When it comes to mammals generally, monogamy has long been known as a rarity. Out of 4,000 mammal species, no more than a few dozen form reliable pair-bonds, although in many cases it is hard to characterize them with certainty, because the social and sexual lives of mammals tend to be more furtive than those of birds. Monogamous mammals are most likely to be bats (a few species only), certain canids (especially foxes), and a few primates, notably the tiny New World monkeys known as marmosets and tamarins, a handful of mice and rats, several odd-sounding South American rodents (agoutis, pacas, acouchis, maras), the giant otter of South America, the northern beaver, a handful of species of seals, and a couple of small African antelopes (duikers, dik-diks, and klipspringers).
Even females in seemingly solitary species such as orangutans, gibbons, and black bears have been found to copulate with more than one male; hence, observations of social organization alone clearly can be misleading . Until recently, lacking the appropriate genetic techniques, we had little choice but to define monogamy by the social relationships involved; only with the explosion of DNA fingerprinting technology have we started to examine the genetic connections, those most important to evolution. Thus, according to the highly-respected book by David Lack, Ecological Adaptations for Breeding in Birds, fully 92% of bird species are monogamous. Socially, this figure is still accurate; sexually, it is way off. The highest known frequency of extra-pair copulations are found among the fairy wrens, lovely tropical creatures technically known as Malurus spendens and Malurus cyaneus. More than 65 percent of all fairy wren chicks are fathered by males outside the supposed breeding group . Here is another eye-opener. Warblers and tree swallows are purportedly monogamous, yet when genetic analyses were conducted on six different offspring in two different species, they were found to have been fathered by five different males!
Although such cases are admittedly extreme, we now know that it is not uncommon for 10 to 40 percent of the offspring in "monogamous" birds to be fathered by an "extra-pair" male; that is, one who isn't the identified social mate of the female in question. (It is much less common for offspring to be "mothered" by an extra-pair female; that is, for an outsider female to slip one of her eggs into the nest of a mated pair. More on this later.)
Given how much we have been learning about non-monogamy and extra-pair matings among animals, and considering the new-found availability of such testing, it is remarkable how rarely genetic paternity tests have been run on human beings. On the other hand, considering the inflammatory potential of the results, as well as, perhaps, a hesitancy to open such a Pandora's Box, perhaps Homo sapiens' reluctance to test itself for paternity is sapient indeed. Even prior to DNA fingerprinting, blood group studies in England found that the purported father is the genetic father about 94% of the time; this means that for six out of a hundred people, someone else is the genetic father . In response to surveys, between 25% and 50% of United States men report having had at least one episode of extramarital sex . The numbers for women are perhaps a bit lower - around 30% - but still in the same ballpark . Many people already know quite a lot - probably more than they would choose to know - about the painful and disruptive effects of extramarital sex. It wouldn't be surprising if a majority would rather not know anything more about its possible genetic consequences, extramarital fatherhood. Maybe ignorance is bliss. (If you feel this way, better stop reading here!)
Until quite recently, multiple mating was hidden from biologists' purview. It wasn't so much invisible as unacknowledged, a perfect example of the phenomenon that even in such a seemingly hard-headed pursuit as science, believing is seeing. More to the point, not believing is not seeing. Sexual infidelities among ostensibly monogamous species, when noticed at all by biologists, were generally written off as aberrant, not worth describing, and certainly not suitable for analysis or serious theory. Distasteful as it may have been, Geoffrey Parker's work changed that, along with this important recognition by evolutionary theoretician Robert Trivers. A "mixed strategy" should be favored, at least among males: Maintain a pair bond with a female, who you might well assist in rearing offspring, but be ready and available for additional copulations if the opportunity arises. The next step was to ask: What about the female? Is she merely a passive recipient of male attentions, an empty tank to be filled with the sperm of various competing paramours? Or does she choose among the eager male prospects? Might she even actively solicit extra-pair copulations, generating sperm competition among different males?
Early work, both empirical research and theorizing, took a decidedly male-centered perspective on multiple mating, emphasizing how males maximize their paternity by being sexually available to more than one female whenever possible, also competing with each other directly (by bluffing, displaying, and fighting) and indirectly by guarding their mates, as well as by using an array of anatomical, physiological and behavioral techniques - such as frequent copulations - to give them an advantage over other males.
More recently, biologists have begun to identify how females partake of their own strategies: mating with more than one male, controlling (or at least, influencing) the outcome of sperm competition, sometimes obtaining direct, personal benefits such as food or protection in return for these extra-pair copulations, as well as gaining indirect, genetic benefits that eventually accrue to their offspring. A penchant for non-monogamy among males is no great surprise, but as we shall see, the most dramatic new findings and revised science brought about by recent demolitions of the myth of monogamy concern the role of females. Freud spoke more truth than he knew when he observed that female psychology was essentially a "dark continent." A well integrated theory of female sexuality in particular still remains to be articulated; perhaps a reader of this book will be suitably inspired.
