Friday Weird Science: You know what they say about guys with big mandibles...

Apr 23 2010 Published by under Friday Weird Science

...they wear big exoskeletons.
Today's Friday Weird Science comes to you courtesy of the talented, handsome, and soon to be no longer stranded in Australia (hopefully), Ed of Not Exactly Rocket Science. Because no one can tell you more about beetles and the size of their...mandibles...than Ed. 🙂 Yamane et al. "Dispersal and ejaculatory strategies associated with exaggeration of weapon in an armed beetle" Proceedings of the Royal Society B, 2009.
And here we have today's male of choice:

As you might be aware if you've studied animal behavior, many different kinds of animals, including insects, like to duke it out for mates. In the case of the flour beetle, this involves having pretty big mandibles relative to body size, and going at it.

(Two beetles fighting, not flour beetles because I couldn't find those. The best part is how the music tries to make it seem all hardcore)
Of course, mandibles that are big carry some pretty heavy evolutionary costs with them. Devoting a lot of energy toward your mandibles means that you can't devote energy to other things. But WHICH other things?
In this study, the authors looked at the relationship between mandible size, wing size, and testis size. Why these three things? Well, the mandible size is obviously important because the males are duking it out for access to a female. But the wing size and ball size are also important. Males who aren't so successful in the big jaw wrestling matches will use their wings to seek new territory where they might have more success. And the balls, well. Female flour beetles are capable of storing semen from more than one male. The bigger the balls, the more semen is likely to get stored, and the more likely that male is to become a daddy.
There were a few hypotheses behind this study. The authors knew that bugs with bigger mandibles win more fights and gain more access to female, so they hypothesized that (1) males who lose fights might select for bigger wings down the line, to make up for a lost fight by searching for new territory, and (2) that males who lose fights are going to have bigger testis. They may not have the initial access to the female, but they need to be able to deliver where it counts. Basically, the smaller your mandibles, the bigger your balls and wings.
To figure this out, the authors bred out beetle LINES for mandible size. They took out of each group of beetles the 12 beetles with the shortest mandible length and bred them, and the 12 beetles with the longest mandible length and bred them. Average length was used as a control. After 12 generations of this careful breeding, they took the resulting beetles and measured their wings, mandibles, and balls. And then they made them fight.
First off: The fights.
Sure enough, the short mandible length beetles in general were the losers, while the long mandible length boys just beat the crap out of them. So the authors confirmed that long mandibles translate to fighting success.
But what about the other body parts?
We'll start with the wings. The short mandibled beetles had significantly larger wings. And they were ALSO capable of dispersing further afield than their heavy jawed brethren.

Here you can see the dispersal rates in two trials for long mandibled (grey), short mandibled (black), and control (white) beetles. You can see that the beetles with short mandibles were able to disperse a lot further. This could be due to both larger wing size AND to the fact they they aren't carrying around as much extra jaw weight.
And now, on to the balls.
The beetles with small mandibles still had it where it counted. In fact, they had it MORE than their long-jawed counterparts. And not only were their testis BIGGER, they also produced more ejaculate.

You can see above there the amount of semen stored in a female after sex with long-jawed, short-jawed, and control beetles. The short mandible beetles ended up with significantly more stored semen, which indicates that their ejaculate volume was larger. They may not get as many ladies, but when they do, they get them good.
So what does all this mean? It means that short mandible beetles have evolved characteristics that allow them to continue to compete with their long mandible brothers, even when they lose the fights. So the next time someone makes fun of your small jaw, tell them you still have it where it counts. Your...wings...are HUGE.
Yamane, T., Okada, K., Nakayama, S., & Miyatake, T. (2010). Dispersal and ejaculatory strategies associated with exaggeration of weapon in an armed beetle Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.2017

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