1 Life emerged on earth about 3.8 billion years ago, but sex did not evolve until more than 2 billion years later. Dirty limericks emerged only quite recently, geologically speaking.
2 Sex—what is it good for? Scientists are not sure, since asexual reproduction is a better evolutionary strategy in some important ways.
3 For those who refuse to commit to one strategy: The hermaphroditic earthworm Dendrobaena rubida has both male and female genitalia. If it cannot find a partner, the worm doubles up so that its female bits and male bits can go to town.
4 Although famously monogamous, female Adélie penguins slip away from their mates occasionally to couple with unattached males. They exact a fee (pdf) for such a dalliance—stones to bolster their nests—not unlike certain people.
5 Some talented penguin teasers can get a gift even without putting out. Again, not unlike certain people.
6 Barbary macaques have a distinctive way to get their mates to make a sperm donation: yelling. If the female does not shout, the male almost never climaxes.
7 How do we know this? German primatologist Dana Pfefferle watched a group of macaques, counting the females’ yells and the males’ pelvic thrusts. She says this work is “quite weird, but it’s science.”
8 Here in the US of A, that kind of stuff ends up on YouTube.
9 Because Barry White sounds terrible underwater: Fish can produce a variety of noises with their bones, teeth, and gas bladders. Grant Gilmore of Estuarine Coastal and Ocean Science Inc. says that male fish probably use some of these sounds to woo females.
10 The spiny anteater, an egg-laying mammal native to Australia and New Guinea, has a penis with four heads, but only two fit into the female at once.
11 The tiny male paper nautilus, an octopus, impregnates the much larger female by shooting his penis (a modified tentacle) into her—and leaving it there.
12 Homosexual behavior is found in at least 1,500 species of mammal, fish, reptile, bird, and even invertebrate.
13 My two dads: When a male goose courts another male goose, a female sometimes slips in and mates with both males. Later, the male partners share paternal duties.
14 Some seagulls practice lesbian mating, although the eggs that result from their liaisons are sterile.
15 Biologists at the University of California at San Francisco have found that male fruit flies exposed to high levels of alcohol become hypersexual and try to court practically anything with wings, including other male fruit flies. Eventually the revelry turns into a dysfunctional orgy, with “a chain of males chasing each other,” says one insect expert (subscription).
16 As the flies get increasingly tanked, their chance for mating success keeps dropping. This is one more reason why the fruit fly is a great model for studying humans.
17 Only a few vertebrates besides humans copulate face to face. Among those that sometimes do this: hamsters, beavers, and some primates, such as bonobos and orangutans.
18 French kissing is rarer still. The only other species known to do it as a prelude to mating is the white-fronted parrot. After the birds open their beaks and touch tongues, the male spews his lunch onto the female’s chest.
19 It is here that the mating habits of the white-fronted parrot and Homo sapiens diverge.
20 Size really does matter: People tend to choose mates of similar race, education level—and chubbiness. A recent British study indicates that obese people usually select partners with comparable levels of body fat.