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Different species vary in their behaviour from living a fairly solitary existence, through clustering into groups, to closely related individuals with a more less complex social organisation. Living in a group increases visibility to predators. On the other hand there are more eyes and ears to sense danger. If a predator takes only one individual at a time, being part of the group reduces the chance of being that individual and, often there is competition to be at the centre. It may help with hunting, as with a pack of wolves, which may kill much larger prey, or be able to surround it. It may assist in defence as when a flock of birds mobs a Sparrowhawk. Living closer together also increases the risk of an infection spreading. Also, a given food source has to support more individuals, and often there is competition within the group.
One might predict that, where resources are sparse or scattered, the young will tend to disperse, indeed they may be driven away by the mother as they become adult. Possibly for reasons given above or because there is some particular focus of attention, they may gather in loosely in often transitory groups.
When considering a resource, one has to take into account the diet. A species with a specialised diet might find resources sparse even in, say a jungle, which would appear at first sight to be rich.
This may be encouraged if their resources are bounded by a more sparse environment, or if there are localised patches that are rich in a particular resource, particularly if their diet is restricted in some way, related females may stay close together in kin alliances capable of exploiting the resource and defending it.
It is the behaviour of the female in her requirements for nutrition and safety for nurturing her young that determines the behaviour of any given species, and hence that of the males. This is entirely our interpretation about how group living may appear, but it is mirrored by Wrangham, who writes "it is selection pressures on female behaviour which ultimately determines the effect of ecological variables on social systems."(1)
The theory here is based on the idea of kin altruism. An individual co-operates with others who are more or less genetically related to her. Nevertheless, any individual has her own needs to meet, therefore each member of the group makes a delicate trade-off between competition and co-operation. There inevitably arises a balance in the group between more or less powerful individuals, typically referred to as a dominance hierarchy. In such groups, disagreements and conflicts between individuals are assuaged by placatory behaviours such as grooming, with complex and shifting alliances between individuals within the group. Animal studies have, until recently, concentrated on the behaviour of the dominant males, and ignored the social interactions within the group which determine its continued cohesion.
For males, who have only themselves to feed and potentially unlimited reproductive potential it tends to be the availability of mates which is the limited resource. Either males may stay within the group and compete with other males, or they may range around a number of groups. In practice there are more complex issues which will be discussed later.
The tendency of one sex to remain with the group is referred to philopatry. Alternatively the tendency of one sex to move out in search of mates is referred to as exogamy. One effect is to maintain genetic diversity and prevent in-breeding.
In most cases, where males stay within the group, and compete aggressively, the biggest and strongest will tend to monopolise the supply of mates, which leads to a selection pressure towards sexual dimorphism. It is a general rule of thumb, which we will meet when we discuss human ancestry, that in species where the males are significantly larger than females, inter-male aggression is implied. As a rider, Martin has noted that in a less favourable environment, small females breed earlier.(2) Other suggestions that have been made is that larger males are more able to protect against predators, and that they are able to utilise different food resources, thus not competing directly with the females. However, these may be incidental consequences, rather than an evolutionary cause.
Footnote:
It might be as well to add some more about altruism. One of the revolutions in thinking which came about with new ideas on evolution, typified by such books as Dawkins The Selfish Gene,3 is that no individual acts "for the good of the species", thus signalling the death knell of what was known as Group Selectionism.
The individual that succeeds through its behaviour in passing on its genes, or more specifically, in producing young, will be more likely to pass that behaviour to future generations. Thus behaviours that enhance individual success are evolutionarily favoured - hence selfish genes. Yet it is quite clear that animals do at times co-operate, the theory suggesting that such co-operation will be between relatives - kin altruism.
There is a phenomenon known as allo-mothering, whereby a daughter will assist in the nurturing of her siblings, rather than having young of her own, where the likelihood is not good that she will be able to do so. The theory suggests that her siblings will be as closely related genetically to her as her offspring would be (about one half)
There are other issues, such as reciprocal altruism, that we won't go into here. Yet, though an individual behaves to benefit itself, its behaviour may benefit others. It may benefit by being part of a group, for instance, but the group may benefit by being of a certain size. Allomothering sometimes occurs among non-related females, particularly in primate groups. However, when assessing behaviours such as alarm or feeding calls, one has to be sure that there isn't a simpler explanation.
Bibliography
- Wrangham, R.W., (1980) An ecological model of female-bonded primate groups, Behaviour, 75:262-300
- Martin, in Lewin, R., (1998) Human Evolution: An Illustrated Introduction, (Fourth ed.) Massachusetts: Blackwell Science.(p69) (bookshelf)
- Dawkins, R., (1989) The Selfish Gene, London, Oxford
Citation
Bland, J.,(2002) About Gender: Living in Groups
http://www.gender.org.uk/about/10ethol/a3_group.htm