Evolution of monogamy Monogamy occurs when one male mates with one female exclusively. A monogamous mating system is one in which individuals form long-lasting pairs and cooperate in raising offspring. These pairs may last for a lifetime, such as in pigeons ,  or it may occasionally change from one mating season to another, such as in emperor penguins.
Zoologists and biologists now have evidence that monogamous pairs of animals are not always sexually exclusive. Many animals that form pairs to mate and raise offspring regularly engage in sexual activities with extra-pair partners.
Sometimes, these extra-pair sexual activities lead to offspring. Genetic tests frequently show that some of the offspring raised by a monogamous pair come from the female mating with an extra-pair male partner. Social monogamy refers to a male and female's social living arrangement e.
In humans, social monogamy takes the form of monogamous marriage. Sexual monogamy is defined as an exclusive sexual relationship between a female and a male based on observations of sexual interactions. Finally, the term genetic monogamy is used when DNA analyses can confirm that a female-male pair reproduce exclusively with each other.
A combination of terms indicates examples where levels of relationships coincide, e. Social monogamy, sexual monogamy, and genetic monogamy can occur in different combinations. Social monogamy is relatively rare in the animal kingdom. The actual incidence of social monogamy varies greatly across different branches of the evolutionary tree. Sexual monogamy is also rare among animals.
Many socially monogamous species engage in extra-pair copulations , making them sexually non-monogamous. But genetic monogamy is strikingly low in other species. Barash and Lipton note: The highest known frequency of extra-pair copulations are found among the fairy-wrens , lovely tropical creatures technically known as Malurus splendens and Malurus cyaneus. They can no longer assume social monogamy determines how genes are distributed in a species.
The lower the rates of genetic monogamy among socially monogamous pairs, the less of a role social monogamy plays in determining how genes are distributed among offspring.
Polygyny in nature Polygyny occurs when one male gets exclusive mating rights with multiple females. In some species, notably those with harem -like structures, only one of a few males in a group of females will mate. Technically, polygyny in sociobiology and zoology is defined as a system in which a male has a relationship with more than one female, but the females are predominantly bonded to a single male.
Should the active male be driven out, killed, or otherwise removed from the group, in a number of species the new male will ensure that breeding resources are not wasted on another male's young. Pheromone -based spontaneous abortion in some rodents such as mice , a new male with a different scent will cause females who are pregnant to spontaneously fail to implant recently fertilised eggs.
This does not require contact; it is mediated by scent alone. It is known as the Bruce effect. Von Haartman specifically described the mating behaviour of the European pied flycatcher as successive polygyny. Males then create a second territory, presumably in order to attract a secondary female to breed.
Even when they succeed at acquiring a second mate, the males typically return to the first female to exclusively provide for her and her offspring. Polyandry[ edit ] The anglerfish Haplophryne mollis is polyandrous.
This female is trailing the atrophied remains of males she has encountered. Polyandry in nature Polyandry occurs when one female gets exclusive mating rights with multiple males. In some species, such as redlip blennies , both polygyny and polyandry are observed. When they find a female they bite into her skin, releasing an enzyme that digests the skin of their mouth and her body and fusing the pair down to the blood-vessel level.
The male then slowly atrophies , losing first his digestive organs, then his brain, heart, and eyes, ending as nothing more than a pair of gonads , which release sperm in response to hormones in the female's bloodstream indicating egg release.
This extreme sexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available. Polygynandry Polygynandry occurs when multiple males mate indiscriminately with multiple females. The numbers of males and females need not be equal, and in vertebrate species studied so far, there are usually fewer males.
Two examples of systems in primates are promiscuous mating chimpanzees and bonobos. These species live in social groups consisting of several males and several females. Each female copulates with many males, and vice versa.
In bonobos, the amount of promiscuity is particularly striking because bonobos use sex to alleviate social conflict as well as to reproduce[ citation needed ]. This mutual promiscuity is the approach most commonly used by spawning animals, and is perhaps the "original fish mating system. The water becomes milky with sperm and the bottom is draped with millions of fertilised eggs. As such, polygamous relationships can be polygynous, polyandrous or polygynandrous.
In a small number of species, individuals can display either polygamous or monogamous behaviour depending on environmental conditions. An example is the social wasp Apoica flavissima. Polygamy in both sexes has been observed in red flour beetle Tribolium castaneum. Polygamy is also seen in many Lepidoptera species including Mythimna unipuncta true armyworm moth.
