Animal of the Week May 7, 2014 -- look at her weiner

This week's animal of the week is Neotrogla, a whole genus of insects (at least four species but probably more—with 10 million species of animal out there, I've got to speed this thing up) in which the females have penis like appendages and the males have vagina like apparatus.

In almost all known animals with intromittent sexual organs, it's the male that does the insertion. But in the case of Neotrogla, Brazilian bark lice that live out their lives in lightless caves feeding on bat guano, the females have evolved the gynosome, an organ used to siphon sperm out of the males' phallosomes. If you are now wondering what defines a male and a female if not their sexual organs, it's actually the size of the sex cells—females produce fewer larger eggs, males produce many smaller sperm.

Girl's on top

In the list of things I find fascinating, sex is pretty high up there—thankfully, a lot of evolutionary biology is either directly or indirectly concerned with the study of sex. When it comes down to it, evolution is about reproduction: survival of the fittest is nothing if the fit genes don't get passed onto successive generations.

Why Neotrogla evolved this role-reversal is unknown. In the nutrient poor caves, the sperm might provide not only genetic material but also vital nutrients without which the females cannot reproduce, so creating competition for access to this resource which the unique arrangement somehow facilitates.

Of course, this also raises the question (although it's not one that keeps me awake at night) of why some male animals have penises? In some insects they are used to aid in competing with other males for delivering sperm: sometimes they are used to scrape out other males' offerings, and in extreme cases, they break off after mating and block any other male from inseminating a female. In mammals, size seems to correlate with the number of males competing for the attention of or access to females. The human penis is one of the oddest—larger in every dimension than those of our close relatives chimps, bonobos and gorillas, they also lack the baculum or penis bone that these species use to achieve erection and instead use blood pumped into spongy tissue. The human penis may not only serve a practical function, but also might be a visual sexual display.

A male penis is clearly an efficient mechanism of introducing sperm to eggs, but Neotrogla reminds us there is more than on way to achieve this, and excites my wholly theoretical interest in the subject.