A new study sheds light on how male Onthophagus orpheus dung beetles use their horns to battle for access to females, and how varying horn sizes lead to varying battle strategies. The findings help researchers “see how evolution keeps working, even in a city park.” Shown here is a male O. orpheus with prominent horns atop its thorax. (Photo by Steven King)

By Grant Bolton, Ph.D.

Grant Bolton, Ph.D.

As cities spread, they reshape the lives of the smallest creatures beneath them, some of which are literally cleaning up after us.

Urban green spaces play an important role in softening the impact of expanding cities, and they aren’t just for joggers and dogs. Beneath the mulch, dung beetles tunnel, bury, and battle, recycling waste and defending breeding sites out of sight. And yet, the same evolutionary forces are at play, but perhaps in ways we don’t quite understand.

In Worcester, Massachusetts, one native tunneler, Onthophagus orpheus, offers a fresh look at how dung beetle sexual selection works in a city. The Onthophagus genus uses fresh dung as a resource to develop its larvae. Males will find a fresh pat of dung and pull pieces of it into a burrowed tunnel underground. A female will eventually form the dung into a brood ball, a carefully crafted place where she can lay a solitary egg.

However, before a male O. orpheus can successfully pass on his genes to the next generation, he will have to defend his home turf from other ambitious males.

Males challenge each other within the tunnels to compete for the female. Evolutionary forces have shaped how the horns on their heads and thorax are used in combat. These weapons can be the deciding factor of whether a male will toss or push his competitor out of the tunnel.

Erin McCullough, Ph.D., an evolutionary biologist and behavioral ecologist at Clark University in Massachusetts, studies how dung beetles adapt to urban landscapes. (Photo by Steven King)

Erin McCullough, Ph.D., an evolutionary biologist and behavioral ecologist at Clark University in Massachusetts, has been studying how these dung beetles are adapting to urban landscapes. “Wherever there are mammals pooping, you’ll find dung beetles,” she says.

She and her team of researchers conducted a study that examines the sexual selection dynamics at play in these urban settings, published in August in Annals of the Entomological Society of America.

“This species, Onthophagus orpheus, is super abundant at our two sites, just off campus,” McCullough says. “I wanted to learn more about how they’re dealing with life in the city—what they’re eating, how they’re competing, and whether sexual selection looks different in that kind of environment.”

To uncover the secrets of city-dwelling dung beetles, McCullough and her team set pitfall traps in two urban forests near Clark University: Hadwen Arboretum and Cascades Park. They baited the traps with cow and dog dung and collected more than 450 beetles over a full breeding season.

Each beetle was measured, identified by sex, and in some cases, filmed in battle under red light to mimic their natural underground conditions. “We used clear tubing to record their fights,” McCullough says. “It let us actually see how the males use their horns, how they push, pry, and defend their tunnels beneath the surface.”

Measurements showed that males display a segmented horn pattern, meaning many fall between the extremes of large and small morphs. This variation may relate to alternative fighting or sneaking strategies. The smaller males, or minor males, often tunnel around competing males and sneak down to the females while the big guys are duking it out.

A new study sheds light on how male Onthophagus orpheus dung beetles use their horns to battle for access to females, and how varying horn sizes lead to varying battle strategies. The findings help researchers “see how evolution keeps working, even in a city park.” Shown here is a female O. Orpheus; females lack the horns featured by their male counterparts. (Photo by Steven King)

The study also found that females far outnumber males throughout the summer, yet the males still carry prominent horns and fight fiercely for territory. Despite the female-biased population, O. orpheus exhibits all the signs of an intense, competitive mating system. In most species, exaggerated male traits evolve under strong competition when males outnumber females. Here, the opposite pattern suggests sexual selection may operate differently in low-density or female-skewed populations.

In their staged fights, males used their horns to hook and heave rivals out of the tunnels, sometimes breaking horn tips in the process. “We found one male with a chipped clypeus and another with a broken horn,” McCullough says. “It’s clear these aren’t just decorative. They are real weapons used in battle.”

Dung beetles may not attract the same attention as butterflies, but they are vital to the health of city ecosystems. By burying waste, they recycle nutrients, aerate soil, and help control parasites. Understanding how sexual selection and survival pressures work in urban populations helps researchers see how wildlife adapts to human-altered environments. “Urban environments change fast,” McCullough says. “They reshape resources, population densities, and even the way animals compete. Studying dung beetles helps us see how evolution keeps working, even in a city park.”

A female Onthophagus orpheus dung beetle burrows in a dung ball held by a researcher. (Photo by Steven King)

What comes next for these urban dung beetles? McCullough would like to see how dog feces is being used by O. orpheus and whether it’s a sufficient resource for them. While dog feces are often more attractive than cow dung, the deworming compounds they contain may pose risks for developing offspring.

In addition, future experiments may use transparent observation tunnels to capture sneaking behavior of minor males in real time.

“We’re just scratching the surface,” McCullough says. “There’s an entire world underground, and every tunnel and brood ball tells part of the story of how life adapts to the city.”

Grant Bolton, Ph.D., is a freelance writer and voice actor with a Ph.D. in entomology based in western Missouri. Email: [email protected].