Sexual selection in mammals is a process the study of which started with Charles Darwin's observations concerning sexual selection, including sexual selection in humans, and in other mammals, consisting of male–male competition and mate choice that mold the development of future phenotypes in a population for a given species.
Elephant seals
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. While the females average 400 to 900 kilograms (880 to 1,980 lb) and 2.6 to 3 metres (8.5 to 9.8 ft) long, the bulls average 2,200 to 4,000 kilograms (4,900 to 8,800 lb) and 4.2 to 5 metres (14 to 16 ft) long. 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 maximum weight of a female is 1,000 kilograms (2,200 lb) with a length of 3.7 metres (12 ft). Males arrive in the colonies before the females and fight for control of harems. Large body size confers advantages in fighting. The agonistic behaviour of the bulls gives rise to a dominance hierarchy, with access to harems and breeding activity being determined by rank. The dominant bulls or "harem masters" establish harems of several dozen females. The least successful males have no harems, but may try to copulate with a harem male's females when the dominant male is not looking. A dominant male must stay in his territory to defend it, which can mean months without eating, living on his store of blubber. Some males have stayed ashore for more than three months without food. Two fighting males use their weight and canine teeth against each other. The outcome is rarely fatal, and the defeated bull will flee; however, bulls suffer severe tears and cuts. Males commonly vocalize with a coughing roar that serves in both individual recognition and size assessment. Conflicts between high-ranking males are more often resolved with posturing and vocalizing than with physical contact.
In the case of intrasexual selection, adorned males may gain a reproductive advantage without the intervention of female preference. This advantage will be conferred by weapons used in the process of resolving disputes, such as those over territorial rights. 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. It is predominantly when two opposing males are so closely matched, as would be found in males not having established themselves in a dominance hierarchy, that asymmetries cannot be found and the confrontation escalates to a point where the asymmetries must be proved by aggressive use of ornamentation. 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.
An uncertain example: the giraffe
The evolutionary origins of the giraffe's (Giraffa camelopardalis) long neck are controversial. The long-accepted "competing browser's hypothesis" originally put forth by Charles Darwin has been put into question. 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. But even though the giraffe's overall height is about 6 meters, it still typically feeds at about 2 meters above the ground. Moreover, the giraffe's kudu, impala, and steenbok competitors do not feed above 2 meters and prefer feeding at shoulder level as well, rather than at the maximum height they could reach. An alternative explanation for the origin of long necks in giraffe is sexual selection. Male giraffe often neck with other males to exhibit dominance. There are six criteria that need to be satisfied for the exaggerated neck to be classified as a result of sexual selection. The characteristic should be more exaggerated in one of the sexes; it must be used to indicate dominance; have no direct survival benefits; cost the organism in terms of survival or other factors (e.g., energetics); positive allometry should be observed. But evolutionary history shows that increased neck length is not correlated to increases in other parts of the body, which would be expected from foraging selection, so sexual selection may be a more satisfactory explanation. Studies have failed to resolve the causes involved: perhaps the neck was a result of both or other forces.
Precopulatory mechanisms Precopulatory mechanisms determine who will father an offspring prior to sex. Male–male competition is the biggest precopulatory mechanism in mammals. Sexual dimorphism is a result of male–male competition that is easily seen in species.
Male–male competition
… excerpt ends here. Continue reading the full article.
