How the Rhino Beetle Got its Horn

For some beetles, size definitely matters. Male Japanese rhinoceros beetles (Allomyrina dichotoma) are equipped with a prodigious forked horn which they use to flip other males off a tree in their competition for a chance to mate. Absurdly large ornaments are not uncommon in the animal world. From the remarkable tails of peacocks and male Long-tailed Widowbirds to the immense antlers of elk, members of one sex often sport exaggerated body parts as a signal of their health and status. The sheer size of these ornaments is meant to impress potential mates and intimidate rivals. For this to work, the signal has to honestly reflect the condition of the animal; without a reliable link to quality, a signal loses its value as a guide in selecting a mate (or picking a fight).

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One way to ensure this link is for the signal to be costly to produce or to expensive to maintain, which limits the opportunity to cheat. Even though large ornaments and weapons might be costly now, they would have been small, inexpensive structures at the start of their evolutionary arms-race. So what could have kept the signals honest then? One possibility is that the signal might be "intrinsically unfakable" because of the way it develops — in other words, it might be impossible to cheat.

Last year, researchers based in the US used the rhinoceros beetle as a model to study the development of signal traits. The size of a male's horn is extremely variable; although some males have horns two-thirds of the length of their body, other have to make do with nothing more than a small bump. The first thing the team did was test how strongly nutrition affected the size of the horn. They fed beetle grubs either a "standard" or "low nutrition" diet and then, once they had metamorphosed into aduts, measured the size of various body parts of the males. The wings were slightly larger in the group which had been fed a standard diet, but the change was about the same as the overall increase in body size. The horns, however, were more sensitive to the change in nutrition, repsonding about three times as strongly as the body or the wings.

The team speculated that the difference might be due to higher sensitivity to insulin/IGF in the horn than in other tissues. The insulin/IGF pathway plays a central role in regulating body size in a most multicellular animals. Insulin and IGF integrate information about the nutritional and physiological state of the animal; the level of these molecules regulates tissue growth. If horns are more sensitive to insulin/IGF than other tissues, the team reasoned, then their growth would be more strongly affected by changes in nutrition — which would also make horn size a reliable signal of status.

To test their hypothesis, the team used a technique called RNA interference to knock down the insulin receptor gene in rhinoceros beetles for 48 hours around the beginning of their metamorphosis. Without the receptor, the insulin pathway wouldn't be able to function properly; the researchers predicted this would have a stronger impact on the growth of the horns than the wings. They were right. While the wings in the knock-down beetles were about 2% shorter than those of beetles with a working insulin receptor gene, their horns were about 16% smaller. In other words, the horns were about eight times more sensitive to insulin levels than the wings.

The increased sensitivity to insulin and IGF makes horn size a reliable signal of the individual's quality; the dependence on insulin links horn size to the individual's nutritional status, meaning it can't be faked. Moreover, the increase in sensitivity also amplifies differences between individuals, just like the difference in size between distant objects is more obvious if you zoom in on them. This means that horn size doesn't just serve as an honest signal; the insulin/IGF mechanism also makes it a more informative one. “We’re proposing that a general way for traits to become exaggerated in animals could be genes evolving heightened sensitivity to these insulin signals,” said Douglas Emlen, the lead author of the study, in a press release. “If you make a trait extra sensitive to this pathway, then you’re going to see some extreme growth. You’re also going to see a reliable signal of male quality.”

References
Emlen, D.J., Warren, I.A., Johns, A., Dworkin, I, and Lavine, L.C. (2012) A Mechanism of Extreme Growth and Reliable Signaling in Sexually Selected Ornaments and Weapons. Science 337 (6906):860-864. doi: 10.1126/science.1224286
The press release from Science

Image Credits
The Japanese rhinoceros beetle is by Flickr user ashung.