How animals define their territory
BEHAVIOUR patterns involved in the distribution of higher animals are usually more varied. The most obvious patterns which govern the number of animals in an area, preventing their numbers from exceeding the limits set by food supplies, are those which cause simple spacing out. Many animals use territories, each territory being held by a single animal, a pair, or a social group. Entry into foreign areas causes fighting, and thus the animals are forced to spread out. As long as each animal can hold on to his own territory, then the total number of animals that can live in a larger area is limited. If there are not enough territories for all the animals in the area, then surplus animals may be expelled from the area, or may even be allowed to stay on as non-breeders.
There are many kinds of territory holdings in the animal world. In more gregarious animals a personal hierarchy often develops. The dominant animals of a group have the first choice on every occasion. This cuts off the lowest members of the hierarchy from their food, and thus again limits the total population. Ordinary hens exhibit this type of hierarchy: the pecking order. Each hen knows where all the others lie in the hierarchy and thus it knows whether to expect submission from another hen, or whether it ought to submit.
Sometimes, when there are large groups of animals, such as in a dancing swarm of gnats, or flock of seagulls, such inequalities in status may show up only in adverse conditions, when the total numbers must be restricted.
Aggression plays an
important part in spacing out animals of a species. It is, in effect,
the opposite of herd attraction. Very aggressive animals are not able to
live in herds, but moderately aggressive animals can do so as long as
they keep a certain distance among themselves. A line of starlings
sitting on a telegraph wire is spaced along it so that each bird cannot
quite reach out to peck at the next. This space, which an animal or bird
needs around it, is, in fact, a tiny version of a territory.
Birth rate is clearly a very important factor in controlling the number of animals of one species in an area. Where there is a high density of animals, the reproductive rate may drop. But this is a physiological effect. Often, under crowded conditions, adults tend to move elsewhere to reproduce. The main effect of high density is thus to promote movement.
Specialised animals have special problems of distribution. Cryptic moths (which mimic some part of their environment in an attempt not to be seen) need a low population density. If there are too many moths close together, it increases the chance that many of them will be eaten up by predators at one go, but if they are well spread out this threat is reduced. To help this, adults lay their eggs singly, and scatter them widely.
Migration is a common phenomenon, which has evolved where it results in a higher reproductive rate, or a lower death rate, than a static existence would allow. Migration is more common in areas with marked seasonal changes, for example, extremes of heat and cold. These regular seasonal journeys from one area to another and back again are undertaken by many birds, whales, seals, bats, fish and some insects.
Migration is a recurring phenomenon occurring in only certain animals. Mass movements can occur which are not regular. If the number of animals in a given habitat becomes excessive something drastic has to happen. Here again the vital factor is food shortage. The suicide pact of the Norwegian lemming, when thousands of animals race westwards, not stopping even for the sea, where most of them drown, is a good example of this.
Perhaps the best-known animal, which erupts under overcrowding, is the locust. It has been shown in the laboratory that crowding will change the locust from a solitary into a highly gregarious insect and that its structure also changes.
All the above examples, of behaviour that distributes animals, have arisen as a result of natural selection. The species must be propagated, but not so much in excess that it leads to overcrowding and starvation. Behaviour patterns have thus developed to temper the propagation, by forcing the species to spread out and find other suitable places to live. Only in a completely constant environment populated by animals that breed evenly all the year-round can a perfect balance be indefinitely maintained. This static balance may possibly be achieved in some marine habitats, but elsewhere the rough and tumble of natural selection forces animals to develop their specialised safety-valve behaviour patterns which ensure the best possible distribution of the species.