It's a tough trade-off for male howler monkeys: a deeper voice, or more sperm?
From an evolutionary and biological perspective, animals are driven to have sex in order to procreate. But there is a lot more to sexual relations between animals. Animal sex is at least as strange and varied as human sex. A male might copulate with many females. And female animals of many species can be choosy, too. Homosexuality is common. Monogamy is rare. And the animal kingdom is full of swingers. In short, with animals, sex can be wild.
Scientists have figured out why some species have more females than males and others have more males than females. Turns out, sex chromosomes are the culprits.
A female water snake in Missouri can do something that no human woman can (no matter how badly she might want to).
A Twitter hashtag, #JunkOff, illustrates the steamier side of biology — and its importance to understanding evolution.
The complicated world of hyena sex involves social hierarchies, weird and potentially dangerous courtship rituals and female pseudopenises that make forced sex impossible.
When it comes to sex, are butterflies as nurturing and gentle as they appear to be, or is there a more aggressive side to their mating?
Black widow males destroy large sections of their mates' webs in order to ward off potential rivals.
Check out these images of the northern giant mouse lemur, Mirza zaza, which has the largest testis relative to body size of any primate.
Rather than mundane, lazy sex, sloths participate in quite the mating game. In fact, sloth sex involves high-pitched screams, promiscuity and eye-gouging battles.
The swollen red bottom of a female balloon may not be the signal of sexual fitness that researchers thought it was.
Valentine's Day may inspire people to buy chocolates for their loved ones or treat their sweetheart to a romantic, candlelit dinner, but animals have entirely different courtship behaviors.
Octopus mating involves shifting body colors, detachable "penises," long-distance mating and sexual cannibalism.