Ecology
Male spiders are losing their reproductive organs, although they do not lose their masculinity at all and are better able to cope with rivals on their territory, scientists say. Professor Daiqin Li from the National University of Singapore and his colleagues have published their findings in the journal Biology Letters.
“Male eunuchs are better at fighting because they get lighter and because of their increased endurance,” said Lee, an expert in animal behavior. “They are trying to discourage other males from mating with females.”
In a large number of spider species, females are much larger than males and are more aggressive.
Lee and his colleagues studied one of the tropical spider species Nephilengys malabarensis (pictured), in which a female eats a male during mating. Scientists are not puzzled by this, but by the fact that after intercourse, a male can dodge the tenacious paws of a voracious girlfriend and leave one of his palps containing sperm in her body.
Palps, which look a bit like boxing gloves, have a tube that males insert into the female's body and through which sperm passes. When a palpa breaks, it stays in the female's body and blocks her genital tract.
This has led some scientists to think that spiders intentionally castrate themselves to create what is known as a copulatory plug that allows sperm not to leak out and act after intercourse completed and the male is in hiding. At the same time, the cork prevents other males from mating with this female.
However, other males can easily remove the copulatory plug, Lee said, so it's odd that males do castrate themselves. Scientists have noticed that 25 percent of males who manage to survive such an inhumane mating process are better at fighting.
“The palps are quite heavy in terms of the spider's body weight,” says Lee. “If one or two palms separate, the spider becomes much lighter than the unharmed spider and can run faster and longer.”
Lee and his colleagues tested this hypothesis by removing one or two fingers from spiders. The palps themselves have such a structure as to break quickly and easily, so scientists easily broke off these organs with tweezers. They weighed spiders with and without palps, comparing the data.
They then tested the endurance of three types of spiders, encouraging them to run in circles, and measured the time it took them to get tired. Scientists found that castrated spiders were 9 percent lighter than unharmed spiders, and those with only one finger removed were 4 percent lighter.
They also found that spiders completely devoid of palps were 80 percent more resilient than unharmed males, and those with only one finger removed were 32 percent more resilient. Experiments have shown that unharmed spiders got tired faster than castrated spiders, which also confirms a new hypothesis about why spiders get rid of palps.