Humans are now mostly monogamous, but this has been the norm for just the past 1,000 years.
Scientists at University College London believe monogamy emerged so males could protect their infants from other males in ancestral groups who may kill them in order to mate with their mothers.
Scientists in Canada recently suggested that a rise in sexually transmitted infections, as social groups became larger among early humans, would have put pressure on staying monogamous in terms of mating behavior.
Photos: Which animals are monogamous?
Primates such as chimpanzees and bonobo monkeys, pictured, do not conform to a mating system and regularly engage in frequent sex with multiple partners.
Emperor penguins usually mate for one year before moving on to a new partner.
Male elephant seals, or "beach masters," protect harems of more than 100 females from other males thinking of moving into their territories.
Swans -- symbols of love and fidelity -- are not monogamous.
Love birds mate and "love" for as long the other mate stays alive. If one dies, the other develops a bond with another individual.
Male lightning bugs entice mates by lighting up the night sky.
Queen bees mate with a very small number of male bees, drones, to produce many eggs.
Jumping spiders are known to "dance" for their mates, performing a complex, zigzagging flamenco-like dance to entice the females. Not only do they make moves, they actually make a rhythmic vibrating song using their body movements.
After sex with your partner, you may just want to relax in bed and cuddle a bit. But would you find it frustrating and a little hurtful if, instead of reaching for you, your partner rolls over and reaches for their phone? Or maybe you're totally spent and craving sleep, but your significant other is offended that you don't want to indulge in intimate, meaningful conversation.
There's no shortage of post-sex behaviors, including touching and cuddling, playing with your phone, smoking a cigarette, falling asleep or even getting up and leaving the room. If you're partial to one type of postcoital approach, it's not surprising to feel vulnerable or take it personally when your partner does something different.
Conversely, if you're satisfied with what happens after sex with your partner, you may be more likely to feel satisfied with the sex itself, as well as with your romantic relationship in general.
Those are the results of a pair of studies that examined the effects of post-sex style among partners. They also found that, although results for similar for men and women, the association between post-sex affection and relationship satisfaction was stronger for women.
That's not surprising, sex therapist RenÃ©e Burwell said. "Women are often socialized that sex and intimacy are one and the same, so they may equate a lack of post-sex affection with their partner not caring about them as a person," she explained. "This affection is often a symbol of intimacy, care and thoughtfulness from their partner."
But that doesn't mean the two are necessarily interchangeable. Indeed, Burwell said, "I am very careful in trying not to equate healthy sex practices with a healthy relationship. Naturally, these practices can help with a healthy relationship. However, great sex and a healthy relationship do not always correlate."
Here are some other points to keep in mind regarding post-sex affection.
Biology may play a role
There may be something to the old stereotype of a man who starts snoring almost as soon as he climaxes.
"Probably the most common complaint I hear is that male partners tend to fall asleep quickly after sex, whereas female partners want to engage in 'pillow talk,' " sex therapist Emily Jamea said. "Several chemicals and neurotransmitters are released following an orgasm, but men have a particular cocktail of chemicals that may result in immediate drowsiness. Just knowing what's happening from a biological perspective can help women take this less personally. That being said, I do encourage male partners to at least attempt a little bit of cuddling."
Sex therapist Ashley Grinonneau-Denton explained that "in my experience working with couples, women are more apt to want to continue a sensual experience, to bask in the glory of oxytocin, otherwise known as the cuddle hormone. Men, on the other hand, are much more likely to want to get up and get something to eat, smoke a cigarette, ingest a drink or do something else that keeps the surges of dopamine rolling now that the body is no longer producing as much in its post-orgasmic state."
Take advantage of this time
"Because we live in a culture that is always on the move, it is not uncommon for couples to rush and have sex to 'just do it,' " Grinonneau-Denton said. "I think the average couple thinks much less about what comes after sex and much more about what occurs during the sexual encounters themselves."
But the post-sex period is an ideal time to strengthen your connection, Jamea said. "We are in one of our most vulnerable emotional and physical states when we are sexual with another person, especially when there are strong feelings with that person," she said. "The time after sex is a critical opportunity to build intimate bonds, not emotional walls."
Try rolling toward, not away
Even if sex makes you sleepy -- or makes you want to get up and go for a run -- it's worth taking advantage of these minutes together. "Given the research, couples should make an effort to connect post-sex," sex therapist Rachel Needle said. "The time you spend post-sex can increase intimacy and bonding, as well as commitment, sexual and relationship satisfaction."