New Study Illuminates Dung Beetles’ Attraction to Death
By Ed Ricciuti
By and large, the titles of scientific research papers can be pretty stodgy. When the research targets dung beetles, on the other hand, there’s rare opportunity to wax playful with words but, as well, the real risk of scatology. Researchers who spent a year baiting dung beetles in traps with dead rats on the Kansas prairie managed to avoid the risk but still come up with a catchy title when they reported their findings this month in the journal Environmental Entomology: “Heads or Tails? Dung Beetle Attraction to Carrion.”
The title refers to whether the beetles are attracted to the head (or cranial) end of the rat or its tail (or caudal) end. The results of the research indicated that, at least in this case, more beetles congregated at the fore rather than the aft end of the carrion, which suggests they were drawn to the carcass itself, not dung in the gastrointestinal tract at its rear, as has been hypothesized in the past.
Dung beetles eat carrion along with their dietary mainstay, which is, not surprisingly, poop. So-called “roller” dung beetles make balls of poop and roll them off, “tunnelers” bury it, and “dwellers” crawl into it, displaying what scientists believe is the original dung beetle behavior. The dwellers departed from the norm and showed no preference for head or tail, distributing themselves evenly over the carcass, suggesting that like tunneling and rolling, attraction to the head of a carcass is a later behavioral development in dung beetle evolution.
Because carrion is a secondary food for dung beetles, they are not considered necrophagous, which means primarily feeding on carrion. Therefore, until now, scientists questioned whether dung exposed by decomposition or the carrion itself attracted the beetles. It appears they no longer have to wonder.
“This research provides evidence that dung beetles have a direct attraction to carrion rather than an indirect attraction to the dung of a decomposing animal, but it is important to remember that this work doesn’t provide evidence of actual carrion feeding,” says Rachel Stone, lead author of the paper. Stone conducted the researcher with fellow forensic ecology graduate student Emmy Engasser during their time at Wichita State University, under direction of Mary Liz Jameson, Ph.D., Stone is now a Ph.D. student at Case Western Reserve University, and Engasser is now a collection specialist of the O’Brien Collection at Arizona State University.
“The arrival of dung beetles to baited traps is merely an indication of initial attraction and cannot be interpreted as evidence of actual feeding. More work needs to be done to provide direct evidence of feeding,” Stone says.
Be that as it may, the team’s research may cause scientists to rethink the role of carrion in dung beetle diet. It is known that carrion also seems to serve as a substitute for dung when and where poop is hard to find. Several dung beetle species living in tropical rain forests, where the large herbivores (and therefore their dung) are scant, depend on carrion and are, in fact, necrophagous. By way of comparison, carrion does not seem to be used as a significant food resource by dung beetle species in tropical grasslands, which teem with large herbivores and their droppings. In general, outside of tropical forest species, scientists have pretty much dismissed the attraction carrion has for the beetles. “The findings of this study indicate that there is more to the dung beetle–carrion interaction than we give them credit for outside the tropics,” says Stone.
Carrion, in fact, may be a far more desirable item on the dung beetle menu than previously believed. “The implications of dung beetles having a stronger head preference over a tail preference is that the carrion resource is more attractive than the dung resource,” says Stone. Maybe it’s because certain dung beetles prefer to feed on carrion, or maybe it’s because the carrion simply emits stronger volatile compounds that attract more dung beetles—we cannot conclude which from this study alone.
The supposition that dung beetles are directly attracted to carrion is supported by a comparison made during the study. Stone and her colleagues also tested whether carrion beetles (Silphidae), which are necrophagous, are attracted to the head, which they were.
Dung beetles are among the insects found on human corpses that are used in criminal forensics to elucidate the circumstances of death. Because they are not purely carrion feeders, however, they are not as useful as, for example, carrion beetles. Further research on carrion feeding by dung beetles may improve their utility to forensic entomologists.
Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.