The title above is misleading, because there isn't any such thing as "normal" sex in the animal kingdom — the variety of sexual strategies and adaptations in nature is simply mind-boggling.
What this page concerns is the portion of our human sexuality that's "normal" in the sense that we share its features with large groups of other life forms, or with smaller groups of our close mammalian relatives.
The next page, by contrast, describes features of our sexuality that are not normal — features that are rare elsewhere in nature or that are even unique to humans. Not surprisingly, such abnormal features are strongly linked, even at a genetic level, to our unusual intelligence.
Resources, parenthood and sex for pay
If there's one thing evolutionary biologists have shown us about our mating behavior, it's the central importance of the sacred whore archetype.
The survival of children in an uncertain world depends heavily on a mother's ability to get enough to eat during pregnancy, and then parents' ability to provide the children with adequate food until they can take care of themselves. In thousands of different species, the gender who has to make the largest investment in reproduction (almost always the female) exacts payment from the other gender going into the deal. Sex for pay is the rule, not the exception, in much of the natural world.
Sometimes things can get tricky, too. In The Evolution of Desire, David Buss describes how hard it is to be a scorpionfly boy: In order to get laid, you have to pay your scorpionfly girl with a dead bug of just exactly the right size. While eating the bug, she'll let you have your way with her. The thing is, if the bug is too small and she finishes it before you're done, she'll toss you off unceremoniously and go on about her business. If the bug's too big, on the other hand, the two of you may get into a lethal marital squabble over what remains of it after you're finished. So, over the course of thousands of millenia, scorpionfly guys have become experts at selecting just the right nuptial payment size.
In transactions of this sort, what a male is buying is not just pleasure or satiation, but access to fecundity: a chance to pass on his genetic message.
Polygyny and polyandry
Among social mammals, including ones quite closely related to us such as gorillas, it's common for a small percentage of the males to father most of the offspring. This is polygyny, and is usually characterized by males of the species being very significantly larger than females. In fact, ethnologists find a measure of such sexual dimorphism to be a useful predictor of polygynous mating.
The reason for the size differential is that when males must compete for the privilege of paternity, larger and stronger ones usually win. Up to a point, this makes sense for selecting the best overall genetic design. However, because it also selects quite directly for larger, stronger males, males keep getting larger and stronger. The process can go too far, in which case the males become better adapted for fighting among themselves than for meeting day-to-day survival challenges.
Common chimpanzees (Pan troglodytes)) and bonobos (Pan paniscus), to whom we are so closely related that we share with them more than 98% of our active DNA, have somewhat different mating patterns. Unlike gorillas or baboons, they are only mildly polygynous — alpha males only have slightly better (earlier) access to females in heat than any other male. Chimps are, however, extremely polyandrous, in the sense that a female in heat often mates with dozens of different males, many times each, over the course of several days. Males literally line up to get a turn. A friend of mine who profoundly enjoys "gang-bang" parties in which she has sex with ten or twenty guys is simply reverting to a pattern that used to be the norm for our ancestors.
In this kind of scenario, males compete through sperm production, and as a result, our chimp cousins have much larger, more productive testicles than, say, gorillas. Judging from the size and productivity of our own balls, humans haven't been competing with sperm quite that intensely for a while.
Bonobos have a much-admired proclivity which is quite unusual in the animal kingdom and of which we humans have clearly retained more than a vestige — the capacity for recreational sex! Unlike most other mammals, bonobos have sex any time, all the time, just for the fun of it. Their most frequent sexual encounters are between males and a female in estrus, but they also have sex with each other that doesn't have anything to do with being in heat, or even with gender at all. Male bonobos often quite contentedly have this kind of sex with other males, and females with other females.
It's important to note that such recreational same-sex interactions among the bonobos are quite different from same-sex pair-bonding (bonobos don't form monogamous pair-bonds). Such recreational interactions don't signify a same-sex preference, and don't result in any reduction in the breeding stock. For that reason, they are better described as "pan sex" (sex without gender preference) rather than as "gay sex" (sex mediated by a strong same-sex preference).
