Moonlight shapes how some animals move, grow and even sing

Crowds of people gather to watch an evening spectacle on beaches in Southern California: Twice a month, typically from March through August, the sand becomes carpeted with hundreds or thousands of California grunion. Writhing, flopping, silvery sardine look-alikes lunge as far onto shore as possible. As the female fish dig their tails into the sand and release eggs, males wrap around females and release sperm to fertilize those eggs. About 10 days later, the eggs hatch and the little grunion get washed out to sea.

This mating ritual is set to the tides, with hatching timed to the arrival of the peak high tide every two weeks. But the ultimate force choreographing this dance is the moon.

Many people know that the moon’s gravitational tug on the Earth drives the tides, and with them, the life cycles of coastal creatures. Yet the moon also influences life with its light.

For people living in cities ablaze with artificial lights, it can be hard to imagine how dramatically moonlight can change the nocturnal landscape. Out in the wild, far from any artificial light, the difference between a full moon and a new moon (when the moon appears invisible to us) can be the difference between being able to walk outside without a flashlight and not being able to see the hand in front of your face.

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And animals respond. The presence or absence of moonlight, along with the predictable changes in brightness across the lunar cycle, can shape reproduction, foraging, communication and other aspects of an animal’s world. “Light is possibly, maybe just after the availability of resources in terms of food, the most important environmental driver of changes in behavior and physiology,” says ecologist Davide Dominoni of the University of Glasgow in Scotland.

Researchers have been cataloging moonlight’s effects on animals for decades and continue to mark new connections. Several recently discovered examples reveal how lunar light influences lion prey behavior, dung beetle navigation, fish growth, mass migrations and even birdsong.

Beware the new moon

Lions of the Serengeti in Tanzania are night stalkers. They’re most successful at ambushing animals (including humans) during the darker phases of the lunar cycle. But how the cats’ prey respond to changing predator threats as the moon waxes and wanes has been a dark mystery.

Meredith Palmer, an ecologist at Princeton University, and colleagues spied on four of the lions’ favorite prey species for several years with 225 camera traps installed across an area almost as big as Los Angeles. Volunteers with the citizen science project Snapshot Serengeti analyzed thousands of images of these animals.

The prey — wildebeests, zebras, gazelles and buffalo — are all plant eaters that need to frequently forage to meet their food needs, even throughout the riskier nighttime. The candid snapshots revealed that these species respond to changing risks across the lunar cycle in different ways, Palmer’s team reported in Ecology Letters in 2017.

Common wildebeests (Connochaetes taurinus), which make up a third of the lion diet, were the most attuned to the lunar cycle. The animals appeared to set their plans for the entire night based on the moon’s phase. During the darkest parts of the month, Palmer says, “they’d park themselves in a safe area.” But as nights got brighter, wildebeests were more willing to venture into dangerous places where run-ins with lions were likely.

Weighing as much as 900 kilograms, African buffalo (Syncerus caffer) are lions’ most formidable prey and were least responsive to changing predation risks. “They just sort of went where the food was,” Palmer says. But as nights got darker, the buffalo were more likely to form herds. Grazing in groups might offer safety in numbers.

The routines of plains zebras (Equus quagga) and Thomson’s gazelles (Eudorcas thomsonii) also changed with the lunar cycle. But unlike the other prey, these animals reacted more directly to changing light levels across the evening, Palmer says. Gazelles were more active after the moon had come up. Zebras “were sometimes up and about and doing things before the moon had risen,” she says. That may seem like risky behavior, but being unpredictable could be a zebra defense strategy to keep lions guessing, she says.

These scenarios playing out in the Serengeti really demonstrate the wide-reaching effects of moonlight, Dominoni says. “It’s a beautiful story, a very clear example, of how the presence or absence of the moon can have fundamental, ecosystem-level impacts.”

Nighttime navigators

For nocturnal dung beetles, moonlight is a compass. How well the insects navigate depends on the phases of the moon.

In South African grasslands, a dung pat is like an oasis, providing scarce nutrients and water that draw a crowd of dung beetles. Escarabaeus satyrus beetles come out at night to grab and go, sculpting dung into a ball that’s often bigger than the beetle itself and rolling the ball away from other hungry beetles. The beetle then buries the ball and itself in the ground.

The most efficient getaway is a straight line to a suitable burial spot, often many meters away, says James Foster, a vision scientist at Lund University in Sweden. To avoid going in circles or landing back at the feeding frenzy, beetles look to polarized moonlight (SN: 7/5/03, p. 4). Some lunar light scatters off gas molecules in the atmosphere and becomes polarized — meaning the light waves tend to vibrate in the same plane. This scattering produces a pattern of polarized light in the sky that human eyes can’t see. But beetles may use this sky pattern to orient themselves, inferring where the moon is without even having to see the orb directly.

In recent field tests, Foster and colleagues evaluated the strength of the polarization signal in the night sky over dung beetle territory. The proportion of light in the night sky that’s polarized during a nearly full moon is similar to that of polarized sunlight during the day, which many daytime insects such as honeybees use to navigate. As the moon gets darker across the lunar cycle, the signal weakens. By the crescent moon, beetles have trouble staying on course, the researchers reported in January in the Journal of Experimental Biology. Polarized light during this lunar phase may be at the limit of what the dung harvesters can detect.

At this threshold, light pollution could become a problem, as artificial light interferes with patterns of polarized moonlight, Foster says. He is conducting experiments in Johannesburg to see if city lights affect dung beetle navigation. Although rural African grasslands may not yet be bathed in an artificial glow, dung beetles are probably not the only nocturnal invertebrates that use polarized moonlight to find their way, Foster says. “Even if [light pollution is] not a problem for this particular species, it could be a problem for many others.”

