Novel study reveals the evolution of daytime sleeping patterns in nocturnal primates

Friday, 20 July 2018

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New research reveals that day-time sleep site selection and predator avoidance are linked unanimously amongst nocturnal primates.

This is the first time research on sleeping behaviours of nocturnal primates has been compiled. The study began more than ten years ago and with data collected in 18 countries throughout Africa and Asia on lorisiform primates – a group that includes slow lorises and bushbabies, distant relatives of lemurs and human beings.

Dr Magdalena Svensson, Oxford Brookes University and lead author of the study said: “Our study has found that nocturnal primates select their sleep sites because of specific features that make the site suitable and safe, such as tree height and presence of tree holes. These sites however, are in short supply. Increasing habitat destruction, which often targets the very trees that are the best ones to sleep in, reduces the number of sleep sites and compromises nocturnal primates during these vulnerable hours."

Scientists compared sleep patterns, sleep site selection and vulnerability to day-time predation among these nocturnal tree dwellers. Through evolutionary analysis, the researchers were able to show that the ancestral loris slept in dense tangles of vegetation and branches, which are most common sleep sites selected by today’s lorisiforms, followed by the more difficult to find tree holes.

Increasing habitat destruction, which often targets the very trees that are the best ones to sleep in, reduces the number of sleep sites and compromises nocturnal primates during these vulnerable hours.

Dr Magdalena Svensson, Primate Conservation Laboratory Technician, Oxford Brookes University

Sleeping during the day exposes these small primates to a wide range of day-active predators, including humans who may capture them for medicinal purposes, for food, or want to keep them as pets.

“One crucial factor in sleep site selection is that it has to be safe from predation – while asleep it is difficult to keep an eye out for those that want to eat you,” said Dr Lydia Luncz from the University of Oxford and co-author of the paper.

“In fact apart from humans, important predators of lorises and galagos are other primates such as chimpanzees and orang-utans."

In addition to these findings, the researchers have found that how our distant ancestors slept may have impacted the evolution of key characteristics in humans such as grasping hands. The evolutionary data gathered during the study found that babies of primate ancestors clung to the fur of their mothers which allowed them to hold on safety during the hours of sleep.

Research Professor of Zoology and galago expert Judith Masters from University of Fort Hare in South Africa, who was not involved in the study, said: “This paper is more than a review of sleeping site selection by lorisiform primates. By linking sleeping site use to the mode of infant transport, and the latter to hind- and forelimb proportions, the authors propose to shed light on infant rearing behaviour among the Palaeogene primates and their relatives. This is truly original.”

The paper, Sleep patterns, daytime predation, and the evolution of diurnal sleep site selection in lorisforms has been published in the July issue of the American Journal of Physical Anthropology and can be found online.

Image: Chimpanzee in Guinea holding a head northern lesser galago, having caught it in the daytime. Photo by Charlotte Houpline, Chimpanzee Conservation Center.