Do Small Rodents in the Desert Really Avoid Moonlight?

The kangaroo rat, from the genus Dipodomys (photo by  E. Reddington)

The kangaroo rat, from the genus Dipodomys (photo by E. Reddington)

We like to believe ideas that make sense, even when we don’t have any scientific reason to prove that they’re actually true. My high school physics teacher once asked our class what would happen if he dropped a heavy rubber ball and a wadded up piece of paper from the same height. Would they hit the ground at the same time, or would one of them fall faster? We all said the ball would hit the ground first because it was heavier, which was wrong of course, because gravity acts the same way on all objects no matter how much they weigh. But none of us believed it until he dropped them and we saw both hit the tile at the same time.

Scientific research often involves debunking wrong assumptions like this. In the study of small mammals, it has been commonly believed that rodents in desert habitats avoid moving around at night during bright moonlight, to avoid being spotted by visual predators like owls. This “moonlight avoidance hypothesis” grew out of laboratory observations beginning in the 1920s, but few large-scale studies had been done in the wild.

Nate Upham, a PhD candidate in the University of Chicago Committee on Evolutionary Biology, and John Hafner from Occidental College in Los Angeles, wanted to find out if small mammals really do avoid moonlight in the real world, so they designed a study to observe the nocturnal movements of all rodent species at a number of sites across the Great Basin Desert in the western United States. The results, recently published in the Journal of Mammalogy, not only call into question the moonlight avoidance hypothesis, but also show that competition is likely more important for the activity of some animals than the presence or absence of moonlight.

“Part of what the study is doing is questioning this lore of moonlight avoidance on the part of people that study small mammals,” Upham said. “That idea is something that’s kind of sexy, and it makes logical sense when you think that since the predator can see them better, then the animals should be less active. But there hasn’t been a study that has really looked at a large geographic area and natural populations.”

Live traps set in the Great Basin desert (photo by N. Upham)

Live traps set in the Great Basin desert (photo by N. Upham)

Over seven years, Upham and his colleagues set live traps at 62 different sites across Nevada, California, Utah and Oregon to track the nocturnal movements of animals.  When they first analyzed the data, they didn’t find evidence that any animals were less active during moonlight. And when they broke it down by species, they found that just one genus, Dipodomys, which includes five different species of kangaroo rats in the Great Basin, avoided moonlight altogether.

Upham said there are two possible explanations for why only this one type of animal holds true to the moonlight avoidance hypothesis. Kangaroo rats are bigger than the other small rodents in the desert. They hop around on two legs instead of scampering around on four, and they often venture into open spaces to feed, so their movements are more obvious to an owl scanning the ground for a snack. Upham called this the “predation risk explanation,” which is the classic explanation for avoiding moonlight. The kangaroo rats don’t want to be caught out in the open and snatched away.

But since they’re bigger and can move around more quickly, they may just be more efficient at gathering food. They don’t need to move around in the moonlight, so they wait until the moon sets. Smaller animals that compete with the bigger kangaroo rats for resources have no such luxury, and are forced to put themselves at risk and be more active during periods of moonlight. Upham said this “luxury explanation,” like the one based on greater predation risk, will need to be evaluated with additional data and observations.

Nate Upham (left) with John Hafner (photo by P. Hafner)

Nate Upham (left) with John Hafner (photo by P. Hafner)

Upham said that the study highlights the fact that perceptions of risk are likely species-specific and subject to selection and evolution like any other complex behavioral trait. Similar animals in the same location can act quite differently because of something as simple as whether they happen to get around on two feet or four.

“The big thing with animals is that they’re trying to balance predation risk on one side with the demand to get food and mates on the other side,” Upham said. “Obviously they can’t be inactive all the time. They have to pick their spots, and so the thinking is that these animals use environmental cues like moonlight or temperature or a predatory urine scent to figure out when it’s risky and when it’s not.”

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Upham, N., & Hafner, J. (2013). Do nocturnal rodents in the Great Basin Desert avoid moonlight? Journal of Mammalogy, 94 (1), 59-72 DOI: 10.1644/12-MAMM-A-076.1

About Matt Wood (503 Articles)
Matt Wood is a senior science writer at the University of Chicago Medicine and nonfiction editor for Another Chicago Magazine.
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