Spring is finally slinking into the northeast, and the backyard wildlife here is shaking off the winter torpor. Our oldest daughter, Charlotte, is now old enough to be curious about this biological exuberence. She likes to tell stories about little subterranean families of earthworm mommies and grub daddies, cram grapes in her cheeks in imitation of the chipmunks, and ask again and again about where the birds spend Christmas. This is, of course, hog heaven for a geeky science-writer father like myself, but there is one subject that I hope she doesn't ask me about: how the garden snails have babies. Because then I would have to explain about the love darts.
Garden snails, and many other related species of snails, are hermaphrodites, equipped both with a penis that can deliver sperm to other males and with eggs that can be fertilized by the sperm of others. Two hermaphroditic snails can fertilize each other, or just play the role of male or female. Snail mating is a slow, languorous process, but it also involves some heavy weaponry. Before delivering their sperm, many species (including garden snails) fire nasty-looking darts made of calcium carbonate into the flesh of their mate. In the 1970s, scientists sugested that this was a gift to help the recipient raise its fertilized eggs. But it turns out that snails don't incorporate the calcium in the dart into their bodies. Instead, love darts turn out to deliver hormones that manipulate a snail's reproductive organs.
Evolutionary biologists have hypothesized that this love dart evolved due to a sexual arms race. When a snail receives some sperm, it can gain some evolutionary advantage if it can choose whether to use it or not. By choosing the best sperm, a snail can produce the best offspring. But it might be in the evolutionary interest of sperm-delivering snails to rob their mates of their ability to choose. And love darts appear to do just that. Their hormones prevent a snail from destroying sperm with digestive enzymes, so that firing a love dart leads to more eggs being fertilized.
Recently Joris Koene of Vrije University in the Netherlands Hinrich Schulenberg of Tuebingen University in Germany set out to see how this evolutionary arms race has played out over millions of years. They analyzed DNA from 51 different snail species that produce love darts, which allowed them to work out how the snails are related to one another. They then compared the darts produced by each species, along with other aspects of their reproduction, such as how fast the sperm could swim and the shape of the pocket that receives the sperm.
Koene and Schulenberg found that love darts are indeed part of a grand sexual arms race. Love darts have evolved many times, initially as simple cones but then turning into elaborate harpoons in some lineages. (The picture at the end of this post shows eight love darts, in side view and cross section.) In the same species in which these ornate weapons have evolved, snails have also evolved more powerful tactics for delivering their sperm, including increasingly complex glands where the darts and hormones are produced. These aggressive tactics have evolved, it seems, in response to the evolution of female choice. Species with elaborate love darts also have spermatophore-receving organs that have long, maze-like tunnels through which the sperm have to travel. By forcing sperm to travel further, the snails can cut down the increased survival of the sperm thanks to the dart-delivered hormones.
Sexual conflict has been proposed as a driving force in the evolution of many species, and this new research (which is published free online today at BMC Evolutionary Biology) supports the idea that hermaphrodites are not immune to it. What's particularly cool about the paper is that all these attacks and counter-attacks co-vary. That is, species with more blades on their love darts tend to have longer rerpoductive tracts and more elaborate hormone-producing glands and so on. Only by comparing dozens of species were they able to find this sort of a relationship.
My wife always tells me that as a science writer, I ought to be well-prepared to give our children the talk about the birds and the bees. But I'm not sure the love darts would send quite the right message.