The Beetle Breaking Point
A novel limit controls the size of an animal weapon, the beetle horn
Some of the most ludicrous and extreme body structures in the animal kingdom—from elk antlers to peacock tails—evolve because they either attract a mate or aid in defending the ability to procreate. In species in which the males compete for females, adaptations for male-to-male combat arise. From the females’ and opposing males’ perspectives, bigger is better. But across most species that have them, these weapon structures are not continually evolving to be bigger, and evolutionary theory says that what limits the extreme of sexually selected characteristics are costs that reduce the ability of the male to reproduce and pass on his genes. So bigger is better, until the bigness is too costly to maintain. Costs can range from being more conspicuous to predators, being more susceptible to disease, stunting the development of other necessary structures or impeding mobility.
Enter the rhinoceros beetle, Trypoxylus dichotomus. Males of this species have a pitchfork protrusion extending from the face that helps dislodge opposing males from a tree to protect the males’ favorite things: food and females. Meet Erin McCullough, a doctoral candidate at the University of Montana who became interested in what costs were limiting the size of the rhinoceros beetle’s incredible horns. In January, she presented her research findings at the annual meeting of the Society for Integrative and Comparative Biology in San Francisco.
To study this beetle species, she traveled to part of their native range in Taiwan and spent a summer studying the nocturnal beetles from 6 p.m. to 6 a.m. She endured swarms of mosquitoes (someone studying insects cannot wear insect repellent), as well as strange looks from the Taiwanese locals as she traipsed around at odd hours in gardening gloves and a headlamp with radiotelemetry and camera equipment. After all this work to understand the beetles’ survival and movement patterns, she “surprisingly found that the horns are not costly.”
After another year of field and lab experiments, McCullough showed that “big males and small males fly at the same speed; there’s no difference in how far they fly. Horns do not impair locomotion.” She also found that the horns were lightweight and did not increase drag significantly, and in the biggest males, flying with a horn required only 3 percent more force than flying without one. Developing big horns did not stunt the development of other body parts and did not have any immune costs, nor were there any differences in survival between males with big horns and males with small horns. All that work, and McCullough still had more questions than answers: “Costs form the basis of the theory of how and why females pay attention to sexually selected traits. If these horns aren’t costly, what’s going on?”
When an expected result is debunked, the scientific method is to make more observations to lead to a new hypothesis. What McCullough observed was that about one-fifth of the beetle population had scratches or gashes on their horns and about 4 percent had broken horns. These observations led her to a new hypothesis: “If the horns weren’t costly in terms of resource allocation or locomotion or immune costs, maybe there is some type of mechanical limit that is setting the upper bound on how big the horns can get.” To test this idea, she turned to engineering techniques, examining what are called safety factors, meaning how safe a structure is from breaking given the loads it experiences. For example, she says, “Steel bridges have safety factors of about two, which means they are twice as strong as they need to be, compared to the loads that they typically experience.” McCullough needed to measure the typical load and the breaking point of the horns. She used a mechanical tester to measure the force it took to break horns of different sizes. She compared this number with the amount of force it took to dislodge each male, measured with a spring scale. Because intense male-male combat is typically between males of the same size, she could assume that the force needed to dislodge another male was the same as the force needed to dislodge that particular beetle. Finally, she found support for her hypothesis: “As the horns get longer, they become more susceptible to breaking.”
Not all rhinoceros beetles have pitchfork horns on their heads. Among rhinoceros beetles alone, and certainly among horned beetles in general, horns vary in their shape, location, orientation and weapon tactic. In a previous study published in Science in 2001, McCullough’s advisor, Doug Emlen, showed that dung beetles’ horns all have the similar function of blocking other males from entering tunnels, but nevertheless have very different forms and locations on the head and back. He showed that this variation was not due to female preference or weapon effectiveness, because bigger was always better in the case of effectively blocking tunnels, but rather due to developmental costs. Bigger horns were related to stunted development of eyes, antennae or wings, depending on the horns’ locations on the body. Different species of dung beetle evolved different structures, depending on which structure was the least costly if stunted.
Rhinoceros beetles are clearly totally different in costs than dung beetles, because their horns do not stunt the growth of other body parts and their fighting tactics are different. McCullough thinks that mechanical limits may play a role in the variation in horns seen in rhinoceros beetles. Next, she’s learning another engineering technique called finite element modeling. With this modeling technique, she can experimentally manipulate the shape and size of the horns to understand the forces different horns experience in different types of fights, using three-dimensional reconstructions of horns from micro-computed tomography scans (also called micro-CT scans). “Rhinoceros beetles have some of the most fantastic weapons morphology that you find in nature,” she says. “I am interested in how costs, or the lack therof, might help us understand the patterns of diversity that we see in animal weapons.”—Katie L. Burke