What are the mechanisms of sexual selection in animals?

Sexual selection in animals is primarily driven by two mechanisms: intersexual selection and intrasexual selection.

Intersexual selection, also known as mate choice, is a process where one sex, usually females, choose their mates based on certain desirable traits. This is often because these traits are indicators of good genes, good health, or the ability to provide resources. For example, in many bird species, females prefer males with brighter plumage, as this is a sign of good health and vitality. Similarly, in some species of deer, females prefer males with larger antlers, as this indicates strength and the ability to defend against predators. This preference for certain traits can lead to sexual dimorphism, where males and females of the same species look different.

Intrasexual selection, on the other hand, involves competition between members of the same sex, usually males, for access to mates. This can involve direct combat, where males fight each other for the right to mate with females. For example, in many species of ungulates, males engage in head-butting contests to establish dominance and win the right to mate. Alternatively, intrasexual selection can involve more subtle forms of competition, such as sperm competition. This occurs when females mate with multiple males, and the sperm of the different males compete to fertilise the female's eggs.

Both intersexual and intrasexual selection can lead to the evolution of exaggerated traits, such as the peacock's tail or the stag's antlers. These traits may not be beneficial for survival, but they confer an advantage in terms of reproduction, and so are favoured by natural selection. This is known as the handicap principle.

It's important to note that these mechanisms of sexual selection are not mutually exclusive, and often operate in tandem. For example, in many species, males compete with each other for access to females (intrasexual selection), and females then choose between the victorious males based on their traits (intersexual selection). This complex interplay of selection pressures can lead to a wide variety of mating systems and behaviours in the animal kingdom.