Cannibalism: A Perfectly Natural History(16)

In some land snails, however, things get bizarre even before the member-munching starts—especially once the partners begin shooting calcified “love darts” at each other, an exchange initiated when the body of one snail touches that of a potential mate. This tactile stimulation triggers the release of built-up hydraulic pressure in a sac surrounding the dart. As a result, the barbed projectile (known as a gypsobelum) explodes outward, embedding itself in the body wall of the second individual. In most instances, the skewered snail responds by shooting a dart of its own, and shortly thereafter the couple appear to remember why they had gotten together in the first place.



Often, though, the exchange proceeds with something less than textbook precision. Since most snails are nocturnal, their visual systems are simple. They can differentiate between light and dark, but an inability to determine details about their slimy targets (or anything else, for that matter) can lead to a serious lack of accuracy. As a result, headshots and similar misfirings are a common occurrence.

The obvious question is: Why do some snails fire miniature harpoons at each other? The proposed function of this behavior (which is rarely observed in humans anymore!) has undergone some revision. Earlier snail experts thought that love darts were the equivalent of an exchange of wedding gifts—in this instance, calcium carbonate, a major component of the snails’ shell and eggs. Another suggestion was that the projectiles might act as an aphrodisiac or that they somehow signaled the shooter’s willingness to mate. But support for these hypotheses never materialized.

I posed the question to McGill University biologist Ronald Chase, whose work in the 1990s helped solve the mystery of this baffling behavior. “The darts serve to increase paternity,” he told me, since snails scoring love dart hits on their partners before mating fathered twice as many offspring as those that didn’t hit their targets. The key to the enhanced reproductive effect was the tiny projectile’s chemical coating. Chase and his colleagues showed that this hormonelike substance prevented digestive enzymes from destroying the majority of incoming sperm, something that occurred in non-skewered snails. Spared the buzzkill of being digested, the snail sperm sped onward, eventually fertilizing a greater number of eggs than those that wound up in non-speared snails.

Additionally, a 2013 study by Japanese researchers showed that snails skewered by love darts delayed re-mating with other individuals, an indication that something in the dart’s mucous coating suppressed subsequent mating behavior—thus reducing the possibility that another male’s sperm would outcompete the dart shooter’s.

According to Chase, “It’s all basically sexual selection.” In other words, in any given population, some individuals outproduce other individuals because they’re better at securing mates, usually by making themselves more attractive to the opposite sex or by beating back the competition. In land snails, explanations for who got the edge and how they achieved it are confounded by the fact that mating individuals not only exchange sperm with each other, but explosive projectiles as well.

Before leaving the topic of snails, if all this talk about love darts has you thinking about one of our most endearing holiday characters, you aren’t alone. Ronald Chase believes that Cupid, the Roman version of the Ancient Greek god Eros, had his origin in land snails and their love darts.

“I think that the Cupid myth arose from Ancient Greeks observing snails mating and shooting love darts,” Chase explained. “The species that we worked on in our experiments is found in Greece and I’m sure they shoot love darts over there as well.” Ever the scientist, Chase added that there was no hard evidence yet and that neither he nor his students was able to find images of snails shooting love darts on Ancient Greek coins or pottery. Personally, I’ve always been a bit creeped out by the idea of a nude, weapon-wielding infant with wings, but considering that the Greeks could have equipped him with turret eyes and a slime trail, I’m willing to cut the current incarnation some slack.





4: Quit Crowding Me


Hunger has its own logic.

— Bertolt Brecht

Overcrowded conditions often coincide with another of Gary Polis’s cannibalism-related generalizations, namely that incidents of cannibalism increase with hunger and with a decrease in the availability of alternative forms of nutrition, a point that will become horribly clear once we begin our investigation of human cannibalism.

Carrying the banner (albeit a tiny one) for crowd-related cannibalism are the Mormon crickets (Anabrus simplex). These insects are native to the North American West and belong to the order Orthoptera, which contains grasshoppers, crickets, and locusts. The fact that A. simplex is actually a form of jumbo katydid also makes them members of an unofficial assemblage composed of misnamed animals like “flying foxes” (which aren’t foxes) and “tree shrews” (guess).9 Attaining a body length of nearly three inches, Mormon crickets are flightless, but like their winged cousins, the grasshoppers and locusts, they’re renowned for their spectacular swarming behavior and mass migrations. According to biologist and Mormon cricket expert Stephen Simpson, favorable early spring conditions like warm weather and moisture can lead to the nearly simultaneous hatching of several million individuals. Almost immediately, the nymphs begin to march, and they do so in a spectacularly well-coordinated manner.

I asked Simpson why Mormon crickets participated in such large-scale movements. He cited studies showing that individuals separated from their swarm suffered 50 to 60 percent mortality from predators. “They got eaten by birds, rodents, and spiders if separated but were safe from predation in a crowd.”