Sexual selection in mammals

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Charles Darwin’s observations led to the continuously growing work in sexual selection. Although Darwin gave thorough evidence for his work, it was nowhere near complete.[1] Sexual selection consists of male-male competition and mate choice that mold the development of future phenotypes in a population for a given species.[2][3]

Precopulatory mechanisms[edit]

Precopulatory mechanisms determine who 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[edit]

Male-male competition is a process often seen in mammals to copulate with the opposite sex. African elephants strongly promote male-male competition. Elephants continuously grow throughout their lifetime. As males grow older, they also experience increasing lapses of musth, which is violent sexual excitement. Most births belong to males that are in musth at the time of copulation because it helps them win fights over other males.[4] A fight between a male in musth and a male not in musth can end up in death of the male not exhibiting musth. Species with intense male-male competition are known to exhibit the most size dimorphism.[5] For example, female bears of the species Ursus americanus are 20-40% smaller than males.[6] Size dimorphism is so significant in this species that only three dominant males fathered 91% of the cubs. Male mammals can compete for harems (a group of two or more females) as well. Bigger harems are monitored by larger males. Elephant seals are a good example of competition for harems. Elephant seals have a secondary sexual trait called a proboscis. As mammals reach sexual maturity, secondary sexual traits arise.[7] The proboscis body part, seen in the adult male of the species, is used to project loud noises that are more frequently heard during the mating season.[8] 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. Mate-guarding, when successful, helps to overlook and court the female.[9] It especially prevents sperm competition from occurring. Mate-guarding ensures that their sperm is the genetic material to fertilize the females’ eggs. This process can be engaged when a post-coital signal is sent to a male to keep guard by the female.[10] Copulatory plugs are a form of mate-guarding that have proved to show precopulatory female choice.[11] Copulatory plugs are commonly acellular and thought to be made by proteins from the seminal vesicles.[12] DNA taken from copulatory plugs show females deter from mating with close relatives.


During the breeding season, mammals will call out to the opposite sex. Male koalas 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.[13] 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.[14] Acoustic signaling is a type of call that can be used from a significant distance encoding an organism’s location, condition and identity.[15] Sac-winged bats 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’.[16] Red deer and spotted hyenas along with other mammals also perform acoustic signaling.[17][18]


Testosterone is a driving factor towards achieving fertilizing success. Bighorn sheep 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.[7] Social rank leads to tending of a group of females to copulate with.[19] Testosterone also appears more in polygynous species than in monogamous species.[20]


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.[21][22]

Sex-role reversal[edit]

Sex role reversal is the change in behavior of a specific species from their ordinary pattern. Sex-role reversal supports sexual dimorphism very strongly.[23] 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.[24] Female-female competitions are observed to gain access for better mates. Meerkat females acquire dominant status because resources for female reproduction are scarce.[25] Dominant females in this species are heavier and win in competitions over other females.

Postcopulatory mechanisms[edit]

Copulating with the opposite sex does not ensure fertilization of an egg. Postcopulatory mechanisms include sperm competition and cryptic female choice.

Sperm competition[edit]

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.[26] Larger midpiece areas in the sperm that contain mitochondria are also observed.[20] Larger testes and bigger midpieces in sperm are seen in females 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.[26] It was found that primates and rodents with longer flagellum fathered more offspring.[27]

Cryptic female choice[edit]

Cryptic female choice is a postcopulatory mechanism that cannot be observed because it takes place inside a female’s body.[28] It enables a female to have some control over who fathers her child even after fertilization.[29]

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.[26] Sexual harassment may be avoided if females give in to males and copulate when they please.[30]


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