6 March 2024

Mammals in the Moonlight

Peter Post sent me this interesting article on one of the great mammalwatching mysteries… does the moon phase affect mammalwatching? Answer – it depends.

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In ancient times, many scholars believed that the moon exerted a powerful influence over people’s minds and bodies. Even the word “lunacy,” historically used to describe mental illness and epilepsy, derives from the Latin lunaticus, which means “moonstruck.” Though modern science has quite thoroughly debunked this theory with regard to humans, the lunar cycle does affect the activity of many other animals. Some coral species, for example, synchronize their spawning with different moon phases, while tiny crustaceans known as ostracods light up Caribbean waters with their dazzling bioluminescent courtship displays—but only on moonless nights.

While this so-called “lunar effect” is well-documented in marine animals, scientists know comparatively little about how moon phases impact the behavior of mammals in terrestrial environments. To find out more, the authors of a new bioRxiv preprint set their sights on forest floors. During the hours between sunset and dawn, these areas become shrouded in near-total darkness. But on clear nights, beams of moonlight can filter through the thick tree canopy and brighten even the understory—one of the darkest places inhabited by land-dwelling mammals.

The international team of researchers behind the study combed through camera trapping data from 17 protected tropical forests on three separate continents, ultimately analyzing photographic evidence of 88 different mammal species. Although moonlight had no obvious effect on most of the animals photographed, the researchers found that roughly 16 percent of species appeared to avoid the light of the moon—a behavior the study authors dub “lunar phobia.” Another three percent, meanwhile, demonstrated “lunar philia,” actively seeking out moonlit patches of forest floor.

Nocturnal mammals, which usually have excellent night vision and are accustomed to hunting—and hiding—in the shadows, were more likely to steer clear of moonlight and showed increased activity during the new moon as opposed to brighter lunar phases. This group included armadillos, opossums, and a large number of rodent species.

Extra illumination proved to be a boon, however, for the pig-like white-lipped peccary, the four-toed elephant shrew, and a species of cottontail rabbit known as the common tapeti. All three of these species are either diurnal or crepuscular, meaning they are active during the day or during the twilight hours of dusk and early morning. The improved visibility afforded by moonlight, the researchers report, likely allowed them to more freely travel and forage at night.

As human activities continue to degrade and fragment forest ecosystems worldwide, the study authors note, decreased canopy cover may have an especially detrimental effect on some moonlight-sensitive mammals. Light pollution could also limit the amount of time that some species invest in hunting, foraging, and other activities that are necessary for their survival. Artificial light at night has already been shown to disrupt coral spawning and could have deleterious effects on other wildlife.

“The prevalence of lunar phobia in our study suggests there may be more losers than winners when illumination increases in tropical forests,” the researchers write.

Read the full paper here.

Jon

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Jon Hall
Genetically Welsh, spiritually Australian, currently in New York City. I’ve also lived and worked in London, Canberra, Paris and Lusaka, and watched mammals in over 100 countries.
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3 Comments

  • 12 March 2024
    KyleFinn

    Thanks for sharing Jon. Interesting article. When I was trapping small mammals in Namibia (rodents, shrews, and elephant shrews) I saw decreased activity in the traps on full moon and new moon nights. Then it increased during other phases. I haven’t gotten around to publishing it… it was 15 years ago now

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    • 12 March 2024
      Jon Hall

      Thanks Kyle. I bet there is a mountain of unpublished trapping data that could be worked on to look at this in more detail. We need some AI help

    • 12 March 2024
      vnsankar

      Interesting. Mistnetting on full moon nights is consistently worse for me (and others who have done a lot more of it than I have) – I’m not sure if it’s just that bats can see the net more easily under a brighter moon, or whether they’re actually less active. But it would be interesting to compile all the data and get at these questions.

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