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Sexual selection in mammals is a process the study of which started with
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
's observations concerning
sexual selection Sexual selection is a mechanism of evolution in which members of one sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex ...
, including
sexual selection in humans The concept of sexual selection was introduced by Charles Darwin as an element of his theory of natural selection. Sexual selection is a biological way one sex chooses a mate for the best reproductive success. Most compete with others of the same ...
, and in other
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
s, consisting of male–male competition and
mate choice Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choi ...
that mold the development of future
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
s in a population for a given
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
.


Elephant seals

A good example of intrasexual selection, in which males fight for dominance over a
harem A harem is a domestic space that is reserved for the women of the house in a Muslim family. A harem may house a man's wife or wives, their pre-pubescent male children, unmarried daughters, female domestic Domestic worker, servants, and other un ...
of females, is the
elephant seal Elephant seals or sea elephants are very large, oceangoing earless seals in the genus ''Mirounga''. Both species, the northern elephant seal (''M. angustirostris'') and the southern elephant seal (''M. leonina''), were hunted to the brink of ...
– large, oceangoing mammals of the
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
''Mirounga''. There are two species: the
northern Northern may refer to the following: Geography * North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating ...
(''M. angustirostris'') and
southern elephant seal The southern elephant seal (''Mirounga leonina'') is one of two species of elephant seals. It is the largest member of the clade Pinnipedia and the order Carnivora, as well as the largest extant marine mammal that is not a cetacean. It gets its ...
(''M. leonina'') – the largest
carnivoran Carnivora ( ) is an order of placental mammals specialized primarily in eating flesh, whose members are formally referred to as carnivorans. The order Carnivora is the sixth largest order of mammals, comprising at least 279 species. Carnivor ...
living today. Both species show extreme
sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...
, possibly the largest of any mammal, with southern males typically five to six times heavier than the females. While the females average and long, the bulls average and long. The record-sized bull, shot in Possession Bay,
South Georgia South Georgia is an island in the Atlantic Ocean, South Atlantic Ocean that is part of the British Overseas Territories, British Overseas Territory of South Georgia and the South Sandwich Islands. It lies around east of the Falkland Islands. ...
, on February 28, 1913, measured long and was estimated to weigh . The maximum weight of a female is with a length of . Males arrive in the colonies before the females and fight for control of harems. Large body size confers advantages in fighting. The
agonistic behaviour Agonistic behaviour is any social behaviour related to fighting, which can include aggressive behaviour, but also threats, displays, retreats, placation, and conciliation. The term "agonistic behaviour" was first defined and used by J.P. Scott ...
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 In the zoological field of ethology, a dominance hierarchy (formerly and colloquially called a pecking order) is a type of social hierarchy that arises when members of animal social animal, social groups interact, creating a ranking system. Dif ...
among males, also known as a
pecking order In the zoological field of ethology, a dominance hierarchy (formerly and colloquially called a pecking order) is a type of social hierarchy that arises when members of animal social groups interact, creating a ranking system. Different types o ...
, 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 Game theory is the study of mathematical models of strategic interactions. It has applications in many fields of social science, and is used extensively in economics, logic, systems science and computer science. Initially, game theory addressed ...
developed for biology, most notably by
John Maynard Smith John Maynard Smith (6 January 1920 – 19 April 2004) was a British mathematical and theoretical biology, theoretical and mathematical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he ...
.


An uncertain example: the giraffe

The evolutionary origins of the
giraffe The giraffe is a large Fauna of Africa, African even-toed ungulate, hoofed mammal belonging to the genus ''Giraffa.'' It is the Largest mammals#Even-toed Ungulates (Artiodactyla), tallest living terrestrial animal and the largest ruminant on ...
's (''Giraffa camelopardalis'') long neck are controversial. The long-accepted "competing browser's hypothesis" originally put forth by
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
has been put into question. Originally, scientists believed that the elongation of the giraffe's neck had been a result of
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...
acting in relation to
foraging Foraging is searching for wild food resources. It affects an animal's fitness because it plays an important role in an animal's ability to survive and reproduce. Foraging theory is a branch of behavioral ecology that studies the foraging behavi ...
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 The neck is the part of the body in many vertebrates that connects the head to the torso. It supports the weight of the head and protects the nerves that transmit sensory and motor information between the brain and the rest of the body. Addition ...
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 Allometry (Ancient Greek "other", "measurement") is the study of the relationship of body size to shape, anatomy, physiology and behaviour, first outlined by Otto Snell in 1892, by D'Arcy Thompson in 1917 in ''On Growth and Form'' and by Jul ...
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 Sexual selection is a mechanism of evolution in which members of one sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (intrasexual select ...
is the biggest precopulatory mechanism in mammals.
Sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...
is a result of male–male competition that is easily seen in species.


