Male wolf spiders that engage in a complex “dance” performance when they encounter a receptive female are more successful in courtship, a new study has found.
Across the animal kingdom, creatures have evolved intricate rituals to win over mates, ranging from elaborate dances of birds-of-paradise to the geometric seafloor-sculpting of the white-spotted pufferfish.
In new research, published in the journal Biology Letters, scientists, including those form the University of Nebraska-Lincoln in the US, found that male wolf spiders (Schizocosa stridulans) encountering a receptive female would unleash an “appendage-scraping, abdomen-quivering, leg-tapping” performance.
Scientists assessed the inputs and outcomes of 44 such performances and found the “dance” produces a range of sounds such as the rasp of fingernails scratching coarse-grained wood, the rhythmic rattle of a mechanical ticker tape, punctuated by flashes of percussive movement.
Researchers discovered that the nine S stridulans males rewarded with sex were the ones who came up with more complex courtship signals.
When a female spider of the species is ready to mate, scientists say it deposits pheromone-laden silk to let nearby males know.
Males then taste the silk and open their pedipalps – a pair of sensory appendages near the mouth that can also hold and eject sperm.
Following this, researchers say the male begins drumming its feet with one or two taps initially, and then its forelegs prestissimo, with blinding speed.
This is then followed by a continual bassline of tremors in the male spider’s abdomen that transmits through its eight legs to the ground and ultimately the female, which feels them rather than hears them.
“Females prefer to mate with males that can produce more complex signals than others,” Noori Choi, lead author of the study, explained.
However, scientists said the complex performance can take its toll on the male spider.
“We wanted to understand why the males use complex signals instead of just simple ones, because the complex signals take a lot of energy and time to produce and can even increase the risk of (attracting) predators,” Mr Choi said.
In the research, scientists conducted an experiment in which they deposited a female S stridulans onto the filter paper of a soundproof chamber a few minutes before introducing a male.
They then recorded the ensuing courtship using a camera and a laser vibrometer.
By shining laser down at a piece of reflective tape stuck to the filter paper, scientists could analyse vibration-driven changes in the frequency and other signatures of the reflected beam.
This way, they captured “every last vibratory signal” that a male sent a female’s way. The camera also caught any visual flourishes accompanying those vibrations.
While earlier studies of the mating spider examined parts of the courtship, the new research looked at the complexity of the whole process.
“Now we’re at the point, with some really talented people who have quantitative skills, of coming up with computational ways to look at how all of these things might interact, and how the entire package might be important in ways that we would never understand if we were just looking at components A, B or C,” said Eileen Hebets, another co-author of the study.
Researchers found that the nine successful males produced more complicated vibratory signals than the 35 males that were turned down.
They said the intricate moves pointed to the spiders improvising, playing the off-beats, and changing up the tempo.
Scientists say males that managed to mate increased their signal complexity even further with heavier females, likely as they had more chances of birthing and rearing a large cluster of spiderlings.
Successful males also notched up the complexity of their signaling as their courtship went on, suggesting they may have been responding to signs of interest from the females.
“I think that’s something people don’t tend to appreciate — that signalers are paying attention to the receivers, they’re paying attention to their environment, and they’re adjusting accordingly,” Dr Hebets said.
“We’ve found this now in several studies, and it really drives home the point that spiders are just as sophisticated as any other animal when you’re talking about communication,” she said.
Researchers suspect the complexity in the moves points to qualities in a male that a female spider would like to see passed along to its offspring.
“Females aren’t necessarily looking for the biggest male or the loudest male or the strongest male. But maybe they’re looking for a male that is really athletic and can coordinate all of these different signals into one display,” Dr Hebets said.
It may also be possible that female S stridulans could be better at perceiving and processing complex signals, leading them to prefer males producing such signals, scientists say.
“Increasing complexity, especially through time, you could almost think of as novelty — the males constantly changing things up to keep the females’ interest,” Dr Hebets added.