MISSOULA –
In the animal kingdom, huge weapons such as elk antlers or ornaments like peacock feathers are sexy. Their extreme size attracts potential mates and warns away lesser rivals.
Now University of Montana scientists and their partners have discovered a developmental mechanism they think may be responsible for the excessive growth of threatening horns or come-hither tail feathers. Published July 25 in the online edition of Science (http://www.sciencemag.org/), the research reveals a mechanism to explain both the size of these traits, and the incredible variation among males of the same species – why some beetles, for instance, grow massive horns while their fellows grow nothing but nubbins.
“Our research explains how these enormous traits get to be so enormous,” said Doug Emlen, a professor and evolutionary biologist in UM’s Division of Biological Sciences. “People have known for 100 years that the best males produce the biggest structures, but nobody has really understood how. Our work looks under the hood to explain why so many sexually selected structures get so massive.”
The researchers discovered when they disturbed the insulin-signaling pathway in Japanese rhinoceros beetles – big insects that can grow horns two-thirds the length of their bodies – the horns were far less likely to grow. In fact, horn growth was stunted eight times as much as growth of the wings, or the rest of the body. They interpret this to mean that the exaggerated structures – the horns – are more sensitive to signaling through this physiological pathway than are other traits.
During the experiment, Emlen and his team injected a cocktail of double-stranded RNA into the beetle larvae to shut down the desired insulin pathway gene. Within 72 hours normal insulin signaling had resumed, but by then horn growth was stunted. Researchers found the genitalia grew normally despite the shutdown, and the wings and bodies were slightly affected. The horns, however, experienced major changes.
“We found an intuitive candidate mechanism – the insulin-signaling pathway – and when we perturbed it with our genetic experiment, we confirmed what we think the pathway should be doing to these beetles,” Emlen said. “There is a hormone signal secreted by the brain that circulates through the whole animal. It communicates to the different cells and tissues and essentially tells them how much to grow.”
Hormone levels reflect the physiological condition of each animal, with high circulating levels in well-fed, dominant individuals and lower levels in poorly fed or less-fit individuals. When tissues are sensitive to these signals, as most tissues are, then their final sizes scale with the overall quality and size of the animal. Because of this mechanism, big beetles have larger eyes, legs and wings than smaller beetles.
Emlen said the horns are exquisitely sensitive to these insulin signals – more sensitive than other structures. Developing horns in big, fit, well-fed males are drenched with the hormone, spurring exaggerated horn growth. On the flip side, a small, less-fit male receive less of the horn-boosting hormone, stunting growth of its weapon.
Emlen said this process explains how horns can range from massive to nonexistent among male beetles of the same species and why the size of such exaggerated, showy traits accurately reflects the overall quality of the males who wield them. He said the results likely are applicable to other species beyond rhinoceros beetles, since additional studies have tied this same physiological pathway to growth of red deer antlers and crab pincer claws.
“Horns and antlers matter,” Emlen said. “Animals pay attention to them when they size each other up for battle. And females pay attention to horns or are attracted to males with really big tails. Why? Because only the best of the best can have really big horns or tails.”
Emlen is the lead author of the Science article, titled “A mechanism of extreme growth and reliable signaling in sexually selected ornaments and weapons.” His co-authors are UM’s Annika Johns, Ian Warren and Laura Corley Lavine of Washington State University, and Ian Dworkin of Michigan State University. Their work was funded by the National Science Foundation.
“We’re the first ones to make the link by explicitly tying the insulin pathway to the evolution of these kinds of male weapons,” Lavine said. “The discovery of the actual mechanism might now open new avenues of study for how exaggerated traits evolved, their genetic basis and the evolution of animal signals.”
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Photo caption: UM evolutionary biologist Doug Emlen with two of his Japanese rhinoceros beetle research subjects (Credit: Laurie Lane)
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