The moon has power over animals | Science News for Students

The moon has power over animals

Consider how critters move, grow and even sing differently when the night’s light peaks
Nov 7, 2019 — 7:41 am EST
grazing wildebeests

Moonlight affects animals in many different ways. Wildebeests (a herd shown) rely on moonlight to avoid nighttime attacks by hungry lions.

Karann Kalsey/iStock/Getty Images Plus

Science News for Students is celebrating the 50th anniversary of the moon landing, which passed in July, with a three-part series about Earth’s moon. In part one, Science News reporter Lisa Grossman visited rocks brought back from the moon. Part two explored what astronauts left on the moon. And check out our archives for this story about Neil Armstrong and his pioneering 1969 moonwalk.

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.

Serengeti mammals
Lions (top) hunt best during the darkest nights of the lunar month. Wildebeests (middle), avoid places where lions roam when it’s dark, camera traps show. African buffalo (bottom), another lion prey, may form herds to stay safe on moonlit nights.
M. Palmer, Snapshot Serengeti/Serengeti Lion Project

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.”

Nighttime navigators

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.

dung beetle
Some dung beetles (one shown) use moonlight as a compass. In this arena, researchers tested how well the insects could navigate under different night sky conditions.
Chris Collingridge

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 ears
Moonlight boosts the growth of some young fish, like the common triplefin (an adult shown, bottom). Scientists discovered this by studying the fish’s otoliths — inner ear structures that have tree-ring-like growth. A cross section, about a hundredth of an inch wide, is shown under a light microscope (top).
Daniel McNaughtan; Becky Focht

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.

Arctic copepods
The moon is the main source of light for life in the Arctic during winter. Zooplankton such as these copepods time their daily up and down trips in the ocean to the lunar schedule.
Geir Johnsen/NTNU and UNIS

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.

Moonlight sonata

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.)

sparrow weavers
Male white-browed sparrow weavers (left) sing at dawn. Behavioral ecologist Jenny York learned that these solos begin earlier and last longer when there’s a full moon. York (right) is shown here trying to catch a sparrow weaver from a roost in South Africa.
FROM LEFT: J. YORK; DOMINIC CRAM

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.

Power Words

(more about Power Words)

archive     (adj. archival) To collect and store materials, including sounds, videos, data and objects, so that they can be found and used when they are needed. The term is also for the process of collecting and storing such things. People who perform this task are known as archivists.

Arctic     A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice. The high Arctic is that most northerly third of this region. It’s a region dominated by snow cover much of the year.

atmosphere     The envelope of gases surrounding Earth or another planet.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

beetle     An order of insects known as Coleoptera, containing at least 350,000 different species. Adults tend to have hard and/or horn-like “forewings” which covers the wings used for flight.

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

behavioral ecologist     A scientist who studies how animal behavior relates to where animals live.

biological clock     A mechanism present in all life forms that controls when various functions such as metabolic signals, sleep cycles or photosynthesis should occur.

biology     The study of living things. The scientists who study them are known as biologists.

calcium carbonate     The main chemical compound in limestone, a rock made from the tiny shells of ancient marine organisms. Its formula is CaCO3 (meaning it contains one calcium atom, one carbon atom and three oxygen atoms). It’s also the active ingredient in some antacid medicines (ones used to neutralize stomach acids).

citizen science     Scientific research in which the public — people of all ages and abilities — participate. The data that these citizen “scientists” collect helps to advance research. Letting the public participate means that scientists can get data from many more people and places than would be available if they were working alone.

colleague     Someone who works with another; a co-worker or team member.

compass     An instrument that uses magnetized substances to show the direction of magnetic north.

copepod     A type of small crustacean found in salt and fresh water. Some species of them are plankton, floating with the currents. Others spend time on the sea floor. These animals aren’t limited to oceans; copepods also are found in freshwater, from ponds to puddles. They often serve as food for larger species, and most eat phytoplankton — single-celled organisms that get their energy from the sun.

coral     Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).

daunting     A description for something that is intimidating and which seems hard to imagine in detail such that you can figure out how to cope with, master or achieve it.

defense     (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)

dung beetle     A type of insect found on all continents except for Antarctica. They feed on a soupy liquid that they extract from animal feces (or dung). Able to fly long distances, these beetles use specialized antennae to sniff out the dung and home in on this food source, 

ecology      A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.

fertilize     (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism. (in agriculture and horticulture) To provide basic chemical nutrients for growth.

field     An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory. ty (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

focus     The point at which rays (of light or heat for example) converge sometimes with the aid of a lens. (In vision, verb, "to focus") The action a person's eyes take to adapt to light and distance, enabling them to see objects clearly.

forage     To search for something, especially food. It’s also a term for the food eaten by grazing animals, such as cattle and horses.

fundamental     Something that is basic or serves as the foundation for another thing or idea.

