The Evolution of Primate Sleep

While I have been on St. Catherines Island studying ring-tailed lemur (Lemur catta) sleeping patterns I’ve learned a lot about the ecological determinants of lemur sleep site choice and nocturnal activity pattern.  While it was not planned, this is quite similar to what I studied as an undergraduate researching the environmental determinants of chimpanzee nesting in Cameroon.  As a consequence, I have become an unintended expert on primate sleeping patterns.  Also, because the two species I have studied are so distantly related phylogenetically within the primate order, it has made me aware of some interesting trends in the evolution of primate sleep.

First off let’s start with the basics.  Ring-tailed lemurs are prosimian primates that are part of a lemur clade of over 100 species.  All known extinct and extant members of the clade live(d) in Madagascar and have been evolving in isolation from all other primate species for almost 65 million years.  They are also one of the most primitive of all primate clades and closely resemble the stem primates, Plesiadapis.  In contrast, chimpanzees are part of the ape clade, which is composed of far fewer species than the lemur clade.  They live in populations that are distributed throughout most of sub-Saharan Africa and are thought to have evolved sometime within the past 6-8 million years (few known fossilized remains prevent a deeper understanding of this evolution).  Chimpanzees are one of the least primitive of all primates, and share only primate universal features with the stem primates, Plesiadapis (e.g., binocular vision, nails, opposable thumbs).

Quick side note: the word ‘primitive’ in primatology does not mean ‘less evolved’ or ‘less complex’ and is not seen as a derogatory term.  Primitive simply means resembling the ancestor.  So in this case lemurs are more primitive than chimpanzees because lemurs share more in common with the common ancestor of all primates than chimpanzees do:

However, primitive is a relative term.  Chimpanzees are more primitive than humans because chimpanzees more closely resemble the chimpanzee-human common ancestor than humans do.

This all has relevance to the evolution of sleep patterns because anatomical similarity can help us piece together the evolution of certain behaviours, including sleeping behaviours.  Throughout my research I have observed sleeping patterns that can tell us a lot about the general sleeping pattern that has developed throughout the 65 million years or so of primate evolution.  I believe some of these observations confirm what previous primatological work has discovered, and some observations may add a novel interpretation to the evolution of these sleeping patterns.

Quick note: Remember, these are patterns, and there are exceptions to these patterns.  I will try and acknowledge some of these exceptions to the general pattern, and try and hypothesize as to why certain behaviour patterns would have re-emerged.

Pattern #1

Throughout primate evolution most primates have been arboreal sleepers.  As humans are one of the most terrestrial primates, we can sometimes forget that most of our order consists of animals highly specialized for arboreal life.  And when it comes to sleep, primates will almost without exception opt to spend their sleeping hours in a tree, but not just any tree.  During my current field research it became obvious that the ring-tailed lemurs would only sleep in very specific trees:

They were always trees that were not only very high off of the ground, but also provided them with an added safety from terrestrial predators: no lower branches.  In my opinion, this is a biologically ingrained behaviour for ring-tailed lemurs (and most other lemurs).  In Madagascar most populations of lemurs are in constant danger from the cathemeral fossa.  Fossa’s are lemur hunting specialists that can climb trees and prey on lemurs in both the day and night.  On St. Catherines Island there are no fossas (or any terrestrial predators to take the fossas place) yet they will still not settle for sleeping trees with lower branches.  As I mentioned, most other primates (including monkeys and apes) behave the same way and much prefer to nest arboreally. For each primate there may be more complex and specific requirements for a sleeping site, but most primatologists agree that arboreal sleep has a specific anti-predator function.

However, for chimpanzees, although arboreal sleeping is certainly the norm in most groups, some individuals and some groups do relax their sleeping site requirements and sleep terrestrially.  This interesting break from the general primate pattern has implications for the development of human sleeping patterns because sometime in our evolutionary past we gradually came down from our arboreal niche and began attempting to live completely terrestrially.

