Yours faithfully... up to a point

Monogamy is one of the rarest behaviours in nature, writes Olivia Judson

Thirty years ago, biologists thought monogamy - as in 'til-death-do-us-part monogamy - was common in nature.

Some 90% of bird species were believed to be monogamous, at least for the duration of a breeding season, and many were thought to mate for life. Wolves and gibbons were similarly admired. But that was before the invention of genetic paternity testing, which revealed the truth about who was really having whose children.

During the 1980s and 90s, the supposedly virtuous came tumbling from their pedestals. These days, when animals live together in pairs, they are said to be socially monogamous, a cautious term that risks no assumptions about their sex lives.

The discovery of true monogamy throughout a population - even for just one breeding season - is rare and exciting news.

This news is now being made by a diverse smattering of creatures. Genetic testing has given the jackdaw a clean record, ditto the chinstrap penguin and the black vulture. The California mouse has an excellent reputation. Kirk's dik-dik - a small African antelope - seems to have a good case.

Several other species lack genetic data, but the circumstantial evidence is good. The woodroach Cryptocercus punctulatus - a wingless cockroach - will probably turn out to be beyond reproach, and several species of shrimp are looking promising. But, beyond that, candidates are few and far between.

Why is true monogamy so rare in nature? It's simple. For monogamy to evolve as a way of life for all members of a population, two criteria must be met. First, there must be a genetic basis for the behaviour. In fruit flies, field crickets and green-veined white butterflies - the only animals that have been considered with regard to this - individuals vary in their tendency for promiscuity, and the trait seems to be heritable.

Second, monogamous creatures must consistently have more surviving offspring than more wanton types. Then, over time, the genes involved in monogamy will spread through the population. In short, true monogamy is rare because it is rarely in the interests of one partner, let alone both.

The circumstances that conspire to produce monogamy are not well understood: in the attempts to understand why most animals are much more promiscuous than expected, the study of strict monogamy has been neglected.

Moreover, the old explanation, that monogamy would evolve whenever two parents were needed to rear offspring, turns out not to be very general. Parental care and monogamy do not necessarily go hand in hand. In Kirk's dik-dik, for example, the male does nothing to help his mate with the childcare, he just trots after her.

Presumably, staying with one mate and keeping other males away is more profitable than trying to seduce other females.

But perhaps the weirdest case of monogamy is the black vulture. Such couples build their nests far away from each other. However, the reason for their monogamy is not a lack of opportunity: black vultures depend on carcasses for food, and many may meet at a single carcass. Yet if a bird tries to solicit sex, he (or she) will be vigorously attacked by other vultures. Black vultures, it seems, do not tolerate lewd behaviour in public.

What, if anything, does this mean for humans? Taken as a species, humans cannot be described as exclusively monogamous. Divorce rates and extra-marital affairs attest to that. Also, non-identical twins occasionally have different fathers. But most humans are not rampantly promiscuous, either. It's hard to tell to what extent social and cultural pressures curtail human appetites for sexual variety and nothing is known about the genetics of monogamy.

If I were a betting woman, though, I'd bet there is a genetic component to human monogamy, and that individuals have different inclinations. In other words, remaining faithful may be a far greater achievement for some of us than for others.