In chronobiology , this includes, for example, the rhythms ( circannual rhythms) that last about a year , such as occur during bird migration , the seasonal change of hair or plumage and the winter rest , but also rhythms that do not have to have a seasonal equivalent in the environment, such as about that of the sexual cycle . In this way, for example, rhythms lasting about a lunar month ( circalunar rhythms) can be observed, which can be synchronized with a moon orbit .
The rotation of the earth around the sun causes the annual cycle , which outside of the tropics is associated with particularly pronounced seasons . The polar-wise increasing variations in solar radiation - as changes in the length of the day and the highest level of the sun above the horizon - lead to seasonal differences with cyclical fluctuations. Changes in the incidence of light, temperature and humidity are the essential factors for plant organisms. The occurrence and growth of plants are there as a food supply for herbivorous animals, and their occurrence and frequency it can then be for animals that feed on them. In most terrestrial and many marine organisms, processes can therefore be observed that change over the course of a year and repeat over the course of the year. Some of these biological processes represent a yearly rhythm organized in the living being.
Circannual rhythms (from Latin about 'approximately' and annus 'year') are chronobiologically called internal rhythms, the period of which is about as long as a year. By external changes as a signal, they can be synchronized with the cyclical fluctuations of the environment. This circannual rhythm is expressed, for example, in seasonal phases of activity or rest and a seasonal reproduction . Often other phenomena are associated with it, such as seasonal color changes , moulting , migration , hibernation or diapause . While breeding birds are often found in all seasons in areas close to the equator , breeding behavior is concentrated in the favorable spring and summer months in regions further away from the equator .
When analyzing the annual rhythm, a distinction is made between ultimate and proximate factors . Ultimate factors are those variables in the environment that have exerted such selection pressure in the course of evolution that a certain behavior, for example reproduction, was restricted within a species to a certain period of the year. Proximate factors are those variables that represent a signal for an individual of this type, for example to initiate reproduction, and can only be inferred through causal analysis. The knowledge of processes that regulate an annual rhythm is also of great importance for animal husbandry and vegetable growing.
For many annual rhythms, the food supply is an important ultimate factor that has led to the timing of behavior in evolution. During the winter months, many insects are in the diapause, a long dormant phase of eggs, larvae or adults. Some mammals constantly suppress their energy consumption and fall into hibernation. Migratory birds leave their breeding grounds and fly to their winter quarters, where they can count on a better food supply. Through the precise timing of such behavior and the associated consequences for the individual, the role of the ultimate factors can be analyzed. The synchronization of the internal clock with an exogenous stimulus is called entrainment .
Timing of reproduction
Roughly three functional principles apply to the timing of the reproduction of birds.
- Synchronization with the supply of food: An example is the exception to the rule. The Eleanor's Falcon ( Falco eleonorae ) breeds on small islands in the Mediterranean Sea in September and feeds its young almost exclusively with migratory birds that are on their autumn migration to Africa.
- Synchronization with conspecifics: This applies, for example, to colony birds according to the rule "The larger the colony, the lower the risk for the individual bird" ( Fraser-Darling principle ). Predators therefore exert selection pressure on the relatively early and late breeders.
- Scarcity of Resources: The earliest returning migratory birds can occupy the best territories, and their prematurely born young are likely to have a better chance of survival. This thesis can only be confirmed through experiments. But these are hardly made yet. There is also the risk that those who return earlier will not find sufficient resources and therefore die. However, recent studies on great tits suggest that an early start of the breeding season is a positive selection factor.
Natural selection in the annual rhythm can only take place if the variations have a genetic basis. Genetic variation in nature could lead to timing correlations in related individuals. In the case of great tits, a correlation was found between the laying times of mothers and daughters. However, attempts at crossing are still pending. In the case of the blackcap ( Sylvia atricapilla ), in which the German population migrates in winter while the population in the Canary Islands does not, restlessness (nocturnal activity) was detected in a moderated form during crossings in the F1 generation in captivity.
