A panel of experts in Nebraska has declared human dung more appealing than that of several other species. These experts didn't so much announce their decision as fall headfirst into baited poop traps while looking for a meal. Still, you won't find a more discerning group of judges than nine thousand dung beetles.
The various species of dung beetle that live together in the Great Plains region have evolved to consume, and share peacefully, its turd piles.* Some species are specialists, preferring one animal's feces to any others. Others will eat anything that falls their way. Two entomologists--Sean Whipple at the University of Nebraska, Lincoln, and W. Wyatt Hoback at the University of Nebraska, Kearney--set out to mess with the balance between these dung beetle species.
The researchers set traps all over a large organic cattle ranch. Each trap consisted of a large bucket sunken into the ground with a pile of dung at the bottom. The bait came from 11 different species that included carnivores, herbivores, and omnivores. Some of these dung flavors were ones the beetles might encounter regularly: bison, moose, cougar. Others were "exotic," from animals that don't normally leave their excrement around Nebraska: zebra, lion, human.
(The animal feces came fresh from a local zoo. As for the human specimens? "I will say this much," Sean Whipple said when I asked. "It is difficult to find volunteers for a study such as this.")
Human and chimpanzee feces were the clear winners of the popularity contest. Almost 9,100 dung beetles stumbled into the authors' traps, belonging to 15 different species. The beetles in the human and chimp dung buckets far outnumbered any of the rest.
Different beetle species preferred different types of dung, which explains how they can all share resources normally. But overall, omnivores were a favorite. Pig dung, while not as wildly popular as human or chimpanzee, still attracted a lot of beetles.
Whipple says the scent of omnivore dung is especially alluring. "Previous research has shown that the dung of omnivores is generally more attractive than that of herbivores, likely as a result of odor," he wrote in an email.
Incidentally, if the dung beetles had gotten a chance to eat all that sweet-smelling human dung, it would have been a good choice nutritionally. Chemical analysis showed that the human dung had the highest nitrogen content, a measure of "dung quality," Whipple says.
Herbivore dung wasn't only less popular than omnivore dung. It was also beaten out by carnivore dung. And the most widely ignored droppings came from bison--a species that local dung beetles evolved alongside, and that would have provided much of their diet just a century and a half ago.
Like five-year-olds who are bored with their green beans and would like some dessert already, please, most dung beetle species in this study were eager to switch from their usual plant-based poops to something new and exciting. They're not likely to start encountering a lot of human or chimpanzee feces on their Nebraska ranch. But a non-native animal that's introduced to an area where dung beetles live (and they live all over the world) could upset the balance between its native inhabitants. Beetles might start competing for the exotic food source, for example, or ignoring piles of poop they would ordinarily clean up.
If you're wondering what makes our own species' dung so appealing, the authors say diet doesn't seem to be a factor. Among the zoo animals whose dung they used, all the carnivores were fed the same diet, and so were the herbivores. But the dung beetles preferred some carnivore or herbivore dung to others, suggesting there's more to poop flavor than the food it started out as.
Though the human subjects may have eaten different diets, their specimens were "thoroughly mixed to ensure homogeneity," Whipple says. Now that's appetizing.
Whipple, S., & Hoback, W. (2012). A Comparison of Dung Beetle (Coleoptera: Scarabaeidae) Attraction to Native and Exotic Mammal Dung Environmental Entomology, 41 (2), 238-244 DOI: 10.1603/EN11285