Same-sex, childless pair bonding is remarkably common in the animal kingdom, occuring in 5% to 15% of the population of many species who form "monogamous" pair bonds. This represents so great a reduction in breeding capacity that it's highly unlikely to be accidental — it has to offer significant evolutionary advantages or it would long ago have been selected out of existence.
As we discuss on our gay sex page, a key evolutionary benefit of gay sex for humans is likely to be found in kin selection that occurs under circumstances of high environmental stress.
No matter what the reasons for its success, though, gay sex is clearly an important natural feature of our world — one we should accept and honor, particularly since we may well owe our own lives to it.
When women live together in close proximity, their menstrual cycles tend to synchronize, mediated by pheromones. This is somewhat unusual but by no means unique among social mammals. Female ring-tailed lemurs in Madagascar, for example, all come into heat together about once a year, usually on the night of a full moon. As you can imagine, this produces an annual lemur orgy of an intensity that would satisfy your wildest fantasies!
Paul Turke and others have argued that such ovulatory synchrony evolved because it gave females more power to select their mates effectively than the common polyandrous model (practiced, for example, by chimpanzees), where a female in heat mates with any and every male in the neighborhood, or the polygynous model (practiced, for example, by gorillas), where the dominant male in a troup is the only one that a female in heat gets to mate with.
The association of synchronized ovulation with more female choice and thus with more female power, is supported by the observation that female ring-tailed lemurs have much greater hierarchical status quite independent of their size than do other primate females.
Human females, like a number of other primates, don't give off any signal when they're fertile, which means that sexual activity can and does happen any time, all the time. Most other animal females, by contrast, broadcast clear signals when they ovulate, and only have sex at those times. Sex all the time is inefficient — a lot of relatively expensive sperm protein goes to waste, and a lot of time is spent courting and copulating that could be devoted to gathering food or to other survival-enhancing activities. We're by no means alone in this inefficiency, however, and there are quite a few conflicting theories that seek to explain what the benefits are of keeping ovulation secret that outweigh the drawbacks.
One, proposed by Sarah Hrdy, is that by confusing paternity, secret ovulation reduces the incentive that males otherwise have to kill children whom they know are not their own, and increases the likelihood that they will protect or help children who might be theirs. Infanticide is quite common in polygynous species such as gorillas, is practiced by raiding chimpanzees, and is a feature of human behavior too — statistics show that human stepfathers are far more likely to injure or kill stepchildren than biological fathers are to kill their own children.
Another theory, proposed by Richard Alexander and Katharine Noonan, is that in a co-operative pair-bond, concealed ovulation reduces a male's incentive to go off and father other children with other females. The idea is that if he knows when fertility occurs, that's the only time he needs to hang around, both to sow his own seeds and to prevent rivals from sneaking their sperm past him. If, on the other hand, he doesn't know when his mate is fertile, he needs to be around the house all month, having sex regularly and driving off seductive rivals.
As is so common with theories of evolution, things turn out to be a good deal more complicated than you might think from considering these two theories (although both seem to have some validity).
Swedish biologists Birgitta Sillén-Tullberg and Anders Møller surveyed 68 species of higher primate (as many as they could get adequate data for) and determined that in 32 of them, males seem to have no idea when a female is fertile. They then examined the correlation between this concealed ovulation and the mating habits of the species in the context of their genetic trees. What they discovered is fascinating.
First, they found that sexual behavior in primates has not been very stable — it has changed back and forth relatively often, presumably in response to changes in the environment. This is no big surprise, since such flip-flopping is common in evolution.
Second, they found that concealed ovulation almost always arose in the context of polyandrous or polygynous sex. This strongly suggests that its initial benefits had to do with confusing paternity so as to reduce infanticide, and perhaps also so as to increase female ability to choose mates.
