High fidelity
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 would be 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 by scientific evidence. Studies on that favourite laboratory organism, the fruit fly Drosophila, seemed to suggest that female flies did not play the field.
These assumptions 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 can ensure that she is fertilised by the male with the best sperm (and hence best genes) – and the benefits of increasing genetic variability in offspring by choosing different fathers.
Humans: born for monogamy?
Where does that leave humans? The evidence to date suggests that we are naturally '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'.
A gene for 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 have a huge impact on behaviour.
The prairie vole is a small rodent that, unusually, mates for life. However, some animals show no such loyalty. Researchers have discovered that the differences between loyal and promiscuous animals lie in a tiny genetic change. This genetic difference does not affect the protein itself – a hormone receptor – but changes where the gene is active.
As a result, the hormone receptor is present in a different part of the animal’s central nervous system. This seems to change the way animals form relationships – including those with mates. In effect, animals do not seem to bond as closely to one another.