Like a grow lamp

In the open ocean, moonlight helps baby fish grow.

Many reef fish spend their infancy at sea — maybe because the deeper waters make for a safer nursery than the predator-packed reef. But that’s just a guess, because these larvae are too tiny to track, so scientists don’t know a lot about them, says Jeff Shima, a marine ecologist at Victoria University of Wellington in New Zealand. He’s recently figured out a way to observe the moon’s influence on these fish.

Larvae of the common triplefin (Forsterygion lapillum) — a small fish that inhabits New Zealand’s shallow rocky reefs — spend about 52 days at sea before getting big enough to go back to the reef. Fortunately for Shima, adults carry an archive of their youth within the inner ear. Calcium carbonate structures called otoliths, or ear stones, grow a new layer every day. So, much like tree rings, ear stones record patterns of growth, with a layer’s width indicating how much growth occurred that day.

By matching otoliths from more than 300 triplefins with a calendar and weather data, Shima and marine biologist Stephen Swearer of the University of Melbourne in Australia found that larvae grow faster during bright, moonlit nights than on dark nights. If the moon is out but covered by clouds, larvae don’t grow as much.

The moon’s effect isn’t trivial. It’s on par with the effect of water temperature, a known driver of larval growth: The advantage of a full moon relative to a new moon is similar to that of a 1-degree Celsius increase in water temperature, the researchers estimated in the January Ecology.

Shima suspects that bright nights enable larvae to better see and hunt plankton. And like a child’s reassuring night-light, the moon’s glow may allow larvae to “relax a bit,” he says. Likely predators, such as lantern fish, shy away from moonlight to avoid the bigger fish that hunt them by light. With nothing chasing them, larvae may be able to focus on foraging. 

But when young fish are ready to return to the reef, moonlight may become a hindrance. In a different study, more than half of over 1,000 young sixbar wrasses (Thalassoma hardwicke) observed arriving at coral reefs in French Polynesia over 11 months did so during the darkness of a new moon. Only 15 percent came during a full moon, Shima and colleagues reported in Ecology in 2018.

Because many predators in coral reefs hunt by sight, a cover of darkness may give young sixbar wrasses the best chance of settling into a reef undetected. In fact, Shima has shown that some of these fish appear to stay at sea several days longer than normal to avoid a homecoming during the full moon. Moonlight might similarly influence larvae of many kinds of reef fish and affect many aspects of the life cycle, Shima says.

Bad moon rising

Moonlight may flip the switch in the daily migration of some of the ocean’s tiniest creatures.

In the seasons when the sun rises and sets in the Arctic, zooplankton plunge into the depths each morning to avoid predators that hunt by sight. But many scientists had assumed that, in the heart of winter when the sun is absent, zooplankton take a break from the up and down.

“People generally had thought that there was nothing really going on at that time of year,” says Kim Last, a marine behavioral ecologist at the Scottish Association for Marine Science in Oban. But the light of the moon appears to take over and direct the migrations, Last and colleagues suggested in 2016 in Current Biology.

Last’s group discovered these winter migrations all across the Arctic by analyzing data from acoustic instruments stationed off Canada, Greenland and Norway, and near the North Pole. The instruments record the echoes of sound waves bouncing off swarms of zooplankton as the critters move up and down in the ocean.

Normally, migrations follow a 24-hour rhythm, with zooplankton, including krill and copepods, descending many centimeters to tens of meters into the ocean around dawn and moving back toward the surface at night to graze on phytoplankton. But winter trips follow a slightly longer 24.8-hour schedule (SN Online: 1/11/16). That timing coincides exactly with the length of a lunar day, the time it takes for the moon to rise, set and rise again. And for about six days around a full moon, the zooplankton hide especially deep, down to 50 meters or so.

Zooplankton seem to have an internal circadian clock that sets their sun-based, 24-hour migrations. Whether the swimmers also have a lunar-based biological clock that sets their winter journeys is unknown, Last says. But laboratory tests show that krill and copepods have sensitive visual systems that can detect very low levels of light, he says. 

Moonlight sonata

The light of the moon also influences animals that are active in daytime. That’s what behavioral ecologist Jenny York learned while studying white-browed sparrow weavers (Plocepasser mahali) in South Africa’s Kalahari Desert.

These brown and white sparrow-sized birds live in family groups. Year-round, family members sing as a chorus to defend territory. But during the breeding season, males also perform solos, waking up at dawn to sing their own tune. These dawn songs are what brought York, now at the University of Cambridge, to the Kalahari.

She awoke at 3 or 4 a.m. to get to her field site before a performance began. But on one bright, moonlit morning, males were already singing when she arrived. “I missed my data points for the day,” she recalls. “That was a bit annoying.”

So she wouldn’t miss out again, York got herself up and out earlier and found that the birds’ early start time was not an isolated incident. Over seven months, she discovered that when a full moon was visible in the sky, males started singing about 10 minutes earlier on average than when there was a new moon, York and colleagues reported in Biology Letters in 2014.

The extra light, rather than some other aspect of the lunar phase, kick-starts the singing, the team concluded, because on days when the full moon is already below the horizon at dawn, sparrow weavers start crooning on their normal schedule. Some North American songbirds seem to have the same moon reaction.

The earlier start time lengthens the males’ average song period by 67 percent, with a lot of variation. Some devote just a few minutes to dawn singing; others go on for 40 minutes to an hour. Whether there’s a benefit of singing earlier or longer is unclear. Although sparrow weavers mate in pairs, they aren’t always faithful. “Shenanigans” go on in the twilight hours, York says. Something about dawn songs may help females evaluate potential mates. A longer performance may very well help the females tell “the men from the boys,” as York puts it.