The Moon Has Power Over Animals
Twice a month from March through August, or so, crowds of people gather on Southern California beaches for a regular evening spectacle. As onlookers watch, thousands of silvery sardine look-alikes lunge as far onto shore as possible. Before long, these small writhing, grunion carpet the beach.
The females dig their tails into the sand, then release their eggs. Males wrap around these females to release sperm that will fertilize these eggs.
This mating ritual is timed by the tides. So are the hatchings, some 10 days later. The emergence of larvae from those eggs, every two weeks, coincides with the peak high tide. That tide will wash the baby grunion out to sea.
Choreographing the grunion’s mating dance and mass hatchfest is the moon.
Many people know that the moon’s gravitational tug on the Earth drives the tides. Those tides also exert their own power over the life cycles of many coastal creatures. Less well-known, 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 night landscape. 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 navigate outdoors without a flashlight and not being able to see the hand in front of your face.
Throughout the animal world, the presence or absence of moonlight, and the predictable changes in its brightness across the lunar cycle, can shape a range of important activities. Among them are reproduction, foraging and communication. “Light is possibly — maybe just after the availability of… food — the most important environmental driver of changes in behavior and physiology,” says Davide Dominoni. He’s an ecologist at the University of Glasgow in Scotland.
Researchers have been cataloging moonlight’s effects on animals for decades. And this work continues to turn up new connections. Several recently discovered examples reveal how moonlight influences the behavior of lion prey, the navigation of dung beetles, the growth of fish — even birdsong.
Beware the new moon
Lions of the Serengeti in the East African nation of Tanzania are night stalkers. They’re most successful at ambushing animals (including humans) during the darker phases of the moon’s cycle. But how those prey respond to changing predator threats as the night’s light changes throughout a month has been a dark mystery.
Meredith Palmer is an ecologist at Princeton University in New Jersey. She and colleagues spied on four of the lions’ favorite prey species for several years. The scientists installed 225 cameras across an area almost as big as Los Angeles, Calif. When animals came by, they tripped a sensor. The cameras responded by snapping their pictures. Volunteers with a citizen science project called Snapshot Serengeti then analyzed thousands of images.
The prey — wildebeests, zebras, gazelles and buffalo — are all plant eaters. To meet their food needs, such species must forage frequently, even at night. The candid snapshots revealed that these species respond to changing risks across the lunar cycle in different ways.
Common wildebeest, which make up a third of the lion diet, were the most attuned to the lunar cycle. These 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 the nights got brighter, she notes, wildebeests were more willing to venture into places where run-ins with lions were likely.
Weighing as much as 900 kilograms (almost 2,000 pounds), the African buffalo are a lion’s most daunting prey. They also were least likely to alter where and when they foraged throughout the lunar cycle. “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 this way might offer safety in numbers.
Plains zebras and Thomson’s gazelles also changed their evening routines with the lunar cycle. But unlike the other prey, these animals reacted more directly to changing light levels across an evening. Gazelles were more active after the moon had come up. Zebras “were sometimes up and about and doing things before the moon had risen,” Palmer says. That may seem like risky behavior. She notes, however, that being unpredictable might be a zebra’s defense: Just keep those lions guessing.
Palmer’s team reported its findings two years ago in Ecology Letters.
These behaviors in the Serengeti really demonstrate the wide-reaching effects of moonlight, Dominoni says. “It’s a beautiful story,” he says. It offers “a very clear example of how the presence or absence of the moon can have fundamental, ecosystem-level impacts.”
Some dung beetles are active at night. They depend on moonlight as a compass. And how well they navigate depends on the phases of the moon.
In South African grasslands, a dung pat is like an oasis for these insects. It offers scarce nutrients and water. No wonder these droppings draw a crowd of dung beetles. One species that comes out at night to grab and go is Escarabaeus satyrus. These beetles sculpt dung into a ball that’s often bigger than the beetles themselves. Then they roll the ball away from their hungry neighbors. At this point, they will bury their ball — and themself — in the ground.
For these insects, the most efficient getaway is a straight line to a suitable burial spot, which can be many meters (yards) away, says James Foster. He’s 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. Some lunar light scatters off gas molecules in the atmosphere and becomes polarized. The term means that these light waves tend to now vibrate in the same plane. This process produces a pattern of polarized light in the sky. People can’t see it. But beetles may use this polarization to orient themselves. It might allow them to figure out where the moon is, even without seeing it directly.
In recent field tests, Foster and his colleagues evaluated the strength of that signal 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 visible moon begins to shrink in coming days, the night sky darkens. The polarized signal also weakens. By the time the visible moon resembles a crescent, beetles will have trouble staying on course. Polarized light during this lunar phase may be at the limit of what the dung harvesters can detect.
Foster’s team described its findings, last January, in the Journal of Experimental Biology.
