Climbing Down Trees Helped Shape Early Primates

Life among the trees profoundly influenced the bodies of early primates. New research indicates that climbing down trees, as well as climbing up them, was important in the evolution of upright postures.

By examining the way present-day tree-dwelling mammals travel along vertical supports, scientists have gained insight into how early primates acquired their distinctive body form.

Tree life shaped early primates

Forests offer trunks, branches and lianas with varying diameters, angles and flexibility. Travelling securely across these supports demands particular body forms and locomotor patterns.

Climbing provides access to food, shelter and safety from predators. Fossils indicate that climbing capabilities emerged very early in mammalian evolution, including during the Jurassic period.

Vertical climbing probably influenced early members of euarchontoglires, the group that includes primates and rodents.

Why descending trees matters

Many researchers consider frequent vertical climbing to have helped early primates evolve grasping hands and feet. Yet studies have largely concentrated on upward climbing. Descent, despite potentially being more hazardous, has received much less scrutiny.

The study’s lead author, Séverine Toussaint, is a specialist at the Center for Research on Paleontology – Paris.

“While not all arboreal mammals traverse narrow terminal branches, they all rely on vertical supports to reach tree canopies,” said Toussaint.

“Their ability to safely descend sloping and vertical supports remains important, yet largely understudied, as most research has focused on their ascent behaviors.”

“We therefore wanted to study how they climb down trees to understand the significance of adaptations that enable this behavior.”

Three ways primates climb down

The researchers examined 21 species, spanning primates, rodents, carnivorans, marsupials and treeshrews.

Altogether, they analysed 1,390 descents and 1,400 ascents. High-speed footage enabled precise assessment of posture, speed, gait and limb contact.

The analysis identified three distinct descent methods. In head-first descent, an animal holds its body parallel to the trunk while facing its head downwards.

In tail-first descent, the body also remains parallel, but the head points upwards. With side descent, the body turns so that the chest faces to the side during the movement downwards.

Primates use varied ways to climb down

Non-primate mammals generally descended head first regardless of branch diameter. Primates, by contrast, displayed considerably greater variation. Lemurs commonly descended tail first.

Many New World monkeys favoured side descent. Smaller primates, including lorises and tamarins, more frequently used head-first descent.

“Primates generally used tail-first and side descents on small vertical supports, suggesting that narrower supports impose constraints that call for more upright postures,” noted study co-author Dionisios Youlatos, a professor at Aristotle University of Thessaloniki (AUTh).

“This marked behavioral divergence highlights the role of evolutionary history and supports recent findings suggesting that shared ecological niches and postural adaptations affect primate locomotion.”

Animals adjust while descending

Descending calls for close control. Among species, head-first descent led animals to lower their absolute speed by roughly 30 to 43 percent compared with climbing upwards. Moving more slowly limits the chance of slipping.

Duty factor, a measure of the time a limb remains in contact with its support, rose during descent. Keeping contact for longer enhances grip and stability.

Gait patterns changed as well. During descent, animals made greater use of asymmetrical gaits, including bounds and gallops.

On vertical supports, such gaits may offer better control because they permit powerful gripping by both the forelimbs and hindlimbs simultaneously.

Notably, primates did not greatly reduce their overall speed when descending side first or tail first. Their relative speed fell, producing shorter distances per stride, but their total speed remained similar.

This finding indicates that upright strategies lessen the braking demands placed on the forelimbs and enhance balance.

Body size shapes descent style

The team recorded body mass, limb length, tail length and relative head size, the latter using encephalization quotient, or EQ.

A number of trends were identified. Greater body mass, longer limbs, longer tails and a larger relative head size each made head-first descent less likely.

The intermembral index, which compares the lengths of the forelimbs and hindlimbs, also had a clear association with descent style.

Primates were distinctive. Most had hindlimbs exceeding 70 percent of body length, tails longer than 90 percent of body length and EQ values above 1.5. Non-primates generally possessed shorter limbs and lower EQ values.

A larger brain adds to head mass and moves the centre of mass forwards. In a head-first descent, this forward loading could heighten the risk of tipping. Upright postures may lessen that danger.

Fossil clues about primates

Using key body measurements, the team created a predictive model. They used it to estimate descent strategies for 13 fossil species, including early primates and related animals.

Most early species were reconstructed as making more than 75 percent of their descents head first. Two fossil primates, however, already had lower head-first proportions, alongside longer hindlimbs and tails.

“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,” said study co-author John Nyakatura from Humboldt University of Berlin.

“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.”

Climbing down helped primates evolve

Regular use of vertical supports in ancient forests may have promoted the evolution of grasping hands and feet.

Upright descent may have left the hands available for handling food while the feet delivered firm support. In time, this division of labour between hands and feet became a defining primate characteristic.

“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,” concluded Toussaint.

The research indicates that upright posture did not emerge abruptly. Gradual changes in body size, limb length, tail length and brain size altered the ways early primates travelled through trees.

Research into modern mammals allows scientists to better understand how life on vertical forest supports shaped the earliest history of primates.