Some male wolf spiders seem to be luckier than others when it comes to courting females. The secret of their courtship? It’s complicated, says a new study led by the University of Nebraska-Lincoln.
From the elaborate mating dances of the birds of paradise to the geometric sculpting of the seabed of the white-spotted puffer fish, males in the animal kingdom have evolved all kinds of intricate behaviors to win over females.
Not to be outdone, a male Schizocosa stridulans spider encountering a receptive female will unleash an appendage-scraping, belly-twitching, leg-tickling feat that can last five minutes or 45. It’s a menagerie of sound—the rasp of fingernails scratching coarse—grainy wood, the rhythmic chatter of a mechanical ticker tape—interrupted by flashes of percussive motion.
After analyzing the input and results of 44 such appearances, the Nebraska-led team found that the nine S. stridulans males who were rewarded with sex also elicited more complex courtship cues. The finding applied to three different degrees of complexity previously used to characterize binary code, birdsong and whale sounds, but never before the vibration signals of arachnids.
“Women prefer to mate with men who can produce more complex signals than others,” said Noori Choi, lead author of the study and a recent PhD in Nebraska.
Whether in the wild or in the lab, a female S. stridulans will deposit pheromone-laden silk to let nearby men know she’s in the mood for love. A male usually opens by tasting her side and moving his pedipalps, a pair of sensory appendages near the mouth that can also hold and eject sperm. Then comes one or two leg taps – the male drums his front paws prestissimo, with blinding speed – followed by a continuous bass line of tremors in his abdomen that travel down his eight legs to the ground and finally the female, who feels them rather than hears them .
But such a showy act can take its toll, Choi said.
“We wanted to understand why the males use complex signals rather than just simple ones,” he said, “because the complex signals take a lot of energy and time to produce and can even increase the risk of (attracting) predators.”
In search of answers, Choi decided to revise an experiment his PhD student, Eileen Hebets, had led years earlier. In that experiment, Hebets’ team deposited a female S. stridulans on the filter paper of a soundproofed room a few minutes before introducing a male. The team captured the ensuing courtship using a camera and a laser vibrometer. By shining the laser on a piece of reflective tape stuck to the filter paper, and then analyzing vibration-induced changes in the frequency and other characteristics of the reflected beam, the team captured every last vibration signal a male sent a female. . The camera, meanwhile, captured all the visual blooms that came with those tremors.
Earlier studies of courtship had mainly examined parts of the whole. Choi was more interested in the complexity itself. To do this, he turned to various methods of analysis, all developed by computer scientists, which no arachnologist ever thought to use. One, which has served as the basis for data compression algorithms, essentially measures the number and repetition of different patterns within a series of signals. Another helps quantify the average amount of information conveyed by a particular signal.
“I’ve been studying complex signaling for more than 25 years,” said Hebets, Charles Bessey professor of biological sciences at Nebraska. “The progress of my work has followed the field well, where it initially focused on one sensory mode (of signaling) and then moved on to the study of multimodal signaling. Even within multimodal studies, we would look at each signaling mode. Finally started We looked at inter-signal interactions, but still treated each sensory modality and their components as separate.”
“Now we’ve come to the point, with some really talented people with quantitative skills, that we’re coming up with computational ways of looking at how all these things interact, and how the whole package can be important in ways that we’d never understand. whether we’re just looking at components A, B, or C.”
Whichever of the three statistical analyzes he applied, Choi found that the nine successful men produced more complicated vibrational signals—similar to improvising, playing the odd beat, upping the tempo—than the 35 men who were rejected.
Other findings suggested that men may also have been rewarded for paying attention. The males that managed to mate increased their signal complexity even further with heavier females, who are generally more likely to give birth to and raise a large, healthy cluster of spiders. Unlike those unlucky in love, the successful males also tended to increase the complexity of their signaling as their courtship progressed, indicating that they may be responding to signs of interest from the females.
“When you talk about spiders,” Hebets said, “I think that’s something that people don’t appreciate — that signalers pay attention to the receivers, they pay attention to their environment, and they adapt accordingly.
“We see that in many other animal groups, but people working on other animal groups are often surprised to see stories of spiders engaging in this sophisticated behavior. We’ve now found this in several studies and it really drives the point that spiders are as advanced as any other animal when it comes to communication.”
Which still leaves a fundamental question that Choi, Hebets and their fellow arachnologists are eager to answer: Why would a woman prefer a man who can, or at least is willing, to court her with a more complex signal?
Perhaps, Choi said, the complexity points to a trait in a man that a woman would like to pass on to her offspring. The team found no correlation between a man’s physical prowess and the complexity of his signaling. But the findings do tie in with an earlier study that suggested women might seek out two traits, strength and skill, needed to coordinate such a complicated courtship.
“Women aren’t necessarily looking for the biggest man or the loudest man or the strongest man,” Hebets said. “But maybe they’re looking for a guy who’s really athletic and can coordinate all these different cues in one screen.”
It’s also possible, she said, that female S. stridulans are simply better at perceiving and processing complex signals, preferring the males who produce those signals. Another, more familiar explanation? That wolf spiders, like humans, can get bored with suitors who aren’t willing to mix up the status quo every now and then.
“There are a lot of studies showing that animals, in a sense, prefer novelty,” Hebets said. “Increasing complexity, especially over time, you could almost think of as novelty — the males are constantly changing things up to maintain the females’ interest.”
The team detailed its findings in the journal Biology Letters.
These male spiders catapult at impressive speeds to flee their mate before being eaten
Noori Choi et al, Increased signaling complexity is associated with increased mating success, Biology Letters (2022). DOI: 10.1098/rsbl.2022.0052
Provided by the University of Nebraska-Lincoln
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