This Male Spider Catapults Itself Into the Air to Avoid Sexual Cannibalism | Science
this male spider catapults itself into the air to avoid

This Male Spider Catapults Itself Into the Air to Avoid Sexual Cannibalism | Science

Philoponella prominens Spiders

A male Philoponella prominens spider (top) mates with a female.
Shichang Zhang

A bad date can be an unmitigated disaster of awkwardness for humans, but for male spiders it often means being eaten alive by the much larger, hungry female. To avoid being cannibalized after sex, the males of some species have resorted to ripping off their own legs to distract their beloved with a bit of food, or even to tying up the female’s legs with silk before mating with her.

But males of the orb-weaving spider Philoponella prominens opt for another strategy that might appeal to anyone who has ever wished for an eject button during a date that’s gone downhill fast. According to research published today in Current Biology, males use their two front legs to catapult themselves to safety as soon as they’ve mated.

The males that don’t launch themselves out of their lover’s embrace in time suffer a grisly fate. Philoponella prominens is part of a family known as the hackled orb weavers. These spiders don’t have venom to help them kill or immobilize their prey. So instead of a quick, venom-induced death, the females mummify the less spry spider-men so tightly that their legs break and they are either crushed to death or suffocate. A 2006 study of another spider in this family found that this crushing wrap-job required some 450 feet of silk.

Shichang Zhang, an arachnologist at Hubei University and lead author of the study, writes in an email that he and his colleagues were studying the sexual behavior of this species, which lives in communal webs of up to 300 individuals, when they noticed tiny males (each one less than a quarter inch long) bounced away from romantic encounters with females.

Zhang wanted to understand three concepts: why the males were jumping off the females; which body parts the males were using to do it; and what kinds of speed and acceleration they managed to achieve. To do this, he brought high-speed cameras and a lot of P. prominens spiders into the lab.

The team observed 155 instances of successful spider mating, and all but three of the males involved flung themselves as far away from the female as they could once mating concluded. The three males that opted to stick around were all subsequently killed and consumed by their sexual partners.

To see what happened if the males couldn’t jump away, the researchers first blocked the ability of 30 male spiders to launch themselves. All 30 of these males were summarily eaten, bolstering the idea that this behavior on the part of males helps them survive mating and mate again.

To zoom in on the mechanics of how these males were flinging themselves, Zhang and his co-authors filmed the spiders mating using cameras capable of filming at 1,500 frames per second. For these spiders, mating takes about 30 seconds, and Zhang says filming such small creatures during such a fleeting act proved challenging.

“Once the spider was mating, people had to adjust the equipment to focus on them,” writes Zhang. “The spider is tiny, so most of the time, males had catapulted before the focus was ready.”

These high-speed, high-resolution videos eventually revealed that the male spiders appeared to be catapulting themselves by folding their two front legs against the female’s body at the spider-equivalent of the knee joint, and then suddenly extending the legs when it was time to leave. Zhang likened the movement to a competitive swimmer starting a backstroke race. The swimmer starts with their legs folded and braced against the pool wall and then they explode backwards when the race starts.

Computer analysis of the high-speed videos revealed that the spiders reached speeds of up to 2.9 feet per second and clocked a peak acceleration of 1734.5 feet per second squared. If the spiders sustained that level of acceleration for longer than the average four millisecond duration the researchers recorded for these jumps, the tiny arachnids would hit 60 miles per hour in about 0.05 seconds. The male spiders also spun a lot while flying through the air—about 174 times per second on average—but it’s unclear what function, if any, that might serve.

To further test whether the male spiders were using their front legs to accomplish their explosive retreat, the researchers set up a series of experiments. In one test, the team removed one or both of the two front legs. Losing one or both forelegs prevented all 60 males involved in the experiment from mating. The suitors courted females, but made no attempts to mount them and consummate the flirtation.

In a second test, researchers removed one or two of the spider’s other six legs, which had no apparent ill effects on mating success or their ability to catapult away—the 20 spiders missing one leg and the 20 spiders missing two legs all mated successfully and leapt to safety.

For the third test the researchers made a tiny hole in the back of the spider’s “knee” joint with a needle. The researchers did this because spiders actually have no muscles responsible for extending their legs, let alone producing the kinds of forces required to launch their owner skyward. To extend their legs, and even to leap into the air like jumping spiders do, these eight-legged lover boys rely on hydraulic pressure.

“Spiders have a big muscle in their thorax and when they compress it they can shoot their body fluid into their legs and cause them to straighten really fast,” says Jonathan Coddington, the Smithsonian National Museum of Natural History’s curator of Arachnida and Myriapoda.

So, when the researchers pierced the spiders’ legs it likely became impossible for them to create enough hydraulic pressure to extend the leg and complete their post-coital launch. All 15 of the males that got this treatment made no attempts to mate with or court the females, instead keeping their distance.

“Jumping spiders use their back four legs to jump, what’s odd about these guys is that the males are using their first pair of legs to shoot themselves into the air,” says Coddington, who was not involved in the study. “This is surprising to see in an orb weaver.”

Saad Bhamla, a biophysicist at Georgia Tech who was not involved in the research, says he finds it “fascinating that they can do this without muscles, just with fluid pressure.” In such a small spider, each leg is likely finer than a human hair, Bhamla says, yet also hollow and able to create a lot of force using only microfluidic action.

Zhang says the study’s findings suggest that this catapulting behavior on the part of males evolved as an adaptation to the female spiders’ proclivities for sexual cannibalism. He adds that one male was able to perform the behavior six times over the course of about eight hours, but was finally killed and eaten after his final performance–perhaps being too exhausted to spring himself to safety. According to the paper, the bounciest males might father more spiderlings by mating with the same female multiple times without being eaten, or by seeking out additional mates.

“This study made me smile and remember that spiders are eternally cool,” says Bhamla. “Spiderman is boring compared to what these spiders are doing.”

Animals

Natural History Museum

Sex

Smithsonian Institution

Weird Animals

wildlife

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