Last week a new species of rove beetle was described, almost two centuries after Darwin had discovered it in a little town on the coast of Argentina called Bahia Blanca. Along with a bunch of other insect specimens and some fossils, this little beetle was packaged up and shipped back to London, where it ended up lost and mislabeled in the Natural History Museum some time around 1935, and since then not a single person has given it any thought.
That was until Stylianos Chatzimanolis, an entomologist and rove beetle enthusiast at the University of Tennessee, got his hands on the collection and recognised this lone rove beetle as something extraordinary. Named Darwinilus sedarisi and described by Chatzimanolis in the latest edition of ZooKeys, the species has a brilliant blue-green iridescent hue lighting up its head and pro- (upper) thorax. There are just two known specimens, and it remains unclear whether a population still exists in Argentina or elsewhere, or if they’ve gone extinct since Darwin’s visit.
They also happen to be related to a species of rove beetle that thrives in muddy situations, both literally and sexually.
Rove beetles of the family Staphylinidae come in a variety of sizes, from 1 to 35 mm long, but they generally all have the same elongated, kind of oval, shape. Individual species can be uniform in colour, or have various segments highlighted in all kinds of colours including orange, yellow, red, blue and green.
The tropical rove beetle (Leistotrophus versicolor) from Mexico, Argentina and Brazil is on the less attractive end of the scale. It’s sort of lumpy looking, and blotchy, with black, gold, and various browns strewn all over its slightly hairy exterior. It basically looks like a shrivelled up, burnt piece of garbage. Which is fortunate, because tropical rove beetles love garbage. Dung, rotten fruit, and dead animals specifically – it’s all awesome in the eyes of a tropical rove beetle. So awesome that they just can’t live without it.
A little over 20 years ago, a couple of researchers observed the ins and outs of the tropical rove beetle societies that would sporadically form on a piece of dung or carrion. Adrian Forsyth from Queen’s University in Ontario, Canada and John Alcock from Arizona State University discovered an array of strange and pretty complex interactions taking place, including exclusion, violent bullying, female mimicry, and unwitting homosexual activity.
When dung, carrion, or rotten fruit comes to rest in tropical rove beetle territory, they’ll congregate on it and create a mini, temporary society of around 20 individuals. Here the bigger males get to mate with the females and eat all the flies that are attracted to the area, while the smaller males are routinely overpowered, mauled and promptly thrown off the mound. This puts the small males in a pretty dire situation, because at no point in their study did Alcock and Forsyth ever witness tropical rove beetles mating anywhere other than on the dung or carrion.
But not all of the small males were suckers. Some of them actively pretended to be females in order to retain access to the dung and its spoils. This is why tropical rove beetles are also known as transvestite rove beetles.
Alcock and Forsyth describe the smaller males’ nifty strategy for staying up on that mound:
“The non-combative male turned and presented his elevated abdomen tip to an approaching rival, which antennated and tapped the abdomen tip with his head. The ‘pseudofemale’ walked slowly forward, waving his abdomen from side to side while the other male followed in a manner typical of heterosexual courtship.”
And no, it’s not just you, that situation does sound vaguely sexy. Dat abdomen tip.
Usually the pseudo-female would wander far enough away from the dung that it would lose the pursuing male – and so avoid a severe mauling. Or the pursuing male would grow tired of courting the pseudo-female and bite it sharply on the abdomen instead, causing it to either run away or turn and fight. But before all this happens, things can get a bit… confused:
“While feeding on a captured fly, a male could not use his mandibles in aggressive defense unless he were to drop the prey. Likewise, when a male was pressing his head on a female’s abdomen tip, he was unable to attack an opponent without abandoning courtship. Accordingly, males that were feeding or courting employed female mimicry.”
So if a smaller male happened to find himself in a mating or eating situation, he would pretend to be female, inviting nearby males to mate with him in the process. Like, in the actual process. This way he could continue mating and eating for as long as the larger male was fooled into thinking he was a female. And you might think that the duped males are the losers in all of this, because they’re not passing on their genes no matter how hard they try, but it’s a man’s world in tropical rove beetle society, which means it’s the females who end up humiliated and forced to clear off within an inch of their lives:
“During their added minutes at the dung resource, three pseudo-females were able to capture flies, and seven other female mimics found and mated with a bona-fide female even while being courted by a deceived male. The dupe remained behind the pseudo-female during the copulation, with the female mimic aggressively chasing the female away after mating was completed.”
The reason female mimicry works so well in this particular species seems to be linked to the sporadic occurrence of dung, carrion, or rotting fruit in tropical rove beetle habitats. These are overwhelming bonanzas of mating and feeding opportunities, and Alcock and Forsyth suggest that the non-mimicking males know that they only have a limited amount of time on the dung – and in life – to mate with as many females as they can. So perhaps they just don’t have the time to be that discerning.
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