The nature of sex

From monogamy to homosexuality, does biology hold any clues to our love lives?

Sex is everywhere. Sex sells. Sex is sold. Sex made you. It’s alternatively known as coitus, copulation, and sexual intercourse. It is performed by humans and all other sexually-reproducing animals. It can take many forms and exist in many different partnerings, ranging from gorilla male-controlled harems to monogamous prairie voles who pair bond for life.

In its most traditional sense, sexual intercourse is for the purposes of procreation. When ovulation in a female coincides with fertilisation from a male, then fertilised eggs can develop into offspring. The vast majority of sexual encounters in nature, determined by inherited behavioural tendencies, do indeed centre around procreation. This is no accident. Since the primordial soup, natural selection has allowed the fittest individuals within a species who survive and pass on their genes. Survival is a means to passing on genes. And passing on genes almost invariably means sex in animals. So individuals who have the genetic tendency to have sex and procreate will pass on this “sex drive”. In some animals, this tendency to engage in sex may be driven by innate pleasure responses in the brain, which would also be selected for by natural selection. We can’t be sure how much pleasure animals experience, but behavioural studies can and have estimated that animals do experience pleasure. In the same way that pain prevents humans from doing things that are bad for our survival, pleasure encourages animals to behave in ways that are beneficial for survival such as feeding, staying warm or cool, or to pass on their genes via mating.

“Monogamy is rare within mammals. Only 3% of species are monogamous.”

Consider typical human sexual characteristics: we are mostly monogamous, we have sex in private, it isn’t obvious when females are ovulating, we have sex all the time, and females also experience menopause, an unusual and abrupt end to fertility. As a population, we predominantly engage in heterosexual encounters, but, to an unknown percentage, also homosexual ones. This complete set of behavioural and physical characteristics are rare within the animal world, but some of the individual traits can be found across the animal kingdom.

Monogamy is rare within mammals. Only 3% of species are monogamous, the rest are a mixture of polygamous relationships such as where a male mates multiple females, or a female mates multiple males, or males and females have multiple mating partners. Within the animal kingdom, up to 90% of bird species form social monogamous relationships, however many of these pairings are not actually sexually exclusive and many chicks are not related to one of their parents. Research has looked into what might make one species monogamous, while another not. A study issued in January of this year delved into what underlies social monogamy and showed that despite arising independently in many different species, monogamous animals show similar patterns of gene expression in their brain. This suggests a universal mechanism underlying the evolution of monogamy across vertebrates.

Studying what contributes to the formation of social bonds in other animals helps us to understand how we function, and even how we might help people with disorders such as autism, which is characterised by deficits in social behaviour. It may seem odd though that we must look to as far away as birds and voles to understand our own behaviour. Our closest primate relatives do not share many of our sexual characteristics. Gorillas, chimpanzees, and bonobos are all polygamous, and, as well as leaving parental care primarily to the females, they will take part in sexual interactions in group social settings.

What really separates humans from the majority of animal species is our lack of defined mating period indicated through the obvious fertility of the female. Many of you may be familiar with references to dogs “in heat”; this relates to physical and behavioural changes associated with the ovulation of a female. These fertility cycles, known as estrus, vary across mammals, but often share similar characteristics. Females in estrus experience behavioural and physical changes, such as the release of pheromones and swelling and/or colour change of the genital region, indicating to the male that they are fertile.

“Many human females are likely glad that their ovulation isn’t apparent, as otherwise it might make for awkward situations or interesting Trinder posts!”

Why is it that during evolution some animals have maintained this outward display of fertility and we have not? The answer, as with most things to do with natural selection, is a balance of risks. Wild animals living in large social groups will want to reduce the amount of sex they need in order to create offspring. Sex is risky; it increases risk of disease, it wastes energy, and may lead to injury or even death, so reducing the frequency is beneficial for survival in the wild. For instance, the rutting season in deer, when females are in season, is associated with high levels of male aggression and fighting for mating rights to groups of females. Therefore most animals have short bursts of “one-night stands” in a season, and then the female cares for the offspring, a less risky and energy consuming solution for all involved.

