Lovebirds and Love Darts: The Wild World of Mating

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Females benefit from promiscuity by improving the genetic quality of their young. A good provider is not necessarily the best sperm donor, so while the female builds a nest with her mate, she may sneak out and go sperm shopping for a male with brighter feathers, bigger body size, or a more glorious singing voice—all indicators of good health.

This widespread lack of fidelity led scientists to coin a new term: social monogamy—living in pairs but sexually unfaithful.

While genetic testing shattered the halos of birds as monogamous creatures, they were not the only species revealed as promiscuous. Only about 3 percent of mammals are thought to be monogamous, mating and bonding with one partner for life. Humans are not one of these naturally monogamous animals. Black vultures, termites, and prairie voles are.

Dueling Love Darts

But back to the arrow-shooting Cupid. Garden snails (Helix aspersa) use "love darts" to get their point across.

The hermaphroditic snails have both male and female characteristics, but do not self-fertilize. Courtship, which could take anywhere from 15 minutes to several hours, consists of circling one another to get in the best position to shoot a love dart but not be shot.

"Love is coming down to war in a way," said Ronald Chase, a biologist at McGill University in Montreal. "Sexual conflict plays out, even though both members of the mating pair are hermaphrodites."

Once the love darts are fired—and about one-third of the time they miss—the snails transfer sperm to one another in a separate process that lasts as long as six hours. What's the point of the love dart if hitting or missing makes no difference to whether the pair copulates?

The mucus covering the dart causes part of the recipient's female reproductive tract to contract, resulting in a significant increase in the amount of sperm stored. Successful dart shooters father more babies than do snails that miss with their darts.

"Successful dart shooting seems to only benefit the dart shooter, not the recipient," Chase said. "It causes damage"—although Chase has seen snails shot through the brain and still live—"it hurts, and it increases the number of babies the shooter is going to get."

Does Cupid enter the picture?

"I believe the myth of Cupid and his arrows has its basis in this snail species, which is native to Greece," Chase said. "The Greeks probably knew about this behavior because they were pretty good naturalists and observers."

The marine flatworm Pseudobiceros hancockanus takes the mating fight one step further, in what scientists call penis fencing. Each flatworm is a hermaphrodite with two penises. The mating ritual consists of dueling penises, each flatworm trying to pierce the skin of the other with its sharp-tipped organ. The winner—the stabber—transfers its sperm. The loser gets what scientists call the burden of motherhood: the considerable energy costs of caring for the developing eggs.

Finally, is there anything in the animal world that parallels the "opposites attract" theory of romance in humans? Yes, according to recent bird research.

A study of the mating habits of great tits (Parus major) in the wild, conducted by scientists at the Netherlands Institute of Ecology, found that the offspring of parents with opposing personalities had much higher rates of survival than the chicks of parents with similar "extreme" personalities.

Niels Dingemanse and Christiaan Both found that when two birds with "extreme" personality traits—very fast and aggressive or very slow and cautious—mated, they produced chicks with more extreme behaviors. These chicks tended to die early and reproduce less. The "average types" resulting from a fast and aggressive bird mating with a slow and cautious bird had the highest survival and reproduction rates.

More Valentine's Day Specials
• Read "Finding a Valentine Can Be Tough for Animals Too, Cameras Show"
• Watch National Geographic Presents: When Animals Attract on Fox TV tonight at 9 p.m. ET.

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