Some dung beetle species, such as Onthophagus lanista, evolved large horns to fight off competitors. (Photo courtesy: Erick Greene)
Big Horns or Large Testes: A Beetle's Dilemma
In a scientific development that begs for corny innuendos, researchers have shown that when the development of horns in a particular dung beetle species is blocked, the insects grow abnormally large testes. The new study verifies a long-standing theory in evolution that some species have no choice but to trade one mating advantage for another.
It all comes down to a basic assumption: males have limited resources to divvy up between developing mechanisms to get the girl and those to make winning sperm. This assumption, according to study co-author Douglas Emlen, biologist at the University of Montana-Missoula, hasn’t been fully explored. “It’s a pretty exciting - pun intended - fertile ground,” said Emlen who adds that his study, published in October in the Proceedings of the National Academy of Sciences, provides clear evidence of this trade-off.
Emlen and his colleague, Leigh Simmons, an evolutionary biologist at the University of Western Australia, studied horn growth in the beetle genus Onthophagus. Typically males grow horns, which can be up to 40 percent of their body length. In species such as O. nigriventris, the beetle in this study, males fall into two categories: large and horned, or small and hornless. Which type a male becomes is dependent on his nutrition level during development.
Emlen and Simmons cauterized the region of developing beetles that would have grown into the horn. The cauterized males grew larger in size and developed larger than normal testes but no horn, indicating that O. nigriventris cannot grow both a large horn and large testes.
Photo courtesy: Hemley & Emlen
Size matters in the species for several reasons. The male beetles use bigger horns to fight other males and block the entrance to breeding tunnels and the females within. (Although some hornless males have adapted by sneaking copulations with the female being guarded.) Bigger testes could convey a reproductive advantage when considering that female beetles mate with several males to ensure they get the ideal sperm to fertilize their eggs. In a theory called sperm competition, sperm that are larger or more numerous maximize their chances of contributing to the next generation.
While this documentation of the basic sperm competition assumption was interesting, Emlen said he was particularly intrigued by examining the significance of the horns-versus-testes trade-off. He and Simmons expected to find the same negative relationship between horn and testis size across the genus, which would indicate that the trade-off determined which traits these beetles developed.
Instead, they found several beetle species that managed to produce both large testes and large horns. “When you’re a scientist, and you stumble on something that doesn’t work the way you think it should, that’s fun,” he said. “That’s when the science really gets exciting.”
Two patterns emerged that show beetles may have evolved ways around the trade-off. Unlike the species Emlen and Simmons first studied, other species can grow horns on the head instead of the thorax. This may avoid the problem of limited resources during development, because the horns and testes are farther apart on the body. Other species have mechanisms that make the testes less sensitive to nutritional deficiencies, allowing the beetle to grow big packages on both ends. “The testes are primary, and it appears they found a way to protect them,” Emlen said.
Still, Emlen’s study “invites future research,” said fellow beetle horn researcher Armin Moczek, biologist at Indiana University at Bloomington. Scientists don’t yet understand the mechanism causing the trade-off, he said. “We’re trying from the outside to deduce what must be going on on the inside.” Moczek also cautioned that the experiment restricting horn development performed by Emlen and Simmons was not completely convincing, because large males were compared to small males, not to those of similar size. However, Moczek does acknowledge the significance of trade-offs in evolutionary trajectories. “The more we look, the more we find evidence of their existence,” he said.