At this threshold, light pollution could become a problem, Foster says. Artificial light can interfere with patterns of polarized moonlight. He is conducting experiments in Johannesburg, South Africa, to see if city lights affect how well dung beetles navigate.
Like a grow lamp
In the open ocean, moonlight helps baby fish grow.
Many reef fish spend their infancy at sea. That may be because deep waters make a safer nursery than does a predator-packed reef. But that’s just a guess. These larvae are too tiny to track, notes Jeff Shima, so scientists don’t know a lot about them. Shima is 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 baby fish.
The common triplefin is a small fish on New Zealand’s shallow rocky reefs. After about 52 days at sea, its larvae are finally big enough to go back to the reef. Fortunately for Shima, adults carry an archive of their youth within their inner ears.
Fish have what are known as ear stones, or otoliths (OH-toh-liths). They are made from calcium carbonate. Individuals grow a new layer if this mineral every day. In much the same way as tree rings, these ear stones record patterns of growth. Each layer’s width is a key to how much the fish grew on that day.
Shima worked with marine biologist Stephen Swearer of the University of Melbourne in Australia to match up otoliths from more than 300 triplefins with a calendar and weather data. This showed that larvae grow faster during bright, moonlit nights than on dark nights. Even when the moon is out, yet covered by clouds, larvae won’t grow as much as on clear moonlit nights.
And this lunar effect is not trivial. It’s about equal to the effect of water temperature, which is known to greatly affect larval growth. The advantage of a full moon relative to a new (or dark) moon is similar to that of a 1-degree Celsius (1.8-degrees Fahrenheit) increase in water temperature. The researchers shared that finding in the January Ecology.
These baby fish hunt plankton, tiny organisms that drift or float in the water. Shima suspects that bright nights enable larvae to better see and chow down on those plankton. 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 dining.
But when young fish are ready to become reef dwellers, moonlight might now pose a risk. In one study of young sixbar wrasses, more than half of these fish coming to coral reefs in French Polynesia arrived during the darkness of a new moon. Only 15 percent came during a full moon. Shima and his colleagues described their findings last year in Ecology.
Because many predators in coral reefs hunt by sight, darkness may give these young fish the best chance of settling into a reef undetected. In fact, Shima has shown that some of these wrasses appear to stay at sea several days longer than normal to avoid a homecoming during the full moon.
Bad moon rising
Moonlight may flip the switch in the daily migration of some of the ocean’s tiniest creatures.
Some plankton — known as zooplankton — are animals or animal-like organisms. 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. Many scientists had assumed that, in the heart of the sunless winter, zooplankton would take a break from such daily up-and-down migrations.
“People generally had thought that there was nothing really going on at that time of year,” says Kim Last. He’s 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 those migrations. That’s what Last and his colleagues suggested three years ago in Current Biology.
These winter migrations take place all across the Arctic. Oban’s group found them by analyzing data from sound sensors stationed off Canada, Greenland and Norway, and near the North Pole. The instruments recorded echoes as sound waves bounced off swarms of zooplankton as these critters moved up and down in the sea.
Normally, those migrations by krill, copepods and other zooplankton follow a roughly circadian (Sur-KAY-dee-un) — or 24-hour — cycle. The animals descend many centimeters (inches) to tens of meters (yards) into the ocean around dawn. Then they rise back toward the surface at night to graze on plantlike plankton. But winter trips follow a slightly longer schedule of about 24.8 hours. That timing coincides exactly with the length of a lunar day, the time it takes for the moon to rise, set and then begin to rise again. And for about six days around a full moon, the zooplankton hide especially deeply, down to 50 meters (some 165 feet) or so.
Zooplankton seem to have an internal biological 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 lab tests, he notes, show that krill and copepods have very sensitive visual systems. They can detect very low levels of light.
The light of the moon even influences animals that are active by day. That’s what behavioral ecologist Jenny York learned while studying small birds in South Africa’s Kalahari Desert.
These white-browed sparrow weavers live in family groups. Year-round, they sing as a chorus to defend their territory. But during the breeding season, males also perform dawn solos. These early morning songs are what brought York to the Kalahari. (She now works in England at the University of Cambridge.)
York awoke at 3 or 4 a.m. to arrive at her field site before a performance began. But on one bright, moonlit morning, males were already singing. “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 that’s when she realized the birds’ early start time was not a one-day accident. She discovered over a seven-month period that when a full moon was visible in the sky, males started singing an average of about 10 minutes earlier than when there was a new moon. York’s team reported its findings five years ago in Biology Letters.
This extra light, the scientists concluded, kick-starts the singing. After all, on days when the full moon was already below the horizon at dawn, the males started crooning on their normal schedule. Some North American songbirds seem to have the same reaction to the moon’s light.
The earlier start time lengthens the males’ average singing period by 67 percent. Some devote just a few minutes to dawn singing; others go on for 40 minutes to an hour. Whether there’s a benefit to singing earlier or longer is unknown. 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.
The feature is written by Erin Wayman