Shouldering: Penis Extraction in Rove Beetles

I don’t “have a thing” for critters with remarkable genitalia. (I swear.) But, while researching barnacle sex, I came across a paper about a male beetle with an intromittant organ (penis) so long and flexible that he has to sling it over his shoulder to keep it safe. Clearly, I couldn’t keep such information to myself.

Meet Aleochara tristis, a rove beetle native to Europe* that typically lives, mates and reproduces on cow poop.

This unassuming poop-populating beetle has what scientists call “exaggerated genitalia.” When fully extended, the male’s flagellum (essentially a guiding rod for the sperm delivery system known as a spermatophore tube) is more than twice the length of the male beetle’s body. While impressive (and disturbing), living with such a long schlong isn’t nearly as glorious as it may sound.

Like any male beetle, A. tristis guys want to maximize paternity by mating with as many females as possible. A male needs that excessively long flagellum to navigate the female rove beetle’s equally long, coiled reproductive tract, but before he can mate with another female, he’s gotta get his rather unwieldy flagellum out of his first lover.

Figure 2. Gack and Peschke, 310

He begins by slowly separating himself from his mate, exposing just a bit of the flagellum, keeping it under tension. He tucks the flagellum between his mesothorax and prothorax (parts of his shoulder) and then backs farther away from the female so that half of the flagellum is free. With the flagellum held taut in his shoulder, the male turns away from the female to extract the rest of the flagellum and then carefully coils it into the aedeagus (the holster for the flagellum and spermatophore tube). If he extracts and stores his equipment properly, he can mate up to five times in one hour.

If he doesn’t follow this innate penis extraction protocol, his flagellum will end up in a tangled (and most likely useless) mess. His baby-making prospects will dwindle and his chances with the lady rove beetles will be shot.

Although awkward and high maintenance, the long flagellum evolved for a reason: to win the sperm wars. The ultra-long flagellum enables a male rove beetle to deposit his sperm right outside the female’s spermatheca (sperm storage site), at the far end of the female’s coiled reproductive tract. Scientists suspect that the ability of a male to essentially leave his sperm on the female’s doorstep increases the male’s chances of fatherhood.

*A. tristis was introduced to the United States in 1965 to control another introduced species: the face fly, named for its habit of collecting on cows’ faces.

CLAUDIA GACK*, & KLAUS PESCHKE (2005). ‘Shouldering’ exaggerated genitalia: a unique behavioural adaptation for the retraction of the elongate intromittant organ by the male rove beetle (Aleochara tristis Gravenhorst) Biological Journal of the Linnean Society, 84, 307-312