Biological Rhythms and Animal Migration

A biological rhythm is a regular, repeating change in the physiology or behavior of an organism, and the main types are circadian (about 24 hours), circannual (about a year) and tidal or lunar rhythms; migration is the regular, long-distance movement of animals between breeding and non-breeding areas, often timed by these rhythms. In brief: rhythms are driven by internal biological clocks that continue in constant conditions, but are reset (entrained) by environmental cues such as day length. Migrating animals use the sun, stars, Earth’s magnetic field, landmarks and smell to navigate.

Bears hibernate in winter, bats sleep by day, and bar-tailed godwits fly from Alaska to New Zealand. This guide defines biological rhythms and the clock that drives them, explains how animals are timed and how they migrate and navigate, compares the types of rhythm in a table, works through a free-running rhythm and a migration calculation and corrects common mistakes.

What are biological rhythms?

Biological rhythms are regular cycles of activity. They let organisms anticipate predictable changes in the environment, rather than only respond to them. Examples include sleeping and waking, hormone levels, body temperature, the opening of flowers, the feeding of intertidal animals at high tide and the breeding of coral at a particular phase of the moon.

How does the biological clock work?

Rhythms are generated by internal clocks. If an animal is placed in constant conditions (constant darkness and temperature), the rhythm continues, but with a period that is slightly different from 24 hours. This is called a free-running rhythm, and it proves that the clock is internal. In humans the free-running period is about 24.2 hours on average.

• Zeitgebers (“time givers”) are external cues, mainly light, that reset the clock every day so that it matches the environmental cycle. This is called entrainment.
• In mammals the main clock is the suprachiasmatic nucleus (SCN) in the hypothalamus, which receives information about light from the eyes. The pineal gland releases melatonin at night under its control.
• At the cellular level, clock genes such as period and timeless in fruit flies make proteins that build up and inhibit their own production in a cycle of about 24 hours. Jeffrey Hall, Michael Rosbash and Michael Young received the 2017 Nobel Prize in Physiology or Medicine for discovering this mechanism.
• Jet lag is the result of a mismatch between the internal clock and local time, and the clock adjusts by about an hour or so per day.

Types of biological rhythms compared

How do animals cope with seasons?

Day length (photoperiod) is a reliable signal of season, unlike temperature. Animals respond in several ways:

• Breeding season: increasing day length in spring stimulates the pituitary and gonads of birds, so that the young are raised when food is abundant. Sheep, which breed in autumn, respond to decreasing day length.
• Hibernation: a state of reduced metabolism and body temperature through the winter, used by ground squirrels, bats and hedgehogs. Bears have a lighter form of winter sleep, with a smaller drop in body temperature.
• Torpor: a short period of lowered metabolism, which may occur daily, as in hummingbirds at night.
• Estivation: inactivity in hot, dry periods, as in some snails and lungfish.
• Molt and color change: the snowshoe hare and ptarmigan turn white in winter.

What is migration?

Migration is the regular movement of a population between areas, usually in response to seasonal changes in food, temperature or breeding sites. It is different from dispersal, which is a one-way movement away from the birth site, and from irregular nomadic movements.

• Birds: the Arctic tern makes a round trip that is often cited at about 70,000 km a year between the Arctic and Antarctic. Bar-tailed godwits have been tracked flying about 11,000 km non-stop from Alaska to New Zealand.
• Insects: monarch butterflies in North America migrate up to about 4,000 km to wintering sites in Mexico, and the butterflies that return north are several generations later than those which left.
• Fish: salmon return from the sea to the stream where they hatched to spawn, and European eels travel from rivers to the Sargasso Sea.
• Mammals: wildebeest follow rains in the Serengeti, and gray whales migrate between Arctic feeding areas and the lagoons of Baja California.
• Reasons: food supply, avoiding harsh conditions, and finding safe breeding places. The costs include energy, risk of predators and storms, and loss of habitat on the route.

Migration has an inherited part. Blackcaps raised in the laboratory show restless activity (Zugunruhe) at the time of migration and orient in the direction typical for their population. Hybrids of birds from populations with different directions choose an intermediate direction (Berthold’s studies), which indicates that genes are involved.

How do animals navigate?

• Sun compass: many birds and insects use the position of the sun, with the internal clock to compensate for its movement. Gustav Kramer showed this in starlings in the 1950s.
• Star compass: indigo buntings learn the pattern of stars around the celestial pole when they are young (Stephen Emlen’s planetarium experiments).
• Magnetic compass: birds, sea turtles, salmon and other animals sense the direction and inclination of Earth’s magnetic field. In birds, a light-dependent mechanism in the eye involving cryptochrome proteins has been proposed, and magnetite is another candidate.
• Landmarks and smell: pigeons use landmarks near home. Salmon remember the smell of their home stream.
• Polarized light and sound: bees use polarized light patterns, and whales may follow sound or the sea floor.

Navigation can be of three kinds: orientation (heading in a particular direction), true navigation (finding a goal from an unfamiliar place, using a “map” and “compass”) and pilotage (following known landmarks).

Worked example: a free-running rhythm and a migration trip

Part A. A mouse is kept in constant darkness, and its wheel-running starts every 23.5 hours. Before the experiment, it began activity at 18:00 each day.

1. Daily shift: 24 − 23.5 = 0.5 hours earlier each day.
2. After 10 days: 10 × 0.5 = 5 hours earlier, so the activity starts at 18:00 − 5 h = 13:00 by the clock.
3. Interpretation: the activity continues without light cues, so the rhythm is internal. The period is shorter than 24 hours, which is why it drifts. In normal light-dark cycles, light resets the clock each day.

Part B. A small songbird flies at an average of 50 km/h for 8 hours each night. It must travel 4,000 km.

1. Distance per night: 50 km/h × 8 h = 400 km.
2. Nights of flight: 4,000 ÷ 400 = 10 nights.
3. Stopovers: the bird must also feed. Suppose it makes 2 stopovers of 5 days each: 10 nights of flight + 10 days of rest = 20 days in total. The numbers are only an illustration, because real birds vary their stopovers with food and weather.

How were rhythms and migration studied?

In 1729 Jean-Jacques d’Ortous de Mairan noticed that a mimosa plant continues to open and close its leaves in constant darkness. Colin Pittendrigh and Jürgen Aschoff established the principles of circadian rhythms in the 1950s and 1960s. Ronald Konopka and Seymour Benzer identified the first clock gene (period) in fruit flies in 1971. Bird ringing (banding) began in 1899 with Hans Mortensen, and modern tracking uses geolocators, GPS tags and radar. The mechanisms of magnetic sensing are still under study.

Which mistakes are common?

Mistake 1: Thinking that rhythms are only responses to light

Light resets the clock, but the rhythm continues in the dark. It is internal.

Mistake 2: Saying that circadian means exactly 24 hours

“Circadian” means about a day. The free-running period is usually a little more or less than 24 hours.

Mistake 3: Confusing hibernation with sleep

In hibernation, the metabolism and body temperature fall much more than in sleep, and the state can last for weeks.

Mistake 4: Assuming that migrants learn the route from others

In many species, the direction and timing are inherited. Learning can improve the route, especially in long-lived social animals such as cranes and whales.