But does that include carnal pleasure too? One way to find out is to study instances of sex that can't possibly result in procreation – for instance, among two or more males, or females; where one or more individual is sexually immature, or sex that occurs outside of the breeding season.

Bonobos, for example, the so-called "hippie apes," are known for same-sex interactions, and for interactions between mature individuals and sub-adults or juveniles. But you don't need to be a bonobo to enjoy "non-conceptive" sex, white-faced capuchin monkeys do it too. In both species, primatologists Joseph Manson, Susan Perry, and Amy Parish, found that that females' solicitation of males was decoupled from their fertility. In other words, they had plenty of sex even when pregnancy was impossible – such as when they were already pregnant, or while lactating just following birth. In addition, interactions among mature and immature individuals were just as common as interactions between two adults, for both species.

If animals indulge in more sex than is strictly necessary for conception, that too might hint at a pleasure-driven motivation to do the deed. A female lion may mate 100 times per day over a period of about a week, and with multiple partners, each time she ovulates. It only takes one eager sperm to begin the road from conception to birth, but the lioness doesn't seem to mind. Could it be that she enjoys it? Similarly high rates of encounters have been observed among cougars and leopards, too.