Cheating is not unusual in the animal kingdom, yet some birds remain surprisingly faithful Birds are among the most monogamous of creatures, Dr Carin Bondar explains in a new book, though that doesn’t stop some bird species […]

Birds are among the most monogamous of creatures, Dr Carin Bondar explains in a new book, though that doesn’t stop some bird species sneaking extra sexual action on the side

Monogamy has long been a topic of ­sexual behaviour that I find both fascinating and frustrating. Divorce and ­infidelity rates are high enough for me to ­seriously question the notion of having only one sexual partner for any kind of substantial ­timeframe, yet our huge brains and ­emotional nature make juggling many ­partners a near impossibility. So when we look to the animal ­kingdom, what do we see? Not surprisingly, we see a wide range of social and sexual relationships, some involving monogamy.

For many decades it was ­believed that social and sexual monogamy were one and the same. Reports of wholesome “family” living and sustained ­relationships with single ­partners were ­described for many ­species of birds, fish, amphibians and ­reptiles. However, a major shift in our understanding of ­monogamy took place with the advent of ­genetic sequencing. Once it was possible for us to examine whose babies belonged to whom, it ­became ­blatantly obvious that social ­monogamy and sexual ­monogamy were two separate things entirely.

Many animals associate with the same partner over a ­breeding season or over consecutive ­seasons. Partners engage in ­social and sexual activities with each other, but the ­majority are also ­involved in ­sexual ­activities outside of the pair bond. When it comes to ­sexual ­reproduction, maximising one’s ­biological ­fitness is the ­ultimate goal, and ­reproducing with only one ­partner is usually not the most ­efficient way to achieve that.

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Monogamy is most often ­observed in birds. For many ­species, a good deal of parental care that requires the efforts of both mum and dad is involved. Here we see deviations from the usual sexual strategies of females being ultra picky and males ultra enthusiastic, because if both ­partners are going to be present for their offspring, both will need to be choosy.

Indeed, ­strategies of mate selection in birds often involve mutual examination. This is not true for all species – we also see many examples of extreme male ­showiness in ­response to extreme female choosiness in ­species where males do not offer any kind of parental care (for example, peacocks). But, for a large number of species, ­monogamy in some form does apply.

At one end of the spectrum are examples of total monogamy, such as in scrub jays in Florida. Despite variations in their environments, scrub jays maintained social and sexual ­monogamy. This is ­surprising and extremely rare. Biologists have ­suggested that high levels of competition for breeding ­vacancies could be the explanation. Intense competition between males leads to high ­quality males in a ­population, such that females won’t encounter males of vastly ­different ­conditions. If all males are relatively equal, it could lead to female ­fidelity because they cannot do “better” by ­seeking out another mate.

High rates of both social and sexual monogamy have been ­observed in Steller’s jays as well. Long-term couples tend to have a higher breeding ­success. The “mate familiarity effect” could be due to familiar ­partners being able to save energy through ­increased coordination of resource-­gathering and ­parental routines. Additionally, staying with the same partner tends to mean ­staying within the same ­territory, so there may also be ­fitness ­benefits through ­familiarity with one’s habitat.

This raises the important ­question: is she staying for me, or for my territory? Female long-tailed manakins and Uganda kob tend to remain faithful to their mating sites rather than their male partners. On the other hand, a female satin bowerbird will not mate if a new male is present at her familiar bower site, while the female great snipe will seek out previous male partners even if she travels to different territories in subsequent breeding seasons.

The “divorce” rates of birds in ­socially monogamous ­partnerships are widely ­variable, from zero per cent in swifts and ­wandering albatrosses to 100 per cent in house martins and grey herons, and are often dependent on operational sex ratios. When females are more abundant, males are more likely to divorce their partners in search of another.

The opposite is true for a male-skewed sex ratio, where females tend to seek extra-pair ­copulations. ­However, when males are more abundant, and therefore not all males in a population are ­guaranteed sexual action, the ­incidence of forced copulations – bird rapes – can also increase.

The jay examples are fairly uncommon in the extent of their monogamy. Generally speaking, in socially monogamous birds we almost always see a low-to-­moderate level of extra-pair ­paternity, indicating the female is getting sexual action on the side while the male is potentially out doing some cuckolding of his own.

Among the bird species with the highest levels of sexual ­sneaking are blue tits and tree swallows, where up to 90 per cent of broods are commonly found to contain offspring sired by males other than the female’s social partner. Females actively solicit ­copulations outside of their social pairs, and maintain a ­commanding level of control over genetic paternity of their offspring.

In times of ­environmental stability, female tree swallows are less likely to seek extra-pair ­copulations; when ­environmental conditions are unpredictable or poor, they make sure their broods are genetically diverse by seeking the genes of several suitors.

The bottom line with respect to almost all “fathers” in socially ­monogamous bird species with high levels of sexual polygamy is this: males provide care to ­offspring they have not sired. This seems, on the surface anyway, a poor choice from the point of view of the male’s biological fitness, but there are several factors that

need to be considered before drawing conclusions.

First, it is possible that males simply cannot tell when they have been cuckolded and are raising ­offspring that belong to another male. Second, though a male may suspect some of the ­offspring in his nest have been sired by ­another male, since he himself may have engaged in extra-pair copulations – leaving another male to provide for that offspring – perhaps it all evens out.

This is an edited extract from ‘The Nature of Sex: The Ins and Outs of Mating in the Animal Kingdom’ by Dr Carin Bondar (Weidenfield & Nicholson, £9.99), on sale now