Researchers have shed new light on the features that enable tree-dwelling mammals to move effectively through their environments, providing insights into the evolution of the distinct upright postures seen in primates.
Two species from the study – a raccoon (Procyon lotor) and mongoose lemur (Eulemur mongoz) – climbing on vertical supports. Credit: Séverine Toussaint (CC BY 4.0)
The study, published today in eLife as the final Version of Record after appearing previously as a Reviewed Preprint, is the first to compare upward and downward climbing behaviours across a broad range of tree-dwelling (arboreal) mammal species. eLife’s editors describe the work as valuable, with convincing analyses that will be of interest to biologists studying animal movement.
Arboreal mammals need to adapt their form (morphology) and movements (locomotion) to climb up and down trees effectively, due to the diversity of tree trunks and branches that vary in their orientation, width and compliance. These critical tree-climbing behaviours likely played a major role in early primate evolution.
“Several existing mammal species have been put forward as models for early primates’ arboreal adaptations, including small strepsirrhines, or lemuriform primates; platyrrhines, or New World monkeys; treeshrews; rodents; and marsupials,” says lead author Séverine Toussaint, Research Fellow at the Center for Research on Paleontology – Paris, at the MNHN, Sorbonne University, and CNRS, France.
“While not all arboreal mammals traverse narrow terminal branches, they all rely on vertical supports to reach tree canopies. 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.”
To do this, Toussaint and the team studied 21 small-to-medium-sized arboreal mammal species and how their postural and motion (kinematic) features change as they climb up and down vertical supports. The species included six strepsirrhine primates; five platyrrhine primates; three rodents; three carnivorans; three marsupials; and one scandentian (treeshrew).
The team began by examining the effect of the vertical supports’ diameter (small, medium or large) on the animals’ posture and kinematics, including their speed and gait, during descents compared to ascents. They then assessed the relationships between how the animals descend and their morphology, such as their body mass, limb proportions and relative head mass.
They identified three distinct strategies of descent: head-first descent, side descent and tail-first descent. There was a clear difference between primates and non-primates in how they descended each type of vertical support: primates showed greater variability in how they climbed down, using both tail-first and side postures more often than other mammals, which mostly descended head-first.
“Primates generally used tail-first and side descents on small vertical supports, suggesting that narrower supports impose constraints that call for more upright postures,” explains author Dionisios Youlatos, Professor in the Department of Zoology at the School of Biology, Aristotle University of Thessaloniki, Greece. “This marked behavioural divergence highlights the role of evolutionary history and supports recent findings suggesting that shared ecological niches and postural adaptations affect primate locomotion.”
In terms of their kinematics, overall the animals studied made several adjustments to stabilise themselves when climbing down compared to up. These included descending more slowly and increasing their use of more asymmetrical gaits.
Additionally, their vertical descent strategies reflected trade-offs among body mass, limb proportions, and head mass. Animals with longer forelimbs and hindlimbs, longer tails, larger bodies and relatively larger head mass are more likely to descend tail-first. In contrast, animals with shorter fore and hindlimbs that are similar in length, including non-primates, are more likely to descend with a head-first posture.
Using a morphology-based model, the team next inferred possible descent behaviours in 13 extinct euarchontoglires – a superorder of placental mammals that may share a common ancestor with primates and rodents. The model reconstructed fossil species as using relatively high proportions of head-first descent, except for the two medium-sized adapiforms Darwinius masillae and Europolemur kelleri, which already displayed relatively long hindlimbs and tail. This suggests that postural adaptations for vertical descent in primates evolved progressively.
“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,” says author John Nyakatura, Professor of Comparative Zoology, Institute of Biology, Humboldt University of Berlin, Germany. “As euprimates evolved better grasping abilities, elongated hindlimbs and tail, and eventually larger brains, they likely began to adopt side and upright vertical descent postures.”
The authors note a couple of key limitations to these findings – for example, in the experimental setting, all the animals did not readily use the vertical supports provided for the study, which may have impacted the proportion of descent behaviours and the kinematics studied for some species. Additionally, the team says it would be useful to compare the kinematics of vertical ascents and descents in more species with a larger body mass and varying morphological proportions, such as apes and large carnivorans.
“Such studies would allow us to develop reconstruction models better suited to large arboreal fossils, which would refine our understanding of the evolution of arboreal adaptations within this group,” Toussaint concludes.
Accompanying photographs for this study are available to access here.
Media contacts
Emily Packer
eLife
e.packer@elifesciences.org
+441223855373
About
eLife transforms research communication to create a future where a diverse, global community of scientists and researchers produces open and trusted results for the benefit of all. Independent, not-for-profit and supported by funders, we improve the way science is practised and shared. In support of our goal, we introduced the eLife Model that ends the accept–reject decision after peer review. Instead, papers invited for review are published as Reviewed Preprints that contain public peer reviews and an eLife Assessment of the significance of the findings being reported and the strength of the evidence. eLife is supported by the Howard Hughes Medical Institute, Knut and Alice Wallenberg Foundation, the Max Planck Society and Wellcome. Learn more at https://elifesciences.org/about.
