séverine Toussaint of the Center for Research on Paleontology in Paris. “Their ability to safely descend sloping and vertical supports remains important, yet largely understudied.” Two species from the study – a raccoon (Procyon lotor) and mongoose …