Dispersion and Philopatry – strategies for the distribution of birds and mammals

Ethologists have noticed a curious trend: the time has come to leave the place where they were born, females are generally dispersed among birds, but in mammals it is more often males who leave their native territory.

Many animals live in community, forming social groups, composed of elements of the same species: flocks, packs, shoals, etc. There are also animals that live in isolation. But even they need to get together to reproduce, if only in the act of mating. In addition, mothers and children form groups, more or less temporary, depending on the species. The Brown Bear female spends about three years with the young. On the other hand, some species of birds are nidifugal, that is, as soon as they are born they leave the nest, which does not mean that the parents, or at least one of them, do not accompany the offspring. Basically, all animals have the need, at least at some point, to share space with other animals of the same species.

Any group obeys internal rules, usually defining social hierarchy, thus maintaining the balance of existing ties. There are countless social behaviors of different species that ethologists try to record and understand. The fact that animals can live in isolation or in community, may be linked to factors derived from competitive pressure: in groups the pressure for food, sexual partner or breeding place increases. The risk of contagion due to disease also increases, in addition to the fact that several animals together are more easily detectable by predators, than when they are isolated. But living in community also increases the number of eyes, noses and ears alert to danger. Among predators, joint cooperation makes hunting easier, in addition to being able to capture prey much larger than it would be possible to obtain in isolation. There are also cases of cooperation in the creation of offspring, with the obvious advantages of this.

Young adults, depending on several factors, can stay in the family group or leave to form their own family or to live in isolation. The habitat, the distribution of food, the mating system and the risks of inbreeding seem to determine, to a large extent, the level of dispersion of young animals in relation to their place of birth. Depending on the species, the factors that most influence dispersion vary, and within each species, there may also be different forms of dispersion.

When young people stay in their home area, sharing the territory with their parents, we speak of native philopatry. This strategy has advantages and costs. The degree of kinship between the members of the group increases the risk of inbreeding, with the consequent reduction in genetic variability, which is an evident evolutionary disadvantage. However, inbreeding can favor the “selection” of genes that determine good adaptation to a given habitat .

Among other costs of group life, we can mention the increase in population density, which will raise competition for resources and sexual partners, as well as for shelters or breeding places.

According to some sociobiological theories, however, life in society leads to a reduction in aggression among members and an increase in altruistic behaviors. Another advantage of the animals’ social life is that they have a better knowledge of where the group lives.

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