Across the animal world there are examples of creatures who use extremely quick speeds to capture prey — or avoid being captured.
Now, for the first time, scientists have observed the use of a superfast action in a very specific scenario: as a way to escape sexual cannibalism.
Scientists discovered male spiders of the species Philoponella prominens can survive encounters with aggressive females because of a unique ability to catapult away. This is facilitated by a newly identified capability described Monday in the journal Current Biology.
Through the use of high-resolution video cameras, the scientists found these spiders initiate their catapult maneuver by folding their front legs against the female. When they release themselves from this fold, the ensuing release of hydraulic pressure causes the legs to rapidly expand, sending the males soaring in a clockwise rotation.
During these catapults, the spiders accelerate so quickly that the action can’t be captured in detail by common cameras. During this display, the males can spin up to 175 times per second during the flight.
Shichang Zhang, a professor at Hubei University in China and the study’s first author, said he first observed this catapulting when observing these spiders out in nature, and he wanted to examine how the action was associated with sexual conflict (when two sexes have conflicting reproduction strategies).
This type of spider is also unusual because it lives in groups of up to 300 individuals. They live together in a mass of linked webs, while maintaining their own individual webs within this complex.
“It is a communal spider, which is not common in nature,” Zhang said. “Most web-building spiders build webs solitarily.”
In his lab, Zhang and his team observed that 152 out of 155 successful spider matings ended with male catapulting. The males that did not jump were killed and eaten. This escape doesn’t occur if the males “don’t sense the approaching danger, or they are exhausted,” Zhang said.
The males that do spring away often return to the scene to try again. Some males end up catapulting several times after their 30-second tries.
To examine the importance of catapulting, the scientists prevented 30 male spiders from catapulting by placing a fine brush on the back of the spiders. All of these spiders were then killed and eaten by the female.
This made it clear that catapulting was essential for survival. To understand how it happens, the team then conducted a series of experiments involving the spiders’ legs. For example, when they removed the spiders’ first pair of forelegs, the male spiders still approached the females — but couldn’t mate. When the scientists removed one or two other legs, mating and catapulting continued as usual.
Further examination with cameras revealed catapulting is possible because the male spider folds its first pair of forelegs against the body of the female spider, and the rapid subsequent extension allows it to spring away.
“This is a well-conducted study presenting a fascinating example of a male trait that most likely evolved as a consequence of sexual conflict due to sexual cannibalism,” said Jutta Schneider, head of the behavioral biology research unit at the University of Hamburg in Germany and who was not a part of the new study.
While sexual cannibalism is generally a rarity, it is common across spiders and scorpions. In turn, a number of male spiders have evolved different means of escape. For example, Schneider found that the males of the golden orb-weaving spider Trichonephila fenestrata “sacrifice their front legs during mating to keep the female occupied,” Schneider said.
Overall, spider sexual canniblism can happen before, during or after mating. Female spiders benefit from eating their partners because it either weeds out unfit males or prevents the best males from mating with other females. Schneider explained that in these cases, the female may just eat the male as well as allow him to live on — although the meal is considered a small one.
“Female cannibalism of males is common in many spider species,” said Eileen Hebets, a University of Nebraska-Lincoln professor who has studied mating preferences in spiders.
“There are all kinds of great stories of strategies male spiders have come up with to escape being eaten by their mates,” Hebets said. “Some males can induce acquiescence in their female partner — for example, knock her out — to escape. Others tie the females up with silk. Others come bearing gifts, like a silk-wrapped prey item, to presumably occupy the female while they attempt to mate.”
In this case, it’s theorized that male Philoponella prominens can jump because of the evolutionary pressures put on them through sexual canniblism. It’s yet certain, but it’s possible that the female spiders may have responded in turn — and now use catapulting as a way to judge the physical fitness of the male.
When Zhang is in the field looking at spiders, people will come over to ask what he and his team are doing. When they reveal they are collecting spiders, “most of them will look at us like we are freaks,” Zhang said. This attitude, he feels, is a mistake. There is much we can learn from spiders, especially when it comes to understanding the relationship between evolution and sex.
“Spiders are not horrible at all,” Zhang said. “If you know them, you will love them.”