evolgen

Spring is in the air. The obvious signs are everywhere: the temperature is rising, the flowers are blooming, and everyone's writing about boners. There's this post from Darren Naish on turtle gonads, and Carl Zimmer has an article on duck phalluses (don't call them penises, as Darren explains) in the NYTimes (and a blog post advertising it). Carl's article summarizes the findings of this study. The basic story is that some ducks have really big penis-like structures (penises are unique to mammals, so a bird schlong is called a phallus), and the females have crazy reproductive organs to match. The following passage captures the tone and content of the piece:

The champion phallus from this Meller's duck is a long, spiraling tentacle. Some ducks grow phalluses as long as their entire body. In the fall, the genitalia will disappear, only to reappear next spring.

Not only does this lead one to recall mid-90s novelty songs, but it also spawned an interesting research project:

Gazing at the enormous organs, she asked herself a question that apparently no one had asked before.

"So what does the female look like?" she said. "Obviously you can't have something like that without some place to put it in. You need a garage to park the car."

When you pull that thing into your garage, does your garage say, "Is it in yet?"

All this talk about mad-crazy-long reproductive structures causes one to think about Drosophila. Well, it causes me to think about Drosophila. Zimmer points out that run-away genital size has coevolved in various invertebrates, but this is the first example from the backboned buggers.

But what about those insane Drosophila genitals? In case you're curious, I have some illustrations and measurements below.

Drosophila males don't have penises, phalluses, or other disproportioned external genitalia. There are some interesting genital shapes amongst drosophilids, but it's not the size that counts in flies (yeah, keep telling yourself that). It's the shape. And the length of the sperm. Oh my god, the length of the sperm. Check out this table from a great paper on sperm length in Drosophila:

Click to embiggen.

I've taken the liberty of emboxening (new word) in red the champion of Drosophila sperm: D. bifurca. At an astounding 58.29 millimeters (see here for more information), these guys produce sperm over one hundred times longer than some other species (eg, D. pseudoobscura emboxened in blue).

Scott Pitnick has been at the forefront of the study of Drosophila sperm for a few years. To the right is a picture from Pitnick showing a D. bifurca male encircled by the testes of another D. bifurca male. These testes produce sperm measuring over 20 times as long as the fly itself. As a point of contrast, human males produce sperm measuring one hundredth of a millimeter -- shorter than the D. pseudoobscura sperm and way shorter than the D. bifurca sperm. If human males produced sperm proportional to the size of D. bifurca sperm, it would stretch 40 yards. That's nearly half a football field. That's one huge gamete.

Long sperm not only mean long testes, they also require long seminal receptacles in females. Here's an illustration from another one of Pitnick's papers:

Click to embiggen.

D. pseudoobscura males make really short sperm (0.36mm), and the females have a tiny seminal receptacle (only 0.41mm). D. bifurca males, on the other hand, produce gigantic sperm, so the females need a really long seminal receptacle (81.67mm). Sort of like the ducks with long phallus and females with large "garages", some Drosophila males produce ginormous sperm, and the females have extra plumbing to handle the long flow through.

Brennan PL, Prum RO, McCracken KG, Sorenson MD, Wilson RE, et al. 2007. Coevolution of male and female genital morphology in waterfowl. PLoS ONE 2: e418. doi: 10.1371/journal.pone.0000418

Pitnick S, Markow TA, Spicer GS. 1995. Delayed male maturity is a cost of producing large sperm in Drosophila. PNAS 92: 10614-10618. [link]

Pitnick S, Markow T, Spicer GS. 1999. Evolution of multiple kinds of sperm-storage organs in Drosophila. Evolution 53: 1804-1822. [link]