Nice Rack

August 31, 2012

Rheanna Sand

Darwin's famous "survival of the fittest" theory usually conjures up images of, say, fast gazelles outrunning a cheetah, or camouflaged insects being ignored by predators. But there is more to evolution that just survival - the genes must be passed on to the next generation. In the animal kingdom, this typically involves the males outcompeting each other and convincing a choosy female to mate with him after a few drinks.

In species as divergent as fiddler crabs, elk, and rhinoceros beetles, ornamental weapons have evolved in males for just this purpose. Males with the biggest weapons defeat other males in battle, and draw the attention of fickle females. But it's not what you might think - the females aren't sitting on the sidelines of the fight, waiting to jump the winner. They actually use the size of the weapon as an indicator of the males' overall health. But what is it about the ornaments and weapons that cause them to grow to such extremes? Other parts of the body, like wings or genitalia, do not vary as much between fit and unfit males.


Examples of male, um... ornaments in the animal kingdom.


A paper published in this week's Science offers a physiological mechanism, looking at the horns of male Rhinoceros beetles. Douglas Emlen and colleagues traced it to a developmental process involving insulin and related proteins called insulin-like growth factors (IGFs). Insulin circulates in your blood and signals your cells to take in sugars, and IGFs work with insulin to tell your cells to proliferate, and therefore allow for tissues to grow in size. Through this system, nutrition (how much sugar is in your blood) is linked to growth.

What Emlen's paper showed was that the ornamental horns of the beetle were more sensitive to the insulin/IGF pathway than other structures. To show this, they reduced the amount of insulin receptor gene in larval beetles using a technique called RNA interference and measured horn size in the adults. By blocking this pathway, they reduced the horns by 16%. Wings were only reduced by 2%, and genitalia were not reduced at all, showing that horn size was the best indicator of overall health during development.

And, because insulin levels cannot be faked, using this pathway ensures that low-quality males can't "cheat" their way into mating. Although, I'm sure if they could, Rhinoceros beetles would buy Escalades and wear Rolex watches, too.



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