While the males of a jumping spider species merely threaten and posture instead of actually fighting each other, fights among females are often fatal.

Scientists have termed their willingness to fight to the death, the “desperado effect,” and they think it’s rather neatly explained by the natural history of the species, Phidippus clarus.

Nomadic males regularly encounter one another in the quest for receptive mates, but the females tend to stick close to a nest, which they need to survive their molting maturation process and to lay their eggs. When they encounter another female, the researchers argue, they have to win the fight or their ability to reproduce will be compromised.

“The bottom line is when you look at the fights between males, they are highly ritualized and the spiders rarely get injured,” said behavioral ecologist Carlos Botero of the National Evolutionary Synthesis Center in Durham, North Carolina. “If you look at females, they have few signals, are very intense, and almost every single fight, one of them dies or gets severely injured.”

Most spider-fighting research has examined the behavior of males competing for mates. The new work, published in the journal of Behavioral Ecology June 4, is one of a small set of studies that has started to look at female fighting behavior.

“When you look at most animal systems, the males are the ones doing the ritualized displays,” said biologist Eileen Hebets of the University of Nebraska. “Males are the ones with the extravagant ornaments.”

And, to a certain extent, it makes scientific sense to focus on the males’ behavior. While the results in the specific Phidippus clarus case may hold, it’s unclear how far they can be generalized. Hebets said that in many spider systems, the females don’t really fight very often. If you take two females and put them into a laboratory setting, it may create an entirely unnatural encounter with little biological significance.

“If you put two predators in a small enough space, they are certainly going to attack each other,” Hebets said.

She thought the most interesting part of the paper was the suggestion that a hormone responsible for the spider’s molting and growing to their adult bodies may also make the females more aggressive the closer they get to their transformation.

“There is work that shows a correlation between hormone levels and cannibalism in spiders,” Hebets said. “So that link is known to be there, and it is also involved in molting.”

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