You might find the lemur mongoose in Madagascar and the raccoon in the deciduous forests of North America, but they have one important thing in common: both species spend a lot of time climbing trees.
The ability to climb up and down trees is more complex than it might first seem. Animals that spend much of their life navigating woodlands must be able to move adeptly across branches and trunks of different widths, orientations, and compliance. Now, research published in the journal eLife shows these behaviors may have influenced evolution.
“While not all arboreal mammals traverse narrow terminal branches, they all rely on vertical supports to reach tree canopies,” lead author Séverine Toussaint, Research Fellow at the Center for Research on Paleontology — Paris, at the MNHN, Sorbonne University, and CNRS, France, said in a statement.
“Their ability to safely descend sloping and vertical supports remains important, yet largely understudied, as most research has focused on their ascent behaviours. We therefore wanted to study how they climb down trees to understand the significance of adaptations that enable this behaviour.”
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Climbing In Living Mammals
Two species from the study by Toussaint et al. On the left, a raccoon (Procyon lotor) and on the right, a mongoose lemur (Eulemur mongoz) climbing on vertical supports.
(Image Credit: Séverine Toussaint (CC BY 4.0))
The team first investigated climbing behaviors in living species. Using high-speed video analysis, the researchers compared upward and downward movements in 21 species of small and medium-sized tree-dwelling mammals. This included 1,390 clips of the animals descending and a further 1,400 clips of the animals ascending.
The movements were then compared to the animals’ physiological traits, such as body mass and limb proportions.
As a general rule, the animals moved more slowly and made more adjustments to stabilize themselves as they descended the vertical supports. However, different species adopted different techniques, which could broadly be divided into three categories: head-first, tail-first, and sideways.
Larger animals were more likely to move tail-first, whereas smaller species descended head-first. Sideways movements were observed but only in primates, who displayed greater variability in approach. The reason, according to the researchers, could be the environments these creatures often inhabit.
According to the study’s authors, the evolutionary success of early primates may be the result of their ability to carve a niche in habitats that other species found more challenging — for example, those with horizontal branches and small supports.
The researchers note that tail-first and side descents were more frequently observed when an animal was descending a small vertical support, suggesting that these objects were more easily navigated in an upright position.
Climbing In Extinct Animals
The team then examined 13 extinct species of euarchontoglires, a group that includes the ancestors of today’s primates and rodents. Using a computer model, the researchers predicted how these species would have climbed trees, discovering all but two appeared to descend head-first.
“Considering that early euarchontoglires were probably small to very small, with shorter hindlimbs, autopods, and reduced brain size, it is plausible that they used mostly head-first descents and asymmetrical gaits on vertical supports,” author John Nyakatura, Professor of Comparative Zoology, Institute of Biology, Humboldt University of Berlin, Germany, said in a statement.
“As euprimates [early primates] evolved better grasping abilities, elongated hindlimbs and tail, and eventually larger brains, they likely began to adopt side and upright vertical descent postures.”
From Raccoons To Robots
The team noted limitations. For example, observations of raccoons in the wild show they descend head-first on occasion — a behavior not seen in the study.
Still, the researchers hope the findings can shed light on how different species adapt to their environment. The research may even inform the design of bio-inspired robots, enabling machines to ascend and descend trees during search-and-rescue missions like mechanized monkeys.
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