Feature

Organic Clocks

Some animals keep sophicticated track of time. How do they do it?

Dr R. Lewis

With rare exceptions, all organisms live in rhythmic environments, and display cyclic patterns of behaviour and physiology which are related to the day-night cycle, the tides, the lunar cycle and the seasons. These cyclic patterns can interact in complex ways and provide the organisms with distinct advantages.

Some organisms are day-active, night-active or crepuscular (active at dawn and dusk). Superimposed on these daily patterns, inter-tidal organisms must take into account the roughly 12-hour cycles of the tides and the spring-neap tide cycles, which recur at 14-day intervals. At the annual timescale, plants flower and animals breed, migrate, hibernate. These temporal patterns of behaviour and physiology enable plants and animals to survive by, for example, avoiding harsh physical conditions and predators, breeding at the best times and hunting when prey is abundant.

How do organisms control the timing of these events in natural conditions? What kinds of timing mechanisms do they have to maintain their activity in synchrony with their environment?

The simplest hypothesis is that timing is a direct response to environmental timing cues, such as dawn or dusk, or the incoming tide. Surprisingly, tests of this hypothesis have generally not supported it -- if we remove all known timing cues from their surroundings, many organisms continue to demonstrate rhythms of activity which closely resemble their natural patterns. This is true at the daily, tidal and even annual time-scales. The implication of these tests is that organisms have internal timing mechanisms -- "biological clocks" -- which act as timers to control, at least in part, their temporal patterns of activity.

The first tests for internal, or endogenous, timing were performed as long ago as 1729, when the daily leaf movements of the mimosa plant were seen to continue in timeless conditions. But it is only over the last thirty years that it has been generally accepted that biological clocks are truly endogenous and have relevance in the control of biological timing.

Biological Timing

In own own tests of the endogenous clock hypothesis in the New Zealand weta Hemideina thoracica, the locomotor activity of individuals maintained in constant darkness at 20^oC has been recorded in some instances for many months. The wetas continue to show distinct rhythms that run initially a little faster than the equivalent day-night cycle, and later slow down. Because the period is almost always different from 24 hours, these free-running rhythms are termed "circadian", meaning that they occur over the period of about a day. In nature, activity is generally on an exact 24-hour time-scale, and so we propose that the endogenous clock is synchronised by natural cycles of light and temperature.

Subjecting laboratory wetas to 24-hour cycles of artificial light (12 hours of light followed by 12 of darkness) shows this to be true. If the clock needs to be "set" by environmental timers, why have a clock?

The answer lies in the anticipation or prediction of significant events. If the weta can predict the time of evening twilight, rather than constantly testing for it, it will be at an advantage. We also believe that the synchrony of all the internal processes, each in time with the others, is a physiological advantage of internal timing.

Just as we have evidence for circadian clocks from constant condition experiments, so too we can find evidence for circa-tidal clocks in the inter-tidal animals of the seashore. When we record the swimming behaviour of rock pool blennies, or juvenile flounder in the laboratory, we find peaks of swimming for several days at the times of high tide on the home shore. It seems that these, and many other inter-tidal species, have internal mechanisms which predict the ebb and flow of the tides.

The inter-tidal sand louse, Cirolana arcuata, is especially impressive in its long-running circa-tidal rhythms. These seem to mirror both the 12.4-hour pattern of tides and the 14-day variations in their amplitude. Hence inter-tidal organisms are made up of a whole complex of clocks of different frequencies synchronised by cycles of wave action, turbulence and moonlight.

Clock Location
and Genetics

The concept of the biological clock would be easier to acknowledge if we could obtain physical evidence of its existence. In some animals this has already been done. In hamsters and cockroaches it has been possible to transplant specific parts of the brain between individuals along with the characteristics of the donor's rhythms. In the hamsters (and also humans), parts of the brain known as the supra-chiasmatic nuclei are the essential structures, and in the cockroaches the optic lobes of the brain are equally important.

These insect results have been supported by our own results with wetas, where the clock has been localised in the inner part of the optic lobes. In some birds, the single pineal gland also has clock-like properties, and can be transplanted between individuals. It seems that these structures are master clocks or pacemakers which keep in synchrony a whole array of sub-clocks scattered throughout the body.

Mutants have been produced of Drosophila fruit flies which have fast clocks, slow clocks, or are lacking in clocks. Implantation of the brains of rhythmic individuals into the abdomens of the clockless mutants bestows the donor's rhythmicity on the recipient. This confirms the role of the brain as a master clock in this species, and also has led to the identification of the period gene and its products. A recent discovery illustrates that the rate of synthesis of the gene protein is also circadian, and the frequency matches that of the locomotor rhythm of the mutant. There seems to be a more or less direct connection between the period gene and the overt locomotor rhythm of the Drosophila. This will hopefully lead to a fuller understanding of the mechanism of the biological clock.

Humans are no exception in the possession of circadian clocks. In fact their properties are very similar to that in other day-active mammals. Evidence for human clocks has come from bunker experiments, and times of isolation in caves and in Antarctica. Human clocks run with a frequency of rather less than the solar day, and comprise two major components -- one regulates the daily rhythm of body temperature and the other the sleep-wake cycle. These are normally kept in synchrony with the natural synchronisers such as sunlight and social cues, but the whole system can become desynchronised by rapid jet travel (well known as jet lag) and shiftwork. Using bright light therapy to resynchronise the shattered array of body rhythms is currently under study and shows great promise.

Dr Lewis is a senior lecturer in chronobiology at Auckland University.

Dr Lewis is a senior lecturer in chronobiology at Auckland University.