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Why Some Sexually Cannibalistic Tarantulas Don't Wait for Sex

Some females want to skip the foreplay to get to the entrée.
A member of the Lycosa genus of tarantula. Image: Joaquim Cazorla/Flickr

There are few phrases more terrifying than “sexual cannibalism," especially when it pertains to tarantulas. This family of jumbo-spiders already boasts venomous giants and jumpers capable of leaping several feet in the air. They were unnerving enough without a bunch of lady cannibals running around in their ranks, too.

But when it comes to freaking everybody out, tarantulas are not to be outdone, and a new study published in Ethology attempts to get at the heart of their bizarre cannibalistic behavior. Researchers based out of the Experimental Station of Arid Zones (EEZA) in Spain were rightly perplexed by the fact that many females don't even wait to copulate before devouring their chosen mates. Why would the females chow down before they've even gotten knocked up?

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The team, led by Rubén Rabaneda-Bueno, wanted to prove whether the personality of the female tarantulas had anything to do with the likelihood of pre-mating cannibalism. To test out the idea, they randomly selected a group of female burrowing wolf spiders—all virgins—and observed their beetle hunting behavior. Some of the females attacked prey at a much higher rate than their more timid counterparts, regardless of size differences.

Lycos sp. with babies on back. Image: Alvaro/Wikimedia

The team noted which females were more aggressive, and then proceeded to phase two: presenting the females with a bunch of randomly selected male tarantulas. The more restrained females ate some males without copulating first, but only the less desirable ones. Once those poor bastards were weeded out, the docile females copulated with the good-looking leftovers. Sure, they eventually dined on those males too, but at least the guys got laid first.

The more aggressive females, however, went on a total rampage. “While docile females attack inferior males and prefer to mate with superior males, aggressive females kill males regardless of their condition, which demonstrates their inability to distinguish males as sources of sperm or food, indiscriminately cannibalizing them," Rabaneda said in a release.

Though the result might seem obvious, it's kind of counterintuitive. "More cannibalistic females were also more voracious towards their prey, and thus the better nourished,” Rabaneda said. “This is surprising since they have more nutritive resources to invest in their offspring before finding the first males. Their priority should be ensuring the sperm's fertilization of their eggs instead of eliminating their potential donors."

The study doesn't speculate on which method is more effective, but it's hard to imagine that the more aggressive tarantulas are as successful at passing on their genes as the cooperative ones. Regardless, the team is proposing that the study proves that these freakish arachnids have evolved distinct personalities and predilections.

"We reached the conclusion that there are aggressive genetics which vary among females and make them act aggressively both when they feed off prey and when they approach a male in courting," said Jordi Moya Laraño, who directed the study. "Others are docile in both contexts, highlighting the existence of different personalities.”

Or, as the authors write in the paper, the results "tentatively support the hypothesis that female aggression levels can spill over on sexual selection through sexual cannibalism." So when it comes to tarantula reproduction, it appears that everything hinges on personal taste.