During my time in Cameroon I observed that some chimpanzees were nesting terrestrially at night.  These happened to be chimpanzees who were also in an area with no human predation pressure (chimpanzees in areas with high levels of human predation pressure always nested arboreally).  This research made me realize that arboreal sleeping throughout primate history may have been primarily driven by predation pressure.  This could mean that our human ancestors began coming down from the trees due to living in an environment with relaxed predation pressure or they became better able at avoiding and preventing predation events (e.g., larger groups, control of fire, better weapons).

Although arboreal sleeping does offer many advantages, terrestrial sleeping allows for greater environmental flexibility and as a consequence enabled our ancestors to move from rainforests and wooded savannahs into different environments because we were no longer dependent on trees for safety.

Pattern #2

A second obvious pattern has to do with the evolution of nest-building or sleeping platform building.  It was no surprise while I was in the field on St. Catherines Island that I did not see any ring-tailed lemur build a nest to sleep in.  Almost no primates build nests to sleep in.  In fact, the only primates that build nests are the great apes.  Nest building is almost certainly a sign of advanced intellect in primates due to the highly social nature of the acquisition of nest building skills (unlike nest building in other species, or damn construction in beavers, which functions more as an extended phenotype).  Primate nests are highly complex structures that require a considerable amount of individual skill and social learning.

Nests also offer the great apes a few very important advantages.  They have been shown to decrease risk of disease when nesting arboreally and terrestrially, aid in thermoregulation in both extreme cold and heat and improve overall sleep quality.  This proved to be a key development in the evolution of primate sleep and may have developed as far back as 20 million years ago.  It was certainly a pattern that characterized our ancestors, as large complex nest-like structures have been found at sites associated with the earliest modern human populations in Africa (all terrestrial of course!).

Pattern #3

A final pattern that I feel deserves mention in this post is the distribution of sleep over a 24 hour period.  Throughout the course of primate evolution more and more primates began adapting to a diurnal existence.  Although ring-tailed lemurs are certainly more primitive than chimpanzees, they seem to have adapted to a primarily diurnal existence as well.  Despite this, there is growing evidence that they also spend a considerable amount of time nocturnally active, which may re-classify them into the odd cathemeral category.  Most of the other 100 or so lemurs are either nocturnal or cathemeral, which reflect their primitive adaptations to the dark half of the L/D cycle.  During my observations the ring-tailed lemurs were certainly night active at times.  They ranged occasionally and there were periods during the night when the entire group would be awake and active within their sleeping tree.

This type of behaviour is not typical of most monkeys and apes (with the notable exception of the aptly titled owl monkey).  At some point in the evolution of primate sleep (probably with the emergence of the first monkeys 40-30 million years ago), primates began occupying diurnal niches with increased frequency.  In contemporary times most monkeys and apes are rarely, if ever, active at night.  While I was in Cameroon this was evident in the ecological patterns left behind by chimpanzee groups.  They had specific nesting patterns for day and night use.  Day nests were simple and mostly terrestrial (even in areas with high human predation pressure).  Day nests were also constructed around termite mounds and ant hills, and surrounded with ‘fishing rods’ and play tools.  In contrast, night nests were far more complex, offering a more stable structure, more padding for comfort and constructed almost exclusively in trees (with some exceptions as previously mentioned).

Final thoughts

It has been an interesting journey learning about the world of primate sleep, and a happy coincidence that my observations and data happen to be on two very different species of primate.  As a consequence I have had a chance to see first hand how primate sleep has developed in distantly diverged clades.  In lemurs sleep is arboreal, with no nest and could be distributed during substantial periods of both the day and night.  In chimpanzees sleep can be both arboreal and terrestrial, always with a nest, and is exclusively distributed during the night.  These experiences have helped me understand our closest relatives, but they have also helped me understand the evolution of our own behaviours.  Now it is time for bed.