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 moons 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.
Special Report: Moonstruck[hhmc]
50 years after Apollo 11, lunar science still surprises and delights
This story is part of a special report celebrating humans enduring fascination with the moon and exploring the many ways it affects life on Earth. More articles will be published in the coming weeks. See all the articles, plus our 1969 coverage of Apollo 11, here.
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.
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 animals 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 moonlights 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[hhmc]
Lions of the Serengeti in Tanzania are night stalkers. Theyre 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.
Candid camera[hhmc]
Lions hunt (top) most successfully during the darkest nights of the lunar month. The big cats favorite meal, wildebeests (middle), avoid places where lions congregate when its dark, camera traps show. The larger African buffalo (bottom), another lion prey, tend to form herds, maybe for safety in numbers.
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, Palmers 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 moons phase. During the darkest parts of the month, Palmer says, “theyd 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 Thomsons 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. “Its a beautiful story, a very clear example, of how the presence or absence of the moon can have fundamental, ecosystem-level impacts.”
Nighttime navigators[hhmc]
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 thats 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 cant 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 thats 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[hhmc]
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 thats just a guess, because these larvae are too tiny to track, so scientists dont know a lot about them, says Jeff Shima, a marine ecologist at Victoria University of Wellington in New Zealand. Hes recently figured out a way to observe the moons influence on these fish.
Growth spurt[hhmc]
Researchers determined that moonlight enhances the growth of young common triplefin fish (an adult shown, bottom) by studying the tree ring–like growth of an inner ear structure called an otolith (a roughly 0.5-millimeter-wide cross section is shown under a light microscope).
Larvae of the common triplefin (Forsterygion lapillum) — a small fish that inhabits New Zealands 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 layers 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 dont grow as much.
The moons effect isnt trivial. Its 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 childs reassuring night-light, the moons 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[hhmc]
Moonlight may flip the switch in the daily migration of some of the oceans 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.
Lasts 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.
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