On a dairy farm in Brookfield, Massachusetts, a dramatic courtship plays out among dung beetles in underground tunnels.
Armed with a pair of devil-like horns, a male Onthophagus taurus stands guard in a tunnel leading to the female with whom he’s just mated. Bowing his horned head like a shield, he blocks any other male from access.
What happens next is an insect version of the famous blues song about the philandering Back Door Man. While the horned male diligently protects his turf, a smaller, horn-less male — possessing big testes full of sperm — sneaks in through a side tunnel and meets up with the female.
“Because of climate change, we have species expansion, but there’s still a lot to learn about why some species are expanding, and why some species are not.”
For evolutionary biologist Erin McCullough, the mating rituals of dung beetles are not only fascinating, they also can provide a window into understanding how climate change might affect biodiversity.
Native to the Mediterranean but accidentally introduced to Florida in the 1970s, the range of Onthophagus taurus has expanded steadily northward across the U.S. All the way to Massachusetts.
“Because of climate change, we have species expansion,” says McCullough, assistant professor of biology. “But there’s still a lot to learn about why some species are expanding, and why some species are not. Understanding how sexual selection plays a role in all of this is not well documented.”
McCullough first began researching Onthophagus taurus during her postdoctoral research at the University of Western Australia. The dung beetle is not only the world’s strongest insect but also the strongest animal; a male Onthophagus taurus “can pull 1,141 times their own body weight: the equivalent of an average person pulling six double-decker buses full of people,” according to a British study.
“Why in some populations is the introduced species the most abundant? And why in other populations is it not? I think that maybe patterns of sexual selection may help explain some of these differences.”
Part of the Scarabaeidae family, whose members are commonly known as scarabs, dung beetles have been celebrated as “nature’s pooper scoopers.” Depending on the species, they eat manure as adults or larvae. Many — like the Onthophagus taurus and tuberculifrons that McCullough captured at farms in Massachusetts and Connecticut — favor dung from herbivores like cows and horses. But some — like Onthophagus hecate, orpheus, and striatulus, all of which she collected from traps set in Clark’s Hadwen Arboretum — dine on dog feces.
Dung beetles fall into three categories — rollers, which form dung into balls; dwellers, which live in manure; and tunnelers, which dig underneath a dung pad to lay their eggs. The genus Onthophagus is a large and diverse group of tunnelers, with more than 2,400 described species, the most of any genus in the animal kingdom.
“They bring balls of dung down into their tunnels and construct what we call a brood ball,” McCullough explains. “The baby beetle feeds on the brood ball and then emerges from the tunnel as an adult.”
In Australia, native dung beetles could not be relied upon to eat cattle manure — they preferred marsupial feces instead — so farmers introduced the tunneling Onthophagus taurus to attack the cow pies and curb flies.
Farmers across the world value dung beetles like Onthophagus taurus because they degrade manure and aerate and transfer nutrients into the soil. In the United States alone, researchers estimate that dung beetle activity is worth nearly $6 billion a year to the country’s economy.
“In Australia, there can be a thousand beetles per dung pad,” McCullough says, “and they’ll shred the dung pad in an hour.”
The downside to Onthophagus taurus’ success in Australia is that “this introduced species is outcompeting native species, and it’s disrupting the ecological balance,” she points out.
McCullough is studying whether that’s the case in Massachusetts. She is examining how Onthophagus taurus’ expansion has affected native dung beetles. And she also wonders whether taurus has changed its sexual selection traits and tactics as it has expanded.
“A lot of biology is about sex and death. That’s really what matters.”
“Why in some populations is the introduced species the most abundant? And why in other populations is it not?” she asks. “I think that maybe patterns of sexual selection may help explain some of these differences.”
In cases where taurus does not dominate, the female may mate only once and then die. In cases where taurus succeeds, the female may mate multiple times with different males before dying, McCullough suggests. And even within the female’s reproductive tract, the sperm of different males compete to reproduce.
“A lot of biology is about sex and death,” she says. “That’s really what matters.”
When McCullough arrived at Clark in 2022, she began setting traps at various sites and collecting varieties of Onthophagus. This summer, she was accompanied by Syd Kochensparger ’25, an environmental science major who works in McCullough’s lab and received funding through Clark’s Penn Family Research Award. Kochensparger already is familiar with Onthophagus taurus, the same dung beetle they had observed eating through piles of manure on their family’s horse farm in St. Cloud, Florida.
Another student, Bea Altopp ’25, a double major in biology and global environmental studies, has joined the lab this fall. McCullough and her two students are now studying the species collected at the Arboretum — orpheus (found to be the most abundant), hecate, and striatulus.
“Over the course of my career at Clark, in real time, I can observe how these populations are changing.”
“These are all native species, and we know very little about them,” she says. “My students and I are raising them in the lab so we can figure out if and how they court and fight.”
McCullough enjoys researching the tunnelers “because they have interesting mating tactics,” she says. “Sometimes they fight, sometimes they court, sometimes they sneak.”
During her postdoc, she explored the role that male-to-male fighting plays in the female Onthophagus taurus’ sexual selection.
“We found that physical strength, horn length, and body mass were significant predictors of male fighting success, but males that won fights were not more attractive to females,” she and her co-authors concluded in a 2016 article published in Behavioral Ecology. “Rather, females preferred males that delivered a high courtship rate, which was not correlated with strength, horn length, or body mass, but previously has been shown to be genetically correlated with body condition.”
Before dung beetles, McCullough studied the sexual behaviors of rhinoceros beetles, whose males sport elaborate horns in the shape of pitchforks, hammers, or crowbars to fight for females. Later, she examined the molecular interactions that occur between sperm and eggs in the reproductive tracts of fruit flies before fertilization.
“I’ve always been interested in biology,” says McCullough, who grew up in Pullman, Washington, where she dissected owl pellets found in her yard. She spent summers playing in the tidepools south of Vancouver Island. But it was in a college biology lecture on bowerbirds’ outlandish courtship rituals that sparked McCullough’s interest in animal behavior.
Now, she feels lucky to be in Massachusetts, at the northern edge of the range for Onthophagus taurus. “Over the course of my career at Clark, in real time,” she says. “I can observe how these populations are changing.”
Photos: Steven King, director of photography / university photographer