They may look harmless, but the snails snacking on your backyard garden boast a sex organ that serves as a sharp, sword-like weapon.

Several species of hermaphroditic land snails fire these so-called love darts into each others' bodies while mating.

Once a snail manages to shoot the dart, which transfers sperm-boosting secretions, that animal can fertilize the other's eggs. (Also see "Wild Romance: Weird Animal Courtship and Mating Rituals.")

Made of calcium, these love darts are an essential tool for promiscuous snails, which mate multiple times with different partners to try to fertilize as many snails as they can.

However, scientists hadn't known whether these bioweapons were harmful. Now, a new study shows love darts can reduce fertility and shorten life spans in darted snails.

Tough Love

For their experiment, study leader Kazuki Kimura, a biologist at Tohoku University in Japan, and colleagues compared the number of egg clutches with rates of longevity in captive populations of Bradybaena pellucida, a species of land snail native to Japan.

The results revealed that a darted snail pays a heavy price for procreation: It will lay fewer eggs and live only three-fourths of a typical snail life span, which is about 60 days, according to the study, published March 10 in Proceedings of the Royal Society B.

It sounds counterintuitive: Why evolve love darts if it will hurt the mother that's carrying your brood?

The answer probably comes down to selfish genes—the darts are known to discourage the mother from mating again, which means the darter carries on their own lineage.

Joris Koene, a zoologist at VU University Amsterdam who wasn't involved with the study, says it isn't clear whether the loss of libido is caused by a chemical on the darts or is a result of physical trauma, but the latter theory could explain why the snails harm each other.

What's more, the study suggests that damaging the fertility of your partner isn't such a bad idea if you're able to fertilize a lot of their eggs. (Also read: "Why Sea Slugs Dispose of Their Own Penises.")

"It has already been shown that snails can strikingly benefit from an increase in 'short-term' male fertilization success," Kimura says by email.

That is, if darts allow a snail to father many more young, a few less isn't a huge deal.

Sexual Arms Race

The study also suggests that these harmful mating habitats are "expected to escalate [the sexual arms race] more drastically," Kimura says.

For instance, snails vying for paternity will evolve new and better darts, from simple pointy cones to elaborately bladed harpoons, as Koene noted in a 2005 study. (See "'Torture' Phalluses Give Beetles Reproductive Edge.")

And they will likely also develop a resistance to the darts. For instance, female organs may evolve a resistance to love darts by becoming better at digesting unwanted sperm. Or, evolving a tougher skin—though a long shot for these soft-bodied animals—could provide an anti-dart armor.

Because when it comes to hermaphroditic snails, all really is fair in love and war.