The strange world of spider sex: it's a popular subject among researchers because, well, it will certainly keep you guessing. That's pretty much how things went recently for a team of scientists studying the Darwin's bark spider (Caerostris darwini), after discovering that males may be performing oral sex to keep females happy and - most importantly - not wondering what their mate might taste like.

Simona Kralj-Fiser, at the Slovenian Academy of Sciences and Arts, recently led a two-week field survey in Madagascar to observe these intriguing spiders in action. The Darwin's bark spider had already made a name for itself among experts as a master of web-building, constructing wide-spanning webs that are about 10 times as tough as Kevlar.

However, little was actually known about this species' reproductive strategies. This is what Kralj-Fiser and her colleagues spent their time observing, reporting the results at this year's Ethological Society's "Causes and consequences of social behavior" conference in Hamburg, Germany last week.

It turns out that like many male bugs, bark spiders are smaller opportunistic maters. They are even seen copulating with younger females whose fang cuticles and armor-like bodies have yet to harden. The males in this species develop faster in order to defend the females and compete against one another for mating rights, but their dominance does not last long.

"When a female's cuticles harden and she can move and attack, she is able to prevent long copulations," Kralj-Fiser explained to New Scientist's Zoo Logger.

The researcher reportedly explained to her peers at the conference that when the observed females reached their deadly adulthood, 76 percent of them were aggressive towards males. A stunning 35 percent even wound up cannibalizing their mates after sex. The infamous black widow spider, by comparison, actually cannibalizes their mates a mere two percent of the time.

And while recent research has found that black widow males can actually smell if a voracious mate might eat then, the bark spider doesn't seem to have this helpful 'spidey-sense.'

So how do they keep the ladies calm? They go down on them. (Scroll to read on...)

"Males nibble on female external genitals using their fangs, and then we observed that there was a liquid coming out of the fangs. We do not know what this liquid is, but it looks like digestive juices, which they usually secrete when eating," Kralj-Fiser said at the conference.

The theory is that this action helps keep the female mate calm both before and after mating, helping to ensure that the male doesn't become a post-sex snack. Supporting this theory is the fact that males don't perform this act on the younger harmless females.

However, it's important to note that there are other explanations for this behavior, too. For example, Darwin's bark spiders often mate with multiple males in their lifetime. Past research has shown that the males of some spider species will actually work to keep their mate happy, even if that mate is not cannibalistic, in order to ensure that she does not go off to mate elsewhere.

Female Leucauge mariana orb weavers also seem to enjoy a hairy kiss. If a male winds up being a good 'kisser,' stimulating his mate with thick hairs on their mouthparts, she is more likely to stick around. She will even sometimes help him form a "genital plug," which essentially prevents other males from having sex with a claimed female spider. It's a strange way of tying the knot, but experts have long compared this kind of mutual plug formation to marriage.

And that could also be what the bark spider's "oral sex" act is really all about. The liquid a male secretes may not be a lubricant at all, but instead a kind of glue, helping facilitate the creation of a plug that ensures the mother of his children doesn't go running off with another man.

However, much more observation and research will be needed to determine what, if not all, of these theories hold true.

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