Greenland     The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Ice covers roughly 80 percent of Greenland. Indeed, the Greenland ice sheet is the world’s largest. Although this is the 12th biggest nation (based on surface area), Greenland averages the fewest people per square kilometer of its surface area.

grunion     Small silvery fish found off of the coast of southern California. They are renowned for flinging themselves onto beaches when the moon is full and the tide is high to spawn in wet sand.

invertebrate     An animal lacking a backbone. About 90 percent of animal species are invertebrates.

krill     Tiny shrimplike crustaceans that live in the ocean and are the main food source of some whales.

larvae     (sing. larva) The young of an insect, which often has a distinctly different form as an adult. (Sometimes this term is used to describe such a stage in the development of fish, frogs and other animals.)

life cycle     The succession of stages that occur as an organism grows, develops, reproduces — and then eventually ages and dies.

light pollution     The intrusion of unwanted light into areas that would naturally remain dark. Light pollution interferes with our ability to view the night sky. It also alters the circadian rhythms of plants, animals and people.

lunar     Of or relating to Earth’s moon.

marine biologist     A scientist who studies creatures that live in ocean water, from bacteria and shellfish to kelp and whales.

migration     (v. migrate) Movement from one region or habitat to another, especially regularly (and according to the seasons) or to cope with some driving force (such as climate or war). An individual that makes this move is known as a migrant.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2); water is made of two hydrogen atoms and one oxygen atom (H2O).

moon     The natural satellite of any planet.

navigate     To find one’s way through a landscape using visual cues, sensory information (like scents), magnetic information (like an internal compass) or other techniques.

new moon     The phase of the moon that appears fully dark, when viewed from Earth. At this time, the moon will sit between the earth and sun. So the lunar face lit by the sun is turned away from us. 

New Zealand     An island nation in the southwest Pacific Ocean, roughly 1,500 kilometers (some 900 miles) east of Australia. Its “mainland” — consisting of a North and South Island — is quite volcanically active. In addition, the country includes many far smaller offshore islands.

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

physiology     The branch of biology that deals with the everyday functions of living organisms and how their parts function. Scientists who work in this field are known as physiologists.

plankton     A small organism that drifts or floats in the sea. Depending on the species, plankton range from microscopic sizes to organisms about the size of a flea. Some are tiny animals. Others are plantlike organisms. Although individual plankton are very small, they form massive colonies, numbering in the billions. The largest animal in the world, the blue whale, lives on plankton.

polarization     (in physics) The condition — or creation of a condition — in which rays of wavelengths of light exhibit different properties when viewed from different directions.

predator     (adjective: predatory) A creature that preys on other animals for most or all of its food.

prey     (n.) Animal species eaten by others. (v.) To attack and eat another species.

proportion     The amount of a certain component of a mixture relative to other components. For example, if a bag contains 2 apples and 3 oranges, the proportion of apples to oranges in the bag is 2 to 3.

range     The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. Also, the distance within which something can be reached or perceived.

reef     A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

sensor     A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly. (in biology) The structure that an organism uses to sense attributes of its environment, such as heat, winds, chemicals, moisture, trauma or an attack by predators.

sound wave     A wave that transmits sound. Sound waves have alternating swaths of high and low pressure.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

sperm     The reproductive cell produced by a male animal (or, in plants, produced by male organs). When one joins with an egg, the sperm cell initiates fertilization. This is the first step in creating a new organism.

swarm     A large number of animals that have amassed and now move together. People sometimes use the term to refer to huge numbers of honeybees leaving a hive.

Tanzania     A nation in East Africa that sits just south of the equator. It’s well known in science communities as home to the East African Rift valley, where many hominid fossils have turned up. With a small population, many big wildlife species remain, including lions, rhinos, elephants, zebras, hippos, cheetahs and giraffes. The country is bordered by the Indian Ocean to the east; Kenya and Uganda to the north; Ruwanda, Burundi and Democratic Republic of Congo to the west; and Zambia, Malawi and Mozambique to the south. It became a country in 1964. It’s official capital is Dodoma.

threshold     A lower limit; or the lowest level at which something occurs.

tides     (adj. tidal ) The alternate rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attraction of the moon and sun.

vibrate     To rhythmically shake or to move continuously and rapidly back and forth.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

weather     Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.

wildebeest     A type of antelope found in Africa. These animals are about 1.5 meters (5 feet) tall, travel in herds, eat grass and are commonly found in Serengeti National Park in Tanzania, as well as on the Masai Mara Game Reserve in Kenya and Liuwa Plain National Park in Zambia.

zooplankton     Small organisms that drift in the sea. Zooplankton are tiny animals or animal-like organisms (such as protozoa) that eat other plankton. They also serve as an important food source for other marine creatures.

Citation

Journal:​ ​​ J.J. Foster et al. Orienting to polarized light at night — matching lunar skylight to performance in a nocturnal beetleJournal of Experimental Biology. Published online January 28, 2019. doi:10.1242/jeb.188532.

Journal:​ J.S. Shima and S.E. Swearer. Moonlight enhances growth in larval fishEcology. Vol. 100, January 2019, p. e02563. doi:10.1002/ecy.2563.

Journal:​ J.S. Shima et al. Born at the right time? A conceptual framework linking reproduction, development, and settlement in reef fishEcology. Vol. 99, January 2018, p. 116. doi:10.1002/ecy.2048.

Journal:​ M.S. Palmer et al. A ‘dynamic’ landscape of fear: prey responses to spatiotemporal variations in predation risk across the lunar cycleEcology Letters. Vol. 20, November 2017, p. 1364. doi: 10.1111/ele.12832.

Journal:​ K.S. Last et al. Moonlight drives ocean-scale mass vertical migration of zooplankton during the Arctic winterCurrent Biology. Vol. 26, January 25, 2016, p. 244. doi:10.1016/j.cub.2015.11.038.

Journal:​ V.V. Vyazovskiy and R.G. Foster. Sleep: A biological stimulus from our nearest celestial neighbor? Current Biology. Vol. 24, June 16, 2014, p. R557. doi:10.1016/j.cub.2014.05.027.

Journal:​ J.E. York, A.J. Young and A.N. Radford. Singing in the moonlight: dawn song performance of a diurnal bird varies with lunar phaseBiology Letters. Vol. 10, January 31, 2014. doi:10.1098/rsbl.2013.0970.

Further Reading