However, Homo sapiens are somewhat different. It is theorised that the hidden ovulation of human females may relate to the time and energy consumption required to raise a human child. It is born defenceless, unable to walk, and doesn’t gain the ability to support itself for well over a decade. Some people hypothesise that by hiding ovulation and therefore needing to have sex regularly to ensure pregnancy, both long-term monogamy and paternal care are selected for – thus ensuring the survival of the next generation. Aside from the evolutionary reasons for our hidden ovulation, many human females are likely glad that their ovulation isn’t apparent, as otherwise it might make for awkward situations or interesting Trinder posts!

“For instance, oral sex is observed in many animal species including hyenas, lions, and a species of bat.”

However we are not alone in the animal kingdom in our regularity of sex or indeed sex for something other than procreation. A number of species have been documented as likely having sex for pleasure or other social reasons. Bonobos are famous for their frequent fornication between members of a social group, mostly between two females. It is thought to be used as a mechanism for cementing social bonds, enabling the dominance of females in the wider social group, and for releasing tension in situations that would normally result in aggression and fights in other species, such as chimpanzees. It is widely agreed that some other primate species, pigs and dolphins for example, take part in sex for pleasure. However, the difficulty in truly understanding animal sexual interactions is that, unlike humans, we can not directly ask animals what they experience or what their motivation is when they behave in certain ways. We instead have to rely on observations of animals and our own interpretation of their actions. This reliance on human observation means that we can often be inclined to project our own motivations onto the behaviour of animals and this leaves room for debate, as to the extent to which animals experience pleasure and exactly what drives them to have sex. Often, biologists are reassured when a particular behaviour is in line with natural selection and evolution. For instance, oral sex is observed in many animal species including hyenas, lions, and a species of bat, either as part of the mating ritual or during copulation, which increase the likelihood and duration of mating so animals can pass on their genes.

“So it seems that across the animal kingdom, sex and procreation can be uncoupled.”

However, applying evolution to sexual behaviour does not always solve the problem. Perhaps the biggest puzzle in the evolution of animals and sexuality is the existence of homosexuality. Homosexual encounters occur in lots of animals, from cows and sheep to macques and lions. In one observational report of giraffes in Tanzania, they recorded that 16 out of 17 sexual encounters recorded were male-to-male. Bonobos and spotted hyenas, who have an enlarged clitoris like a penis, regularly engage in female-to-female genital stimulation in order to maintain their matriarchies. However, as with its exact occurrence rate in humans, the number of species which exhibit homosexuality is unknown. As suggested by the significant occurrence of homosexuality across the animal kingdom, areas of the human genome have been identified as associated with homosexuality, indicating an innate origin of the behavioural trait. Despite its existence as a reliably recurring trait, an exact evolutionary explanation remains to be elucidated. One possibility is that rather than being something that is incompatible with the survival of a species, so long as there is some heterosexual mating and procreation happening at the right point of the estrus cycle across a population, the remainder of sexual interactions are inconsequential. So it seems that across the animal kingdom, sex and procreation can be uncoupled.

It is even possible to procreate without having sex in some species. Known as parthenogenesis, some females have evolved to have the ability to lay eggs that produce exact clones of themselves and do not require fertilisation. This type of asexual reproduction has the advantage that you pass on all your genes to the next generation, but the disadvantages of this are that the next generation are more susceptible to disease and environmental change as they are all genetically identical. Parthenogenesis has been recorded in marbled crayfish, lizards, komodo dragons, aphids, and many more species. In the whiptail lizard, which is all female and asexual, sexual behaviour is still seen between the females which increases reproductive output from a female about to lay eggs. You may be thinking that this must be among the more unusual ways to reproduce in the animal kingdom, but not to be outdone, in some species of spiders, females mate with a male and then either eat him during or after sex.

Sexuality in the animal kingdom comes in a great manner of weird varieties, some with clear evolutionary support and others with unclear benefits. While we might not learn a great deal about how to attract members of the desired sex in an appropriate manner from the animal kingdom, I think we can definitely be reassured that we humans fit into the wide, wonderful, and wacky array of sexual behaviours that are out there.

Maeve McCann

Maeve McCann is a former Deputy SciTech Editor of Trinity News.