Moonlight and Mortality


Collared brown lemur (Eulemur fuvius collaris). Credit: Luke Dollar

Say you’re a small mammal that enjoys being out and about after dark–such as a kangaroo rat in the sandy grassland of California, or a brown lemur in the forests of Madagascar. What should you fear most: A bright full moon? Or a dark, moonless sky?

“Ecologists have long believed that moonlight increases predation risk for small prey species,” says Laura Prugh, a wildlife biologist at the University of Alaska Fairbanks. In the dark, it’s harder for lurking predators to spot them and turn them into midnight snacks.

But things may not be that simple. In a recent study published in the Journal of Animal Ecology, Prugh and colleague Christopher Golden of Harvard University, compiled the effects of moonlight for 58 nocturnal mammal species. “Contrary to common belief, the results were very variable,” Prugh says.

For instance, position in the food chain played a minor role in their affinity for moonlight. What seemed to matter most was whether a species used vision as their primary sensory system. Prey species that mainly relied on vision–such as primates–were generally more active in the moonlight, whereas prey species that use smell or echolocation–such as rodents or bats–tended to be less active.

One study found that people in Africa were a lot more likely to get killed by lions on nights following the full moon. Credit: Wikicommons/^demon

Moonlight makes it easier for species that rely on vision to detect predators, and it also makes it easier for them to find food. “It’s about the net effect,” Prugh says, “and the gains may outweigh any increases in the risk of predation.”

What’s more, moonlight may hinder those predators that rely on stealth and ambush to capture their prey. Carnivorous predators like European badgers and African lions were less active on moonlit nights. On the other hand, nocturnal avian predators– such as owls–may be more active since they may rely more on speed. “Our results suggest that moonlight alters predator-prey relations in more complex ways than previously thought,” said Prugh in a release.

But the effects of moonlight aren’t limited to small preys like lemurs or kangaroo rats. A study by Craig Packer from the University of Minnesota two years ago had shown that the frequency of attack by lions on humans were also tied into the moon cycles. The study had compiled nearly 500 lion attacks on Tanzanian villagers between 1988 and 2009, where more than two-thirds of the victims were eaten. In particular, people were a lot more likely to get killed by lions on nights following the full moon—nights when there are a few hours of darkness between sunset and moonrise.

Earth’s moon has had an influence on the evolution of species. Image Credit: Gordon

According to Packer these dangers could explain the role of the full moon in human folklore. As he wrote, “The full moon accurately indicates that the risks of lion predation will increase dramatically in the coming days. [It] is not dangerous in itself but is instead a portent of the darkness to come.”

The moon cycles may have also played an important role in evolution. “Nearly half of all mammal species are nocturnal, and it’s widely thought that the ancestor of all mammals was nocturnal,” says Prugh. She thinks moonlight may have selected for good vision in nocturnal species, and that adaptations to nighttime activity may have been a driving force in the evolution of mammals.

In other words, nocturnal life may be very different in a world with no Moon, relying less on vision and more on other senses like smell or hearing, or perhaps even developing other senses that are completely alien to us.

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