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In most species, it is seen that either males or females have evolved to be aggressive towards approaching members of the same species of the same sex and maintaining strict territorial boundaries. The logical explanation is that such behavior is advantageous in the evolutionary context. If you increase competition, then the chance of genes suited to the environment getting passed on increases. Obviously, if one male wards of another male from his territory then the genes possessed by him are positively selected for.
In such a scenario, why haven't both the sexes developed a similar level of aggressive competition (Wouldn't this also increase the chance of genes that are suited to the environment getting passed on?)
The reason for this discrepancy is mainly a consequence of Bateman's principle. Citing wikipedia:
Bateman's principle suggests that in most species, variability in reproductive success, or "reproductive variance," is greater in males than in females. This is ultimately a consequent of anisogamy. Females, especially mammalian females, almost always invest more energy into producing offspring than males invest. Bateman's principle anticipated and is consistent with Robert Trivers's theory of Parental investment-in most species females are a limiting factor over which males will compete. This competition results in some males being more successful than others, leading to greater reproductive variance among males than females
Variation in competition abilities for access to a mate is called sexual selection. As a consequence of Bateman's principle, sexual selection is more common in males than in females. You can split sexual selection into
inter-sexual selection. These terms are poorly chosen IMO. They mean:
- Inter-sexual selection: One gender try to look sexy and the other gender chooses the mate. Fighting to impress females is one type of behaviour that can evolve under inter-sexual selection
- Intra-sexual selection: Individuals of one gender fight for the access to an individual of the other gender. This individual cannot chose who to mate with.
In both case, fighting can be involved. As a consequence, you will see among-males fighting is more common than among-females fighting.
To put it in layman's terms, it's the female's job to tend to the young and the male's job to defend their turf. After all, a hen can't ward off an intruder and sit on its eggs at the same time. Or imagine a mother deer leaving its fawn to fight off a wolf pack. The deer would be leaving its young unguarded. Moreover, if the wolves killed the deer, its young would then perish.
That's a gross generalization (and a little simplistic to boot), as females of some species can be aggressive as well as males.