Super big is super sexy in the animal kingdom, even if you’re a Japanese rhinoceros beetle sporting large horns.
A team of scientists at the University of Montana discovered a growth mechanism that may be responsible for big horns and colorful tail feathers throughout the animal kingdom.
The paper, “A Mechanism of extreme growth and reliable signaling in sexually selected ornaments and weapons,” was published Wednesday in the online edition of Science with Washington and Michigan state universities contributing.
Doug Emlen, a professor and evolutionary biologist at the Division of Biological Sciences at UM, took the lead in writing the paper. The research explains how certain growth traits result in the biggest, flashiest things possessed by males.
“People have known for 100 years that the best males produce the biggest structures, but nobody has really understood how,” Emlen said. “Our work looks under the hood to explain why so many sexually selected structures get so massive.”
The key was found in the insulin signaling pathways of the Japanese rhinoceros beetle, a rather large and nasty looking insect with black horns that grow tines resembling those of an elk.
When researchers disturbed the pathway, the horns were less likely to grow. The results suggest that exaggerated structures, such as horns, are more sensitive to signaling through the insulin pathway than other traits.
Emlen said he and his team made the discovery after injecting double-stranded RNA, or certain growth molecules found in all life forms, into beetle larvae to shut down the insulin pathway gene.
Within three days, normal signaling had resumed but horn growth was stunted, even while genitalia grew normally.
“There is a hormone signal secreted by the brain that circulates through the whole animal,” Emlen said. “It communicates to the different cells and tissues and essentially tells them how much to grow.”
In the animal kingdom, large weapons like antlers and horns, or ornamental traits such as the flashy colors in a peacock’s feathers, are sexy because they’re big or bright. Their extreme size and coloration, Emlen said, helps attract mates while deterring rivals.
“Horns and antlers matter,” he said. “Animals pay attention to them when they size each other up for battle. Females pay attention to horns or are attracted to males with really big traits. Only the best of the best can have really big horns.”
Emlen said the function of the insulin signaling pathway can leave beetles of the same species with giant horns or nubbins, reflecting the overall quality – or lack thereof – of the males who possess them.
The results may apply to other species beyond the rhinoceros beetle. Additional studies have already tied the same pathway to the pincer claws on crabs and antlers on deer, caribou, elk and moose.
“We’re the first ones to make the link by explicitly tying the insulin pathway to the evolution of these kinds of male weapons,” said Laura Coley Lavine, a fellow researcher at Washington State University.
“The discovery of the actual mechanism might now open new avenues of study for how exaggerated traits evolve, their genetic basis and the evolution of animal signals.”