Beetle Battles
The Strange World of Horned Combatants

BY CARY SHIMEK

Though he’s only the size of a pencil eraser, a male onthophagus beetle, at his scale, is one of the most formidable creatures on earth. Like a medieval knight, he’s sheathed in body armor — a black exoskeleton — and he carries a lance — a huge rhinoceros-like horn protruding from his head.

This brute guards his castle, which in this case is a straw-sized tunnel leading downward through dung into the moist earth below. There lives his princess, a lady beetle busily sculpting nutritious dung into balls where she will lay her eggs. When these heirs to the kingdom hatch, the larvae feast in underground safety on the meal their mother provided.

But lo, other beetles often lay siege to this minute fortress, hoping to gain access to the lovely, sequestered princess, who is fickle and bestows her favors on any knave who reaches her. If a smaller male challenges the guardian at the gate, the big beetle braces his spiny legs against the tunnel walls and uses his horn to lever the fool out in seconds. If the combatants are evenly matched, they lock weapons and engage in epic battles that can last half an hour. These living tanks are too heavily armored to hurt each other, although the larger male with the bigger horn almost always wins.

But lady fair sometimes mates with small males with stunted or nonexistent horns — males whom her protector would crush in single combat. These little males somehow manage to sneak past her horned protector — perhaps while he is away eating — or they dig secret side passages into her subterranean lair. Perhaps one in 10 of these clandestine quests actually succeed, but to the victors go the spoils, especially in the beetle world.

Doug Emlen, a UM evolutionary biologist, didn’t set out to study onthophagus. “I promise you that no normal person wakes up one morning and goes ‘DUNG BEETLES!’” he says. But the more he learned of them — that they have distinctive horns, can be bred in the lab and have a fast generation time — the more he realized they would be perfect for his research.

Now ten years have passed, and Emlen’s beetle studies have given insight into some of biology’s most basic questions: Why is there so much diversity in life? How can there be so much diversity among closely related species or even within the same species?

Biologist Doug Emlen

Emlen started studying beetle battles and their other behavior using glass observation chambers resembling ant farms. These containers were placed in dark rooms lit with red light. Since the beetles literally can’t see red light, they went about their natural, hidden lives while patient researchers watched and videotaped.

Beetles are the most diverse group of animals on the planet. Many varieties dig tunnels near their food source, such as dung, so they can pull the stuff underground away from competition and hoard it. This behavior is successful for females, and later their offspring, but it makes it easy for a jealous male to restrict access to his mate if he guards the tunnel entrance. Biological weapons — horns — help males fight off competitors and pass on their genes. Horned beetles are more successful than those without horns, and over the eons tens of thousands of beetle species have wound up looking like miniature bull moose.

These horns fascinated Emlen, and he was soon doing beetle-breeding experiments to examine the genetic underpinnings of the weapons. The beetles he works with grow from egg to adult in a month, so he can study many generations in a year. He mated certain males with certain females, and he soon had a detailed parentage analysis of his beetles, measuring and quantifying the genetic variation of the horns.

What he learned is that the horns aren’t inherited in a traditional genetic sense. Horn size depends on the environment. For example, a larva that encounters good conditions, such as lots of food, grows into a large insect with a big horn. But kids from the same beetle family that are similar genetically can grow up small without horns if they don’t get enough to eat. Emlen says the environment can make siblings look totally different, like different species. Not only do they look different, they act differently. The hulking, horned beetles start guarding females in tunnels and battling. The “wimpy” males start digging secret entrances and sneaking to mate with females.

“It’s like a switch mechanism, a type of physiological decision as they grow,” Emlen says. “There is a sophisticated developmental mechanism that assesses the environment and dictates the genes that get expressed.”

Horned onthophagus beetles guard the tops of tunnels, attempting to keep out sexual rivals, while smaller hornless males may reach females by digging side tunnels of their own. Female beetles drag in dung within which to lay their own eggs.

He says a sneaky male isn’t necessarily genetically inferior to a big male — he just didn’t have an easy childhood. A female, by digging a tunnel and staying put, passively but effectively chooses big, dominant males to mate with. But she will mate with any tiny sneaker that reaches her inner sanctum. Only the best sneakers are likely to reach her without getting caught by the resident guarding male, and for the female, transmitting good “sneak” characteristics may be just as beneficial as passing on effective “guardian” characteristics. After all, her offspring may encounter harsh growing conditions and have only the resources to grow into sneakers.

Emlen says many of his experiments study to what extent genetics or environment contribute to the expression of horns. Using an artificial selection experiment, he caused beetle populations to change over time across generations.

He says all male beetles grow horns with abundant food, and all are hornless if deprived of food, but in the mid-range there is a lot of variation. So his researchers selected animals that were more and more sensitive to diet. Eventually they were able to measure and quantify the evolvability of the traits they were interested in — especially the horns.

Not only did Emlen study beetle behavior and genetics, he also probed beetle development by manipulating hormones. For instance, he and his associates discovered a hormone that makes small males produce horns despite their small size.

“I was the first person in the world ever to study the development of horns in beetles,” he says. “There are probably 100,000 species of horned beetles, and nobody had ever tried to study horn development before. So we had to start from scratch. Now, by linking behavior studies with genetics and development, I think we have come up with a comprehensive picture of what these horns are and how they evolved.”

Emlen says big horned males don’t get all the benefits in life. Growing large weapons — and the variation is astounding among the different beetle species — doesn’t come without a price. Beetles that grow horns from their heads often have smaller eyes. Those with horns protruding from their thoraxes grow smaller wings. Such drawbacks may offset somewhat the advantages of having huge horns, giving the beetle sneaks of the world more of a chance.

Emlen’s beetle research and multidisciplinary approach have gained attention from the broader scientific community. Teachers are using his beetles as examples in biology classrooms, and he recently earned a $650,000 Faculty Early Career Development Grant from the National Science Foundation. He’s also published articles in recent issues of Natural History and Science.

“As we tease apart the factors shaping the horns,” Emlen says, “we are realizing just how intricate and numerous are the connections among ecology, evolution and development.”

For more information about Emlen’s research, call (406) 243-2535 or e-mail demlen@selway.umt.edu.