Spiders Risk Exposure for Love

Spiders that pretend to be ants to fool predators have an unusual problem when it comes to sex. How do they get the attention of potential mates without “breaking character” to birds that want to eat them?

University of Cincinnati biologists say evolution might provide an elegant solution. Viewed from above, the mimics look like skinny, three-segmented ants to fool predators. But in profile, the adult mimics retain their more voluptuous and alluring spider figure to woo nearby mates.

Most birds avoid ants and their painful stingers, sharp mandibles and habit of showing up with lots of friends. Try to eat one and you’re likely to get chewed on by 10 more. That’s why nearly every insect family from beetles to mantises has species that mimic ants.

By comparison, spiders are delicious and nutritious, said Alexis Dodson, a UC doctoral student and lead author. “That’s what a lot of natural selection is all about — to convince other species not to eat you and convince members of your species to mate with you and to do so at the least cost possible,” Dodson said.

Alexis Dodson

Lots of insects and arachnids mimic ants because they’re so formidable. Some plants, too, have evolved a mutually beneficial relationship with aggressive ants to discourage hungry leaf-eaters.

Nathan Morehouse, assistant professor of biological sciences in UC’s McMicken College of Arts and Sciences, will use a $2 million National Science Foundation grant to study spider vision around the world. But for this study, he didn’t have to go far. He and his students collected mimic spiders by spreading a sheet under trees and whacking limbs at UC’s wooded Center for Field Studies a few miles off campus.

Spiders occupy a three-dimensional world. But whether they’re on the ground or climbing a tree, potential predators are likely to get a dorsal view. “Thinking of vantage point is essential,” Morehouse said. “From the top juveniles and adults both look like ants. And juvenile spiders look very much like ants from the side. But adult spiders shift away from the ant profile toward a more traditional spider-like profile.”

But it’s not enough to look like an ant, Morehouse said. To fool clever predators, you have to act like one, too. The spiders have enormous back legs like ants. Spiders have an extra pair of legs compared to ants and no antennae. But ant mimics will wave their small forelegs in the air like ant antennae.

“The level of mimicry we encounter in jumping spiders is incredibly detailed,” he said. “When ants follow a trail, they weave their heads back and forth. The ant is trying to cast back and forth over a chemical trail that’s hard to find.”

“Remarkably, jumping spiders also perform this weaving behavior even though it has no functional significance for them,” Morehouse said. “They’re trying to be convincing actors. They’re trying to look like an ant.”