Romain Boisseau

Thorny devils grow giant legs to pin rivals

PROVIDENCE—True to their name, stick insects are famous for their spindly legs and lithe brown or green bodies that let them blend in with their environments. Males are typically much smaller than females. But tree lobsters—which include New Guinea’s thorny devil (Eurycantha calcarata) and the Lord Howe Island stick insect (Dryococelus australis)—are a glaring exception. Giant, cigar-size males sport thick hind legs tipped with powerful spines. Now, researchers know why these tree lobsters bulk up: to make sure they get their gal.

Female tree lobsters can reproduce all on their own, so some researchers have proposed that males evolved their powerful legs to grab and hold unwilling mates. Others assume they use them to fight off predators. Still others wonder whether the legs are the equivalent of an elk’s rack—a weapon for fending off rivals.

Researchers studying thorny devils in Papua New Guinea soon found that males and females are at equal risk of being attacked and eaten, meaning male-only bulking would make no sense. Video evidence of sexual encounters revealed females did not resist male advances, putting a nail in the coffin of the “unwilling partner” theory. But the females’ appetite for sex—they quickly mate multiple times with multiple males—suggests the rivalry theory could explain the males’ need to be big and strong, the researchers reported here this week at the joint meeting of the American Society of Naturalists, the Society for the Study of Evolution, and the Society of Systematic Biologists.

Unlike their slimmer cousins, tree lobsters spend their days crowded into tree cavities. Females emerge after dark and hang out on the tree trunk for about an hour before heading out to hunt for food. It’s during this cocktail hour that males have their best chance to mate. So, they come out even earlier and jockey for position, sometimes fighting for the best spot. The bigger males wrap a hindleg around smaller rivals, convincing them to move on, the researchers report. And having that extra bulk really makes a difference: The bullies mate twice as often as their less macho peers.