A seasonal periodicity in reproduction can also be demonstrated in humans. The annual peak in conception is in June in the northern hemisphere and in March for births. The peak in infant mortality is in winter. The amplitude of the reproductive rhythm has, however, decreased since the birth of the birth statistics (beginning of the 19th century).
Many season-dependent activities cannot take place suddenly, but require longer preparation time. If the ultimate factors in the environment reach a critical value, it can be too late. For example, before migratory birds can reproduce, they first have to fly to the breeding area, occupy a territory, find a partner, and build a nest. Therefore, the ultimate factor usually does not offer a good stimulus. Many organisms have therefore developed strategies to use signals from their environment as a harbinger for the expected optimal moment of reproduction, migration, etc. Such trigger signals are called proximate factors. Only in very variable, unpredictable environments is the proximate factor normally equated with the ultimate. For example, birds in tropical areas will start breeding when the first rain has fallen and the food supply has improved accordingly. Sometimes the rain itself, as in the case of the zebra finch , can also be the proximate factor. Most animals of the temperate latitudes use the length of day or photoperiod as a proximate factor, the most accurate indicator of the season. The response to the length of the day is called photoperiodism . A combination of proximate factors can also be used, such as the length of the day and the amount of food available.
Innate or learned?
As with the circadian rhythms, the question of whether the annual rhythms are exogenous or endogenous was asked . And as with the 24-hour rhythms, experiments have shown that the rhythms are maintained under constant conditions. Using the example of ground squirrels that were born and raised in captivity under constant conditions, it could be shown that the annual rhythm is innate and not learned. The same applies to many species of birds.
The annual rhythm of ground squirrels and migratory birds behaves like an internal oscillator . However, very little is known about the mechanisms behind it. The theory that the animals count the days could not be confirmed. So far, no endogenous pacemaker like the circadian rhythms could be detected. The organism as a whole may possibly function as an oscillator, with different stages following one another, but the few experiments on this could in no way support this thesis.
In songbirds, day length is a reliable timer for the endogenous circannual rhythms. In deer , too , the formation of the antlers could be influenced with light as a timer. However, the length of the day does not seem to play a role in the hibernation cycles of ground squirrels.
Functional meaning of circannual rhythms
Endogenous programming of an annual rhythm is probably especially important for species that are heavily dependent on precise timing but have no access to reliable information about the season. For example, in the case of a winter sleeper like the golden-mantled ground squirrel ( Spermophilus lateralis ), which spends six months underground and then has to find a sexual partner within the first week after awakening in order to reproduce in the short summer season, an exact time determination can determine the reproduction .
Moon, oestrus, population cycles
This has developed in connection with the phase cycle of the moon , which currently lasts about 29.5 days . Some species of bristle worms in the Mediterranean mate under a full moon in summer . The reproductive behavior of the Palolo worm from the Pacific is also linked to circalunar rhythms. During the Palolo times in the old light before the new moon , an individual rejects the rear part of the body containing germ cells. The former abdomen then moves actively to the surface of the water and meets there with others of the same kind; at sunrise the sperm and egg cells are released for fertilization.
The New World earfish Leuresthes tenuis or Grunion , which is endemic to California's coast, spawns in the sand of the beach. To do this, he uses the nights immediately after the spring flood , as occurs with the high tide at full moon and at new moon, i.e. roughly every two weeks. These tides , also called semilunar , can flush the spawning grounds half a month later, whereupon the developed larvae slip into the sea water. In this respect, the tides and the moon phase synchronize their life cycle.
In mammals, the oestrus or estrus is the time of increased sexual activity. During the oestrus, ovulation occurs in the female sex . The mating times and thus also the rut of the male sex are based on this. The female sexual cycle represents an infradian rhythm, which in many mammals is linked to other infradian rhythms, for example a circannual one. Such a link to an annual cycle cannot be found in humans (see also menstrual cycle ).
There are a number of hypotheses for the causes of population cycles, such as in the case of lemmings . Feedback mechanisms are often discussed. It is likely that predator - prey relations or parasites play a role in the regulation. In comparable taxa, there are often allometric relationships for the population cycles .