When animals want to prove their reproductive fitness, they engage in some pretty fancy ornamental displays (things like peacock feathers and deer antlers). Few organisms pull this off as well as the male Japanese rhino beetle, however — an insect with a disproportionately massive horn on its head. And new research indicates that this horn is far from a cosmetic trick to lure mates, and is in fact a potent indicator of good health — one that cannot be faked.
Writing in his Not Exactly Rocket Science column at Discovery, Ed Yong, points us to a recent study conducted by Doug Emlen from the University of Montana. Emlen has discovered that the growth of the horns is closely tied to molecules that show how well-nourished the beetles are. In fact, it's a very special body part — one that can only respond to these particular molecules; no nourishment, no horn growth.
Specifically, the beetles are sensitive to insulin — or lack thereof. Emlen's study shows how rhino beetle horns are as much as eight times more sensitive to the disruption of this chemical than other parts of its body.
Consequently, Emlen has concluded that the ornament is a bona fide indicator of a beetle's health — it can't be faked. A weak and starving beetle cannot produce anything that remotely resembles the horns of a healthy rival. As Yong notes, "females can rely on the size of the horns to judge a potential partner's health."
Yong says that this is a bit different than some explanations that try to explain the prevalence of animal ornamentation, what's called the "handicap principle." He explains:
It states that low-quality individuals can't bear the cost of, say, a long tail or a magnificent set of antlers. They would be too conspicuous or heavy. They need strength and health to pull off. Cheats couldn't bear the burden.
You can understand how the handicap principle would work for a signal that's already exaggerated, but obviously, those signals didn't start off that way. They would have had much humbler and smaller origins, when the costs of bearing them would have been low. So, at this early stage of evolution, why didn't weak individuals cheat by producing larger ornaments?
Emlen's rhino beetles provide an answer. The signals can't be faked not because they're a drain, but because they're intimately tied into an individual's physical condition. It's not that cheaters can't carry the burden of big ornaments. It's that cheaters can't exist.