Male–male competition

Male–male competition to copulate with the opposite sex is often seen in
mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...
.
African elephants African elephants are members of the genus ''Loxodonta'' comprising two living elephant species, the African bush elephant (''L. africana'') and the smaller African forest elephant (''L. cyclotis''). Both are social herbivores with grey skin. ...
strongly promote male–male competition.
Elephants Elephants are the Largest and heaviest animals, largest living land animals. Three living species are currently recognised: the African bush elephant (''Loxodonta africana''), the African forest elephant (''L. cyclotis''), and the Asian ele ...
continuously grow throughout their lifetime. As males grow older, they also experience increasing lapses of
musth Musth or must (from Persian, ) is a periodic condition in bull (male) elephants characterized by aggressive behavior in animals, aggressive behavior and accompanied by a large rise in reproductive hormones. It has been known in Asian elephan ...
, a violent sexual excitement, and most
reproductive success Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves. Reproduct ...
happens to males in musth as it helps them win fights. A fight between a male in musth and one not can result in the death of the latter. Species with intense male–male competition are known to exhibit the most size dimorphism. For example, female
American black bear The American black bear (''Ursus americanus''), or simply black bear, is a species of medium-sized bear which is Endemism, endemic to North America. It is the continent's smallest and most widely distributed bear species. It is an omnivore, with ...
s (''Ursus americanus'') are 20–40% smaller than males. Male mammals can compete for
harems A harem is a domestic space that is reserved for the women of the house in a Muslim family. A harem may house a man's wife or wives, their pre-pubescent male children, unmarried daughters, female domestic Domestic worker, servants, and other un ...
as well with
elephant seals Elephant seals or sea elephants are very large, oceangoing earless seals in the genus ''Mirounga''. Both species, the northern elephant seal (''M. angustirostris'') and the southern elephant seal (''M. leonina''), were hunted to the brink of ...
competing fiercely for harems. As mammals reach sexual maturity,
secondary sexual characteristic A secondary sex characteristic is a physical characteristic of an organism that is related to or derived from its sex, but not directly part of its reproductive system. In humans, these characteristics typically start to appear during puberty ...
arise. Elephant seals have a proboscis in the adult male, which is used to project loud noises, frequently heard during the mating season. Elephant seals with a bigger proboscis emit lower sounds than males with a smaller proboscis and are the bigger of the males in a colony. Mate-guarding is an important factor in male–male competition to ensure fertilization of an
offspring In biology, offspring are the young creation of living organisms, produced either by sexual reproduction, sexual or asexual reproduction. Collective offspring may be known as a brood or progeny. This can refer to a set of simultaneous offspring ...
, and, when successful, helps to overlook and court the female. It especially prevents
sperm competition Sperm competition is the competitive process between Spermatozoon, spermatozoa of two or more different males to fertility, fertilize the same Egg cell, egg during sexual reproduction. Competition can occur when females have multiple potential m ...
from occurring as it ensures reproductive success. This process can be engaged when a post-coital signal is sent to a male to keep guard by the female.
Mating plug A mating plug, also known as a copulation plug, vaginal plug, sperm plug, or sphragis (Latin, from , "a seal"), is a gelatinous secretion used in the mating of some species. It is deposited by a male into a female genital tract, such as the vag ...
s are a form of mate-guarding that have proved to show precopulatory female choice. Copulatory plugs are commonly acellular and thought to be made by proteins from the seminal vesicles. DNA taken from copulatory plugs show females deter from mating with close relatives.


