Many studies have suggested that exaggerated ornamentation or weaponry, like beetle horns, deer antlers, or extremely long bird tails, are symbols of a male’s virility. Now, a new study demonstrates how this might work: Researchers have found that the size of a beetle’s horn is more sensitive to insulin signaling than other features, like the insect’s wings or genitalia.
Since insulin signaling has been tied to nutrition and growth in a wide range of animals, the finding implies that such exaggerated traits are indeed honest signals of male quality. Only healthy, high-quality male beetles should have large horns while weak, low-quality males should have small ones, according to the researchers.
Douglas Emlen from the University of Montana, along with colleagues from Washington State University and Michigan State University, studied male rhinoceros beetles that have forked horns jutting out from their heads. Some of the beetles’ horns are just tiny bumps while other horns grow to two-thirds the length of a beetle’s body.
Their findings appear in the 27 July issue of the journal Science.
Rhinoceros beetles (Trypoxylus dichotomus) filmed in the field at the National Chi Nan University, Taiwan. A female is feeding on oozing sap on the side of a tree, and males compete for chances to mate with her. Males use their enlarged horns to pry and flip rival males from the tree. Beetles were individually color-marked (dots) as part of this 2011 study by Erin McCullough.
[Video courtesy of Erin McCullough]
“This is one of the most pervasive and striking patterns observed in nature: Ornaments of sexual selection tend to be really big and flashy,” Emlen said during a phone interview. “But, very few researchers have looked under that hood at the mechanisms involved. Here, we’re proposing a candidate mechanism in which a trait has evolved to become more sensitive to a signal than other traits.”
Emlen and the other researchers used gene-silencing experiments to discover just how much more sensitive the beetles’ horns were to changes in the insulin pathway compared to other sexually selected traits. In light of their findings, they suggest that an increased cellular sensitivity to this pathway may be responsible for the exaggerated growth of rhinoceros beetle horns.
“We took a gene at the top of this pathway—the receptor—and knocked down expression of the gene to test whether it affected our exaggerated trait,” explains Emlen. “We perturbed the pathway and then looked at several different traits. We knew that the beetles’ genitalia wouldn’t respond, but we saw that the wings did. And then we added horn size into the equation and found that it made even more of a difference.”
“The horns are extremely sensitive,” he adds. “They’re about eight times more sensitive than the rest of the body.”
Insulin and insulin-like growth factors are known as essential regulators of tissue growth and body size in many animals, where they control cell proliferation in response to nutrition and physiological condition. Since the growth of rhinoceros beetle horns is especially sensitive to these signals, the researchers propose that size of the exaggerated weapon accurately reflects the quality of an individual. Because of this mechanism, variation in male quality is reflected in the relative sizes of the male weapon, they say.
Male rhinoceros beetles (Trypoxylus dichotomus) wield an enormous pitchfork-like horn on their heads, which they use in battles with rival males over access to females. | Image courtesy of Will Freihofer and Douglas Emlen
“A high-quality animal is generally one that has an active immune system and is resistant to infection,” explains Emlen. “Some studies have shown that pathogen infection, for example, reduces circulating insulin levels in an organism. So, an animal’s overall quality and condition is going to translate to its circulating insulin levels.”
“We’re proposing that a general way for traits to become exaggerated in animals could be genes evolving heightened sensitivity to these insulin signals,” he says. “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.”