Most polygamous species present high levels of tournament behaviour, with a notable exception being bonobos [ citation needed ]. Parental investment and reproductive success[ edit ] Main article: Bateman's principle Mating grey slugs , suspended from a slime thread Female and male sexual behaviour differ in many species. Often, males are more active in initiating mating, and bear the more conspicuous sexual ornamentation like antlers and colourful plumage.
This is a result of anisogamy , where sperm are smaller and much less costly energetically to produce than eggs. This difference in physiological cost means that males are more limited by the number of mates they can secure, while females are limited by the quality of genes of her mates, a phenomenon known as Bateman's principle. Thus, females are more limited in their potential reproductive success. In hermaphroditic animals, the costs of parental care can be evenly distributed between the sexes, e.
In some species of planarians , sexual behaviour takes the form of penis fencing. In this form of copulation, the individual that first penetrates the other with the penis, forces the other to be female, thus carrying the majority of the cost of reproduction. Seasonal breeder Brain corals typically spawning in connection with the full moon every August Many animal species have specific mating or breeding periods e.
In marine species with limited mobility and external fertilisation like corals , sea urchins and clams , the timing of the common spawning is the only externally visible form of sexual behaviour. In areas with continuously high primary production , some species have a series of breeding seasons throughout the year.
This is the case with most primates who are primarily tropical and subtropical animals. Some animals opportunistic breeders breed dependent upon other conditions in their environment aside from time of year. Mammals[ edit ] Mating seasons are often associated with changes to herd or group structure, and behavioural changes, including territorialism amongst individuals. These may be annual e. During these periods, females of most mammalian species are more mentally and physically receptive to sexual advances, a period scientifically described as estrous but commonly described as being "in season" or "in heat".
Sexual behaviour may occur outside estrus,  and such acts as do occur are not necessarily harmful. For these species, the female ovulates due to an external stimulus during, or just prior, to mating, rather than ovulating cyclically or spontaneously. Stimuli causing induced ovulation include the sexual behaviour of coitus, sperm and pheromones. Domestic cats have penile spines. Upon withdrawal of a cat's penis , the spines rake the walls of the female's vagina , which may cause ovulation.
This breeding season is accentuated in temperate regions, in boreal climate the breeding season is typically concentrated to a few short days in the spring. Some species, such as the Rana Clamitans green frog , spend from June to August defending their territory. In order to protect these territories, they use five vocalizations. In a group of clownfish, there is a strict dominance hierarchy. The largest and most aggressive female is found at the top.
Only two clownfish, a male and a female, in a group reproduce through external fertilisation. Clownfish are sequential hermaphrodites, meaning that they develop into males first, and when they mature, they become females. If the female clownfish is removed from the group, such as by death, one of the largest and most dominant males will become a female.
The remaining males will move up a rank in the hierarchy. Motivation[ edit ] Various neurohormones stimulate sexual wanting in animals. In general, studies have suggested that dopamine is involved in sexual incentive motivation, oxytocin and melanocortins in sexual attraction, and noradrenaline in sexual arousal. In contrast, montane voles have a polygamous mating system. When montane voles mate, they form no strong attachments, and separate after copulation.
Studies[ citation needed ] on the brains of these two species have found that it is two neurohormones and their respective receptors that are responsible for these differences in mating strategies.
Male prairie voles release vasopressin after copulation with a partner, and an attachment to their partner then develops. Female prairie voles release oxytocin after copulation with a partner, and similarly develop an attachment to their partner. Neither male nor female montane voles release high quantities of oxytocin or vasopressin when they mate.
Even when injected with these neurohormones, their mating system does not change. In contrast, if prairie voles are injected with the neurohormones, they may form a lifelong attachment, even if they have not mated. It's believed[ by whom? Prairie voles have a greater number of oxytocin and vasopressin receptors compared to montane voles, and are therefore more sensitive to those two neurohormones.
It's believed that it's the quantity of receptors, rather than the quantity of the hormones, that determines the mating system and bond-formation of either species. Oxytocin and rat sexual behaviour[ edit ] Mother rats experience a postparum estrus which makes them highly motivated to mate. However, they also have a strong motivation to protect their newly born pups.