Thirdly, they discovered that 10 out of 11 "monogamous" species they studied (ones in which males and females formed co-operative pair bonds) concealed their ovulation! From this we can conclude that concealed ovulation is actually an important precursor of successful monogamous behavior in primates, which confirms its importance as a means of keeping pair-bonded males at home.
Monogamy and pair-bonding
Co-operative parenting based on a pair-bond occurs in species where offspring require more parental nurturance than a single parent can provide. This is true of some primates (particularly New-World ones such as marmosets), of many birds, and of a variety of other animals.
Humans, it almost goes without saying, form strong pair-bonds and engage in co-operative parenting. This makes a lot of sense, given how much care a human child requires in order to survive to maturity. We can thus classify ourselves as a species that practices "monogamous" sexual behavior.
What exactly does that mean, from a biological perspective? Does it mean, as some of us were taught in school, that monogamous animals form bonds for life and never have sex with anyone but their chosen mate? No. With the exception of a few creatures such as the prairie vole, which bonds so thoroughly to its mate that if the mate dies, its sex life is over, most "monogamous" species form pair bonds that last only through the period during which offspring need both parents' care, and which may be renewed thereafter, but which are not necessarily renewed.
That's definitely the human pattern. We may mate for life (and very nice it can be when things work out that way), but very often our pair bonds dissolve at the end of a breeding cycle. As Helen Fisher describes in her wonderful book, The Anatomy of Love, when she was looking in the U.N. database for correlations between cultural attitudes and divorce patterns, she found to her amazement that across all human cultures with remarkably little variation, divorces tended to clump at roughly four-and-a-half-year intervals, which is approximately the length of time a human infant is absolutely helpless.
Also, in every known human culture and in many "monogamous" animal species, it is a common practice among pair-bonded couples to have surreptitious sex outside of the pair-bond. The reasons for this in terms of reproductive advantage have been analyzed at length — Jared Diamond articulates some of them with his usual clarity in the second chapter of Why is Sex Fun?, and Helen Fisher describes the advantages of such behavior for women in The Anatomy of Love.
It's important to realize that extra-marital sex is attractive to both genders. Males are better-known as the philanderers, because they usually initiate extra-marital sex, while females are are merely "receptive," but this distinction obscures the fact that both genders do it remarkably often. In The Third Chimpanzee, Jared Diamond cites an unpublished study conducted by a friend of his in the 1940's of 1,000 newborns delivered at a respected U.S. hospital. The study's focus was the inheritance of blood type, but its unintended discovery was that at least 10% of the infants had not been fathered by the husbands of the women who bore them. Later published genetic studies of American and British newborns confirmed this figure by finding that between 5% and 30% of the children studied were conceived during extra-marital sex.
Implications of "normal" animal sex for your behavior
What can we learn about our own sexual behavior from the information presented above?
One thing that's clear from looking at both animal patterns and human patterns is that we do not naturally mate for life. I feel strongly that to serve us better, our marriage institutions should take that observation into account more fully than they now do. As things stand, divorces are usually conducted in an atmosphere of blame, failure and acrimony that helps no one and is usually devastating for any children involved. We need to be far more aware going into a marriage that it may not last, and as a society, I believe we should provide far better support for couples who find their union coming apart. I don't believe this would weaken marriage as an institution — I think it would strengthen it.
Unfortunately, the statistics about children fathered in extra-marital affairs are not ones to encourage men to trust women in marriage, or women to trust men. They might, however, encourage you to think about whether maintaining the intimacy and honesty of your marriage is important enough to you that you would agree to conduct any extra-marital liasons openly and honestly, and to let your spouse do the same. Many people have found that fidelity in marriage — the ability to trust their spouse and be worthy of their spouse's trust — is far more important to them than whether the spouse has sex with someone else from time to time. For more information about this possibility, see our polyamory page.