Claim: Bonobos and dolphins are the only animals other than humans who engage in sex for pleasure.
Origins: The above claim revolves around a very specific definition of "sex for pleasure," which in this case is copulation between the male and female of a species where such activity is completely separated from the purpose of fertilization. That is, in only a couple of animal species other than humans will males and females willingly (and regularly) engage in sex with each other even when there is no possibility that offspring will be produced as a result. (Note that this definition specifically excludes homosexual and masturbatory activity, as neither of those categories involves male-female pairings.)
Females of most animal species give off detectable signals when they are fertile: a change in appearance, a distinctive smell, the production of particular sounds, or specific signaling behaviors. They do this because a number of factors makes engaging in sex an expensive and risky proposition:
Males need to expend precious energy to produce sperm, hunt for potential mates, attract females, fight off rivals, and complete the mating process.
The mating process takes time away from other vital activities, such as hunting for food or caring for previous offspring.
Mating pairs are vulnerable to attack from predators or enemies.
Competition for females often results in injury or death to males (and sometimes even the females themselves).
For reasons such as these, many animals cannot afford to "waste" the effort of engaging in sex when there is no possibility that their genes will be passed on to another generation through the production of offspring. (Reproduction may be impossible when the female is not yet sexually mature, is not ovulating, is already pregnant, is still caring for young from
a previous pregnancy, or is past reproductive age.) Therefore, in most species females produce signs detectable by males that indicate when they are ovulating and fertilization is possible. Both sexes can then avoid the risks associated with the mating process when the effort will be fruitless. (If males try anyway, their attempts will be rebuffed by the females).
Human females, however, are one of the few members of the animal world to undergo what is known as "concealed ovulation." This means that the human male has no reliable method of discerning when a particular female is fertile (and, outside the use of modern technology, women themselves can't really tell when they are ovulating). We don't know for sure the reasons why human females developed concealed ovulation through the evolutionary process, but the general belief is that it helped to ensure that males stayed with their mates and assisted with the child-rearing process. (The theory is that a male would be more likely to stay with one female if she were receptive to sex all the time and not just when she was fertile; that a male could not be certain whether he had successfully impregnated a female who engaged in sex even when she was not ovulating and therefore had to stick around to make sure he completed the task; that a male was less likely to go off in search of other mates when he had no way of knowing when they were fertile either; and that since a male did not know when his mate was ovulating and subject to being impregnated by other males, he had to stay around to defend against the intrusion of rivals.)
Therefore, we say that we humans engage in "sex for pleasure" not just because sex is something we consider to "feel good," but because it meets the criterion specified above: both men and women willingly engage in sex even though neither one of them may know for certain that the woman is fertile, and even at times when fertilization is impossible (such
as when the woman is already pregnant or is post-menopausal). We know that a few (and only a few) species other than humans behave in a manner consistent with our definition of "sex for pleasure." Female bonobos (also known as pygmy chimpanzees), for example, are receptive to sex for several weeks before and after ovulation, and male bonobos will engage in sex with them even though they know the females are not fertile. Dolphins, too, frequently mate even when the female is not fertile.
Of course, we have to make many seemingly artificial distinctions to arrive at our conclusion. Animals other than humans have no awareness that their sexual activities are connected with reproduction: They engage in sex because they're biologically driven to do so, and if the fulfillment of their urges produces a physical sensation we might appropriately call "pleasure," it isn't the least bit affected by the possibility (or impossibility) of producing offspring. We are also discounting cases in which animals do engage in sex even though reproduction is an impossibility, because we claim there are other "purposes" (of which the animals themselves are unaware) at play. (For example, the females of some species of birds will invite males to mate with them even after they have laid their eggs, but we ascribe a purpose to this behavior: this is a biological "trick" to fool males into caring for hatchlings they didn't father.) We also employ subjective terms such as "willingly" and "regularly" in claiming that bonobos and dolphins are the only other animals who "willingly (and regularly) engage in sex with each other even when there is no possibility that offspring will be produced as a result," and even then it may be the case that these species have some other "purpose" for doing so that we haven't yet discovered.
Perhaps the most important concept to be learned here is that although humans naturally tend to think of their behavior as "normal" and consider the habits of other animals to be departures from the norm, in many ways — especially in our sexual behavior — we are quite exceptional.