Wolf spiders and jumping spiders don’t weave webs, relying instead on their excellent vision to capture prey. The males carry on a courting dialog using vibration and, in some species, by waving their arms in a sort of sexual semaphore. Scientists wondered what would happen if a female watched a video of a male courting. Many animalspigeons and bees, for examplesee video as a confusing slide show rather than a smooth movie. But wolf spiders, it turns out, not only perceive video on even tiny TV screens smoothly, they act like it’s real. University of Cincinnati behavioral ecologist George W. Uetz and colleague David L. Clark showed videos of potential mates to wolf spiders. The spiders replied to the sexual signal, waving their legs and rotating their bodies.
Uetz has studied two closely related wolf spider species. One, called Schizocosa rovneri, uses only vibration to communicate its courtship messages. Females of this species ignore any leg waving. The other, called Schizocosa ocreata, uses both vibration and semaphore. Although the two species live in the same forests, their microhabitats differ. Semaphore spiders live in loose leaf litter, which dampens their vibratory signals. Loose leaves don’t affect light, however, and the leg waving apparently allows the male ocreata to send his message over longer distances. To enhance its semaphore ability, the species sports small flagstufts of hairson the forelegs to make them more visible.
“We wondered if the visual courtship displays and tufts on the forelegs of Schizocosa ocreata evolved as a means of increasing the detectability of their signal.” Uetz says. So he and a student shaved the males’ legs to see if the female would still respond. Shaving slashed the male’s success in winning mates only when the female could see but not hear him. But the shaving operation itself may have changed the behavior of the males, Uetz worried.
Being able to send spider signals in video gave Uetz and colleagues the key to addressing Uetz’s concern and answering his question. Using techniques borrowed from high-tech Hollywood movies, Uetz digitally shaved a virtual spider’s legs, then presented the video to a female of the same species. She failed to respond, clinching the conclusion that females pay close attention to the spider’s semaphore.
Then Uetz tried one more manipulation. He made videos of male Schizocosa rovneri spidersthe ones that don’t semaphore while courtingwaving their forelegs and showed the tapes to females of the same species. As expected, the females showed no interest. Then Uetz digitally added tufts to virtual Schizocosa rovneri forelegs and played the videos to females. This time they responded. Uetz guesses that the semaphoring ocreata split evolutionarily from the older rovneri, evolving semaphore signaling to compensate for the move to the loose-leaf-litter habitat. “Using the video method, we are able to test this hypothesis about how evolution might have happened by adding traits to a species that doesn’t have them. “The visual courtship displays exploit the female’s sensory bias (visual prey detection mechanism), allowing males to identify themselves; the tufts make their message easier to detect,” Uetz concludes.
[1996, edited 04 25 03]