Faithfulness (or the lack of it) can place great strains on a relationship. Are humans supposed to mate for life? What about other species? And is one sex more promiscuous than the other?
Although comparisons with other organisms should be made with caution, there is much to be learned from comparing human biology with that of other animals. The nature of sexual relationships in the animal world gives us much to chew on.
A central question is that of monogamy – having one sexual partner for life. It is often held up in human culture as an ideal. In nature, though, monogamy is very rare. Bonding for life is seen in a few birds (e.g. some penguins), the dikdik (a type of antelope), beavers, otters and a few other rodents, and some primates such as the gibbon.
It used to be thought that birds in particular made loyal partners. In the film ‘Heartburn’, for example, the female lead character complains about men to her father, who responds, “You want monogamy? Marry a swan!” In fact, most birds (including swans) are far from monogamous.
Flying the nest
Popular misconceptions also surround the promiscuity of the sexes. For a long time it was assumed that females were less promiscuous than males. Partly this reflected deep-seated assumptions about women’s motivations – that they mainly wanted a stable family group to provide safety and support – but it was backed up by scientific evidence. Studies on that favourite laboratory organism, the fruit fly Drosophila, do suggest that female flies play the field less than males.
Wider assumptions about promiscuity in the animal kingdom were held for many decades, until a series of experiments in birds forced a radical rethink. Advances in genetic fingerprinting techniques made it much easier to identify the parents of chicks. Surprisingly, the father of chicks in a nest was not always the female’s partner. While the male was away, the female was indulging in ‘extra-pair couplings’ with other males.
In fact, it now appears that promiscuity is the norm in females. The evolutionary explanations are based mainly on the idea of sperm competition. A female has a better chance of ensuring that she is fertilised by the male with the best sperm (and hence the best genes) if she mates with more of them. The mating rituals that exist in some species are a way for females to choose the best males – female birds might choose the males displaying the brightest plumage, for example. But this does not mean they can only choose once.
Female cats can even have a litter of kittens in which offspring have different fathers. This is more common in city cats, since in rural locations it’s easier for a male to monopolise the litter.
Humans: born for monogamy?
Where does that leave humans? The evidence to date suggests that we are naturally only ‘partially monogamous’, but with some commitment to long-term relationships as well.
For example, males in monogamous species tend to have smaller sexual organs (relative to body size) than those in polygamous species, where size can be a way of attracting females. Compared with other primates, we are somewhere in the middle.
But before we claim our promiscuous behaviour is simply a drive to reproduce whenever possible, it is worth recalling that we also have a powerful cerebral cortex. We can think about our actions and their impact on others, and have at least some control over most of our ‘natural urges’.
The genetics of loyalty
It might seem difficult to imagine how genes can affect something as complex as sexual behaviour and relationship building. Yet research on a small American rodent illustrates how tiny genetic changes can potentially have a huge impact on behaviour.
The prairie vole is a small rodent that, unusually, mates for life. Researchers have recently discovered that the differences between loyal, life-mating voles and those that have yet to mate lie in a tiny genetic change. This change acts as a switch determining the level of certain hormone receptors in the vole’s brain.
As a result, a vole that has already mated is more sensitive to the hormones oxytocin and vasopressin, which play an important role in pair bonding. Scientists hope that by further studying these molecular changes, they may understand more about monogamy in other animals, including humans.Lead image:
Christopher Michel/Flickr CC BY NC ND
- Male fruit flies sleep around but females keep it in the family
- Testis weight, body weight and breeding system in primates (1981)
- Nature: Gene switches make prairie voles fall in love
- High variation in multiple paternity of domestic cats (Felis catus L.) in relation to environmental conditions (1999)