Biological rhythms, biological clocks and migration – Behaviour and Response in Mammals (Senior 5 Biology) | Ganzaa

Behaviour and Response in Mammals

Lessons

Senior 5 · Biology

Behaviour and Response in Mammals

4 lessons

Lesson 4 of 4

Biological rhythms, biological clocks and migration

25 min

Travellers that know the time

Every year, from about October, white storks that bred in Europe and western Asia appear across eastern and southern Africa, and in March they head north again. Each stork must know when to leave, which way to fly, and how to find its way over thousands of kilometres. Closer to home, your own body temperature, alertness and hormone levels rise and fall on a daily cycle, and your body "knows" the time even in a room with no clock. Both depend on biological clocks. This lesson looks at biological rhythms, the clocks that drive them, and one of their most impressive uses: migration.

Definition

Biological rhythm: a regular, repeated change in an organism's behaviour or physiology.

Circadian rhythm: a rhythm with a period of about 24 hours (Latin circa diem, "about a day"). Circannual rhythm: one with a period of about a year.

Biological clock: an internal timing mechanism that drives a rhythm, even without outside cues.

Zeitgeber (German, "time-giver"): an outside cue, such as light, that resets a biological clock each day.

Migration: the regular movement of animals between two areas, usually seasonal and returning.

Daily rhythms in humans

Many body functions follow a circadian rhythm: core temperature (Figure 6), sleep and wakefulness, the hormone melatonin, released by the pineal gland at night and making us sleepy, and cortisol, which peaks in the early morning. These rhythms let the body prepare in advance for the regular changes of day and night, instead of only reacting once they happen.

The master clock

In mammals the master clock is the suprachiasmatic nucleus (SCN), a tiny group of about 20 000 neurones in the hypothalamus, just above the point where the optic nerves cross. Special light-sensitive cells in the retina send it information about light, and it controls the release of melatonin by the pineal gland. Inside each clock cell, clock genes make proteins that build up during the day and then switch off their own genes; as the proteins break down, the genes switch on again. This negative feedback loop takes about 24 hours. The 2017 Nobel Prize in Physiology or Medicine was awarded to Jeffrey Hall, Michael Rosbash and Michael Young for discovering this mechanism, first in fruit flies.

Free-running rhythms

Volunteers who live for weeks in rooms with no daylight and no clocks keep a regular cycle of sleeping and waking, but its period is slightly longer than 24 hours in most people, so they drift a little later each day: this is a free-running rhythm (Figure 7). In normal life, morning light acts as a zeitgeber and resets the clock every day, keeping it in step with the 24-hour day. When we fly across several time zones, the clock is out of step with local daylight for several days: jet lag.

Worked example 1: reading an actogram

Use Figure 7. (a) By how much does the start of activity shift each day from day 5? (b) What is the free-running period? (c) How late will activity start on day 14 if the conditions continue?

(a) On day 4 activity starts at 07:00; on day 10 at 10:00. Shift $= \frac{3\,\mathrm{h}}{6\,\mathrm{days}} = 0.5\,\mathrm{h}$ per day, 30 minutes.
(b) Each cycle takes $24 + 0.5 = 24.5\,\mathrm{h}$ .
(c) Day 14 is 4 days after day 10: $10{:}00 + 4 \times 0.5\,\mathrm{h} = 12{:}00$ .

Meaning: the rhythm continues without light cues, so it must be driven by an internal clock; the light–dark cycle normally keeps it at exactly 24 hours.

Other rhythms

Tidal rhythms, of about 12.4 hours, in shore animals such as crabs that feed when the tide is out.
Lunar rhythms, of about 29.5 days: many corals and marine worms release their gametes on particular nights of the lunar cycle.
Circannual rhythms: breeding seasons, moulting, hibernation and migration. Far from the equator, the changing length of the day is the main cue. Near the equator, as in Rwanda, day length hardly changes through the year, so many animals instead time their breeding to the rainy seasons, when food is plentiful.

Migration

Animals migrate to reach food, to escape a harsh season, or to breed in a safer or richer place. The journeys are costly and dangerous, so migration evolves only where the benefits outweigh the costs. Before leaving, migrants store fat, and caged migratory birds become restless at the time of year when they would leave: a sign of a circannual clock.

How do migrants find their way?

