Sexual selection in mammals
[2][3] A good example of intrasexual selection, in which males fight for dominance over a harem of females, is the elephant seal – large, oceangoing mammals of the genus Mirounga.
There are two species: the northern (M. angustirostris) and southern elephant seal (M. leonina) – the largest carnivoran living today.
Both species show extreme sexual dimorphism, possibly the largest of any mammal, with southern males typically five to six times heavier than the females.
[5][6] The record-sized bull, shot in Possession Bay, South Georgia, on February 28, 1913, measured 6.85 metres (22.5 ft) long and was estimated to weigh 5,000 kilograms (11,000 lb).
The agonistic behaviour of the bulls gives rise to a dominance hierarchy, with access to harems and breeding activity being determined by rank.
A dominant male must stay in his territory to defend it, which can mean months without eating, living on his store of blubber.
Males commonly vocalize with a coughing roar that serves in both individual recognition and size assessment.
[10] In the case of intrasexual selection, adorned males may gain a reproductive advantage without the intervention of female preference.
The use of sexual ornamentation as a signaling device to create a dominance hierarchy among males, also known as a pecking order, allows struggle to proceed without excessive injury or fatality.
How often males will physically engage each other, and in what manner, can best be understood by applying game theory developed for biology, most notably by John Maynard Smith.
Originally, scientists believed that the elongation of the giraffe's neck had been a result of natural selection acting in relation to foraging behaviour, where it was supposed that longer necks enabled favoured individuals to gather food inaccessible to other animals.
[24] Elephant seals have a proboscis in the adult male, which is used to project loud noises, frequently heard during the mating season.
Mate-guarding is an important factor in male–male competition to ensure fertilization of an offspring, and, when successful, helps to overlook and court the female.
Females choose sires because of indirect benefits that their offspring could inherit, like larger bodies.
[31] Acoustic signaling is a type of call that can be used from a significant distance encoding an organism's location, condition and identity.
Females invest into choosing the best possible mate because they have more of a part in bringing up their offspring than males (gestation and lactation).
As a result of sperm competition, some males in a given species can develop bigger testes and seminal vesicles.
[37] Larger testes and bigger midpieces in sperm are seen in males that mate with multiple partners.
A female that has been with multiple partners will most likely give birth to an offspring fathered by the male that produced the most or faster sperm.
