Diurnality, Nocturnality, and Cathemerality

Diurnality, nocturnality and cathemerality are all concepts that define different observed activity patterns in the animal kingdom.  In the classroom these concepts were all very clear and made perfect sense to me.  If an animal was primarily active during daylight hours and its circadian rhythm was intimately tied to the light half of the L/D cycle, it was diurnal.  If an animal was primarily active during the night and its circadian rhythm was intimately tied to the dark half of the L/D cycle, it was nocturnal.  And if an animal was not wedded to being active during either the day or the night, it was cathemeral.

However, the field is not the classroom and these concepts all become quite a lot more confusing when your observing and recording the activity patterns of a different species (in this case, ring-tailed lemurs (Lemur catta)).  But first, a little history.

Understanding activity patterns

For several decades early primatologists (and other animal behavioural specialists) assumed that all animals were either diurnal or nocturnal.  Furthermore, they believed that these two activity patterns were uniquely distinctive characteristics of the two primate suborders: strepsirrhini (lorises, galagos and lemurs) were nocturnal and haplorhini (tarsiers, monkeys, apes and humans) were diurnal.  However, this was largely a product of researcher bias and the way primatologists divided up their field seasons.  Typically, it was convention for a primatologist to study an animal either in the night or the day.  Rarely, would researchers decide to track and follow a primate throughout a 24-h period or design a more erratic observational time schedule that was dispersed both during the day and the night.

Nor was this seen as necessary.

Primatologists who studied bonobos or baboons (for example) assumed that their animals were active only in the day and that they only slept at night.  Likewise, researchers who studied mouse lemurs or pottos (for example) knew that their study subjects were active during the night and were sleeping during the day.  The need for the concept of cathemerality was not thought to be necessary until it was discovered that members of the genus Eulemur did not fit into either category.

This proved to be an evolutionary riddle for biologists and anthropologists as well.  Why would an animal become adapted to both the day and the night?  These are vastly different temporal niches that require exceptionally specialized sensory structures (primarily optical).  Being active during both the night and day would require adapting partially to both time periods at the expense of becoming biologically specialized in one.  How could such a generalist compete with diurnal and nocturnal specialists?  In some ways these questions have not been answered and are still being debated by contemporary researchers.

Cathemerality was also problematic because it destroyed the simplistic notion that all of the most primitive primates were nocturnal and that divergent evolution had provided our order with a very linear progression of ‘higher’ primates that had become steadily more diurnal.

Either way, over the past few decades all animal behavioural researchers have had to accept that the world of animal activity patterns is not black and white and different evolutionary pressures can force any animal into either temporal niche, with cathemeral behaviour possibly being a transition stage into complete diurnality or complete nocturnality.

So why so confused?

The reason this makes studying activity patterns confusing is because of the inherent limitations and subjectivity of concepts and definitions.  Without releasing exact data, I can say that I have definitely observed periods when the lemurs have been awake at night.  They will do anything from lifting their head and surveying the area around them to social grooming to play to switching sleeping trees to a nearby tree.  So there is a range of activity, and there may even by a pattern to this activity.  Is that cathemerality?  Am I right to conclude that thus far I don’t believe I’ve witnessed it?

Unfortunately, it may depend on the definition of the concept of cathemerality.  Some researchers will say that in order for an animal to be classified as cathemeral, their activity needs to be distributed ‘fairly evenly’ throughout the 24-h period, whereas others will say cathemerality is an activity pattern comprised of ‘distinct periods’ of nocturnal and diurnal behaviour.  Of course, these definitions are likely used by different researchers in order to gain a more favourable research conclusion or to make publication easier.  If in order to classify as cathemeral activity the animal must distribute activity ‘fairly evenly’ throughout the 24-h period, then I certainly have not witnessed that pattern.  However, have I witnessed ‘distinct periods’ of activity in both the diurnal and nocturnal temporal niches?  I think I could conclude ‘yes’ objectively.  But then I would become troubled because couldn’t most animals be considered cathemeral?  In my mind that interpretation sort of distorts the usefulness of the category.

But then again I have further questions about these categories while observing at night.  Is an animal cathemeral if they continue foraging and travelling past dusk but then go to sleep shortly after dusk and then sleep continuously until dawn?  Is an animal cathemeral if an animal is awake for parts of the night but their behaviour is radically different and substantially less active than during the day?  Is an animal cathemeral if they are only active at night during a few days or weeks out of a year?

This seems to bring me back to a problem that I’ve acknowledged before in The Ratchet about concepts and definitions.  Humans universally attempt to make our world discontinuous, when really it is continuous and cannot be categorized.  There is a range of activity patterns in the animal kingdom that cannot be nicely grouped into three categories: diurnal, nocturnal and cathemeral.  Even if, by current primatological standards, I would be able to claim in a research article that ring-tailed lemurs are cathemeral, with the current data I have it would be obvious that their activity budget would be drastically different than previous species who have been classified as cathemeral.  For me this is not only a problem in the field, but it raises questions about the way knowledge is produced and disseminated.  And I am still unsure about how I will solve this problem internally when I begin writing my thesis.

 

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About Cadell Last
I am a science educator, freelance science writer, and founder of The Advanced Apes based in Toronto, Ontario. In the past my academic research focused on the evolution, ecology, and behaviour of non-human primates (i.e., chimpanzees, gorillas, ring-tailed lemurs). Currently, my official blog, The Ratchet, can be found via The Advanced Apes and Svbtle. I enjoy exploring recent research in human evolutionary sciences, as well as biology, ecology, astronomy, physics, and computer science. My work has been featured in Scientific American, American Humanist, Richard Dawkins Foundation for Reason and Science, and Jane Goodall Institute of Canada. I am also exploring science popularization in new mediums in collaboration with PBS Digital Studios with an animated YouTube channel. You can contact me on Twitter (@cadelllast) or via email: cadell.last@gmail.com

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