Cue	Evidence and example
Sun compass	Birds and bees use the sun's position, corrected for the time of day by their internal clock. Pigeons whose clocks are shifted by keeping them on an altered light–dark schedule fly off in the wrong direction by a predictable angle.
Star compass	Night migrants, such as indigo buntings in a planetarium, orient to the star patterns around the celestial pole, which they learn while young.
Earth's magnetic field	Many birds, sea turtles and salmon can detect it; magnets attached to pigeons upset their homing on cloudy days.
Landmarks	Coastlines, rivers and mountain ranges, as storks follow the Nile valley.
Smell	Salmon return to breed in the very stream where they hatched, recognising its smell.

Worked example 2: a clock-shifted pigeon

The sun's direction moves across the sky by about $\frac{360^\circ}{24\,\mathrm{h}} = 15^\circ$ per hour on average. Pigeons are kept for a week with lights on and off 6 hours later than the real day, so their clocks run 6 hours behind. They are then released at noon. By about how many degrees will they misjudge the direction home?

The pigeon's clock reads 06:00 when it is really noon.
It interprets the sun's position as if it were 6 hours earlier: an error of $6 \times 15^\circ = 90^\circ$ .
So the pigeons fly off about $90^\circ$ away from the correct direction.

Conclusion: this kind of result, found in real clock-shift experiments, shows that birds use the sun as a compass and that they correct for its movement with an internal clock.

Warning

Common mistakes

"Rhythms are caused by the light–dark cycle." They are driven by an internal clock; light only resets it. The proof is that rhythms continue, free-running, in constant conditions.
"Circadian means exactly 24 hours." It means about a day; the free-running period is usually a little different from 24 hours.
"Migration is just wandering." Migration is regular and directional, and the animals usually return.

TED-Ed: "How does your body know what time it is?" by Marco A. Sotomayor. Watch for the master clock in the hypothalamus, the role of light, and the feedback loop of clock genes and proteins.

Tip

Memory hook: an internal clock is like a watch that runs slightly slow or fast; the zeitgeber (light) is like checking it against the radio each morning. Without the check (constant conditions), the watch drifts: a free-running rhythm.

✦

Practice: rhythms, clocks and migration

Define a circadian rhythm and give two examples in humans.
Where is the master clock in mammals, and how does it receive information about light?
What is a zeitgeber? Give an example.
In constant darkness, a hamster begins running 15 minutes earlier each day. What is its free-running period?
Explain why a traveller flying from Kigali to a city 7 time zones away suffers jet lag.
A bird's internal clock is shifted 4 hours behind. By about how many degrees will it misjudge directions using the sun?
Explain why breeding in many Rwandan birds is timed by the rains rather than by day length.
Evaluate the costs and benefits of long-distance migration for a white stork.

Show answersHide answers

A rhythm with a period of about 24 hours. Examples: the sleep–wake cycle, core body temperature, melatonin release, cortisol release.
In the suprachiasmatic nucleus of the hypothalamus; light-sensitive cells in the retina send it nerve impulses.
An outside cue that resets a biological clock; for example, light (also temperature or meal times).
$24\,\mathrm{h} - 0.25\,\mathrm{h} = 23.75\,\mathrm{h}$ .
Their biological clock is still set to Kigali time, so sleep, temperature and hormone rhythms are out of step with local day and night until daylight resets the clock over several days.
$4 \times 15^\circ = 60^\circ$ .
Near the equator day length hardly changes through the year, so it is a poor cue; the rains bring the food that young need, so breeding at that time is favoured.
Benefits: plentiful food in Africa during the northern winter, and long summer days with abundant food for raising young in Europe. Costs: great energy use, risk of death on the journey (exhaustion, predators, hunting, power lines), and time spent travelling. Migration persists because, on balance, more young survive than if the storks stayed in one place.

Key points

Biological rhythms (circadian, tidal, lunar, circannual) are driven by internal clocks and reset by zeitgebers such as light.
The mammalian master clock is the SCN in the hypothalamus; clock genes form a 24-hour feedback loop.
In constant conditions rhythms free-run with a period close to, but not exactly, 24 hours.
Migrants navigate by the sun (with clock compensation), stars, the magnetic field, landmarks and smell.

1Travellers that know the time

2Daily rhythms in humans

3The master clock

4Free-running rhythms

5Other rhythms

6Migration

7How do migrants find their way?