Callings

During the
breeding season Seasonal breeders are animal species that successfully mate only during certain times of the year. These times of year allow for the optimization of survival of young due to factors such as ambient temperature, food and water availability, and ch ...
, mammals will call out to the opposite sex. Male
koala The koala (''Phascolarctos cinereus''), sometimes inaccurately called the koala bear, is an arboreal herbivorous marsupial native to Australia. It is the only Extant taxon, extant representative of the Family (biology), family ''Phascolar ...
s that are bigger will let out a different sound than smaller koalas. The bigger males which are routinely sought out for are called sires. Females choose sires because of indirect benefits that their offspring could inherit, like larger bodies. Non-sires and females do not vary in their body mass and can reject a male by screaming or hitting him. Male–male competition is rarely exhibited in koalas. Acoustic signaling is a type of call that can be used from a significant distance encoding an organism's location, condition and identity.
Sac-winged bat Emballonuridae is a family (biology), family of microbats, many of which are referred to as sac-winged or sheath-tailed bats. They are widely distributed in tropical and subtropical regions around the world. The earliest fossil records are from ...
s display acoustic signaling, which is often interpreted as songs. When females hear these songs, named a 'whistle', they call onto the males to breed with a screech of their own. This action is termed 'calling of the sexes'.
Red deer The red deer (''Cervus elaphus'') is one of the largest deer species. A male red deer is called a stag or Hart (deer), hart, and a female is called a doe or hind. The red deer inhabits most of Europe, the Caucasus Mountains region, Anatolia, Ir ...
and
spotted hyena The spotted hyena (''Crocuta crocuta''), also known as the laughing hyena, is a hyena species, currently classed as the sole extant member of the genus ''Crocuta'', native to sub-Saharan Africa. It is listed as being of least concern by the IUC ...
s along with other mammals also perform acoustic signaling.


Testosterone

Testosterone Testosterone is the primary male sex hormone and androgen in Male, males. In humans, testosterone plays a key role in the development of Male reproductive system, male reproductive tissues such as testicles and prostate, as well as promoting se ...
is a driving factor towards achieving fertilizing success.
Bighorn sheep The bighorn sheep (''Ovis canadensis'') is a species of Ovis, sheep native to North America. It is named for its large Horn (anatomy), horns. A pair of horns may weigh up to ; the sheep typically weigh up to . Recent genetic testing indicates th ...
display curved horns on the rams of the species which are big compared to the small horns displayed on the females of the species. The bigger the horns are, the more testosterone there was found to be in the male. This is important because social rank has a positive correlation with the length of the horns. Social rank leads to tending of a group of females to copulate with. Testosterone also appears more in polygynous species than in monogamous species.


Polyandry

Polyandrous females have two or more mating partners while they are in heat. Females are more likely to find a new mate when their current male had a high number of paternities the year before or their current male was old. This is presumed to have an effect on offspring and giving them more genetic diversity.


Sex-role reversal

Sex role reversal is the change in behavior of a specific species from their ordinary pattern. Sex-role reversal supports sexual dimorphism very strongly. Female–female competition is a common abnormality within animals with accepted sex roles. 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). Gestation and lactation are energy-consuming, which means their competition for resources is high. Female–female competitions are observed to gain access for better mates.
Meerkat The meerkat (''Suricata suricatta'') or suricate is a small mongoose found in southern Africa. It is characterised by a broad head, large eyes, a pointed snout, long legs, a thin tapering tail, and a brindled coat pattern. The head-and-body ...
females acquire dominant status because resources for female reproduction are scarce. Dominant females in this species are heavier and win in competitions over other females.


Postcopulatory mechanisms

Copulating with the opposite sex does not ensure fertilization of an egg. Postcopulatory mechanisms include
sperm competition Sperm competition is the competitive process between Spermatozoon, spermatozoa of two or more different males to fertility, fertilize the same Egg cell, egg during sexual reproduction. Competition can occur when females have multiple potential m ...
and
cryptic female choice Cryptic female choice is a form of mate choice which occurs both in pre- and post-copulatory circumstances in which females of certain species use physical or chemical mechanisms to control a male's success of fertilizing their ova or ovum; i.e. by ...
.


Sperm competition

Sperm competition involves male gametes trying to fertilize eggs first. As a result of sperm competition, some males in a given species can develop bigger testes and seminal vesicles. Larger midpiece areas in the sperm that contain mitochondria are also observed. 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. It was found that primates and rodents with longer flagellum fathered more offspring. The length of the
baculum The baculum (: bacula), also known as the penis bone, penile bone, ''os penis'', ''os genitale'', or ''os priapi'', is a bone in the penis of many placental mammals. It is not present in humans, but is present in the penises of some primates, ...
is also influenced by sperm competition in some mammal species.


Cryptic female choice

Cryptic female choice is a postcopulatory mechanism that cannot be observed because it takes place inside a female's body. It enables a female to have some control over who fathers her child even after fertilization. In some species, females may choose to mate with more than one male to prevent infanticide or harassment. Infanticide can be prevented by confusing the males in a given colony. If the female mates with multiple males, then the males will not know for sure who fathered the offspring. Infanticide can also be prevented by choosing a male that will protect her and the offspring. Sexual harassment may be avoided if females give in to males and copulate when they please.


References

{{Sex (biology) Sexual selection Mammalian sexuality