Monday, May 9, 2022

Male spiders use catapult mechanism to avoid sexual cannibalism

Close-up of communal orb-weaving spiders mating.
Photo credit: Shichang Zhang

In the animal word, numerous mechanisms have been described that allow for extremely fast actions or reactions via the slow storage of energy, typically in elastic structures which is then nearly instantly released, similar to the operation of a catapult. Many of these mechanisms are employed for prey capture or predator avoidance, however such superfast actions have not yet been reported as a means to dodge sexual mechanism.

Associate Professor Li Daiqin from the National University of Singapore (NUS) and a team of scientists have discovered a mechanism in the legs of male spiders enabling them to undertake a split-second catapult action immediately after mating to avoid being cannibalized by their partner. This is the first time a catapult mechanism to escape sexual cannibalism has been observed in any animal.

The scientists demonstrated that male communal orb-weaving spiders (Philoponella prominens (Family: Uloboridae)) activate the catapult mechanism by extending a joint that lacks extensor muscles, called the tibia-metatarsus, on their forelegs via hydraulic pressure. The rapid expansion of the legs greatly reduces the likelihood of the male being sexually cannibalized.

“Our novel discovery shows that the catapulting behavior is a direct adaptation to female cannibalism among communal orb-weaving spiders. Males with superior locomotory performance and physical capacity may be able to perform the rapid catapulting behavior multiple times, thereby increasing their chance of paternity. This creates the stage of coevolution in the male and female spiders, where females can afford high levels of sexual cannibalism while still having mating opportunities as some males are able to escape from being killed,” said Assoc Prof Li who is from NUS Biological Sciences.

The findings were first published in the journal Current Biology.

Catapulting to safety

Close-up sequence of male communal orb-weaving spider catapulting away from female.
Photo credit: Shichang Zhang

Sexual cannibalism is observed among spiders and insects, such as praying mantises, for a variety of reasons. Females may cannibalize males as a means to choose high quality mates or obtain nutrients. As a way of escaping the femme fatales of their species, the male communal orb-weaving spider has a special catapulting mechanism that allows them to escape being cannibalized after mating.

The researchers conducted 155 successful mating, among which 152 males performed the catapulting behavior after their first mating and survived. However, three males that did not perform the catapulting behavior were captured, killed and consumed by the females. In addition, the scientists found that male spiders which were physically blocked from catapulting were eventually consumed by the females.

As an additional adaptation to improve their chances of paternity, the male spiders spun silk “safety lines” that were connected to the females’ web which prevented them from falling to the ground, and allowed them to climb back to the web for further mating attempts. A male can mate up to six times with the same female.

Harnessing leg power

Close-up of the tibia-metatharsus joint with the theca identified in the red box.
Photo credit: Shichang Zhang

The researchers identified that the tibia-metatarsus joint of the spiders’ forelegs was crucial in triggering the catapulting behavior. During mating, the angle of the males’ tibia-metatarsus joint, which is the second joint from the front of the leg, was about 56.8° and extended to almost 180° during post-mating catapulting through hydraulic pressure.

The legs of the spiders are also covered by thecae, which are enveloping sheaths covering the joints with elasticity for extension and contraction. In particular, the surface area of the theca in the tibia-metatarsus joint for each leg is significantly larger than any of the other joints, ranging from 1.8-110 times in size. Adding to this, theca surface area on the tibia-metatarsus joint of the forelegs is significantly larger, up to 5 times, than that in other legs.

The researchers found that the male spiders were unable to successfully mate with the females without the first pair of legs, or if they had damaged theca in the tibia-metatarsus joint of their forelegs. However, the male spiders were able to mate successfully and catapult away without one or two of their other legs.

Remarkable speeds and accelerations were achieved by the catapulting males. The main peak speed was up to 88.3cm per second, peak acceleration went up to 528.7m per second squared, and the spin rate went up to 469 revolutions per second.

Further research

The researchers observed that the theca surface area of tibia-metatarsus joints in the forelegs of the male spiders varied. However, it is unknown whether this variation correlates to the kinematic performance of the catapulting. Based on this observation, potential future research could investigate the relationship between theca surface area, kinematic performance of catapulting, and male fitness.

Source/Credit: National University of Singapore

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