Stink beetles’ threat posture precedes repugnant spray
By Jeff Mitton
I was setting up my tent in a remote area of the Vermillion Cliffs when I was distracted by an unusual desert stink beetle emerging from a mammal burrow.
In red rock country, desert stink beetles are a common sight, for there are numerous species and some of them are quite abundant. But this beetle was distinctly different from all of those that I had seen previously.
Desert stink beetles are in the genus Eleodes, a name derived from a Greek term meaning “olive like,” which describes the shape and the black coloration. Stink beetles are known for their characteristic habit of responding to a threat by standing still with their butts held high. This pose is worth noting, for it is a threat display that precedes the release of a malodorous, repugnant fluid.
Some species simply evert the scent glands, spilling the fluid on themselves, but other species can direct a spray 10 to 20 inches in the direction of the threatening creature. Chemical analyses show that the brown fluid is a mixture of quinones, all noxious to the nose and irritating to the skin.
Approximately 30 percent of the species of desert stink beetles stand on their head, butt in the air, giving fair warning prior to spraying their awful stuff. These are the beetles that have given all the other black beetles a bad name. But about 70 percent of the beetles in the genus Eleodes are not capable of either oozing or spraying. Among these, about one third will display the threat posture even though they cannot spray. These species are mimics, fending off predators and naturalists by imitating closely related stink beetles that spray.
The world is full of crafty predators, and even for species able to spray blinding, choking, vile fluids, there are usually a few predators that that have found a way to get around the defense.
Grasshopper mice, Onychomys arenicola, are plucky predators in the desert Southwest. They chase other species of mice and even lizards, grab them and bite through their necks to paralyze their prey. They have learned to eat stink beetles with impunity. They pounce on a stink beetle, grasp it in their forepaws, and jam its butt end into the sand to avoid the spray. They restrain the planted beetle and eat the head first, then the thorax, and then the abdomen until they reach the scent glands, which they leave in the planted and hollowed carcass.
Desert hairy scorpions, Hadrurus arizonensis, also prey on stink beetles. They snare a beetle by grabbing a leg and then quickly maneuvering the beetle with their claws, holding it with the butt pointing away. They sting the beetle between its thorax and abdomen, immobilizing it. Then they break the beetle in half, and eat the tastiest of its organs.
The desert stink beetle that caught my eye is named Eleodes caudiferus, for its distinctive caudal or butt end. It is not truly a butt, but a projection of fused wing covers. A few closely related stink beetles have projecting butts, but this beetle is the most extreme. I have searched the literature for an explanation of this perplexing end, but no one has studied it or even speculated about it.
Unconstrained by expert opinions on the subject, I offer two hypotheses. Given that this group of beetles sprays toxic chemicals from its butt to dissuade tentative predators, and given that a substantial proportion of beetles protect themselves by mimicking the threat posture, I propose that the extended butt is an exaggerated advertisement of the deterrent. Some beetles may strike a threat pose, but this one strikes a threat pose and waves a fire hose at the same time.
Several species of desert stink beetles have extended butts, and in each species, males have longer butts than females. This consistent pattern of difference suggests sexual selection, a form of natural selection focusing on the ability to compete for mates. In a variety of species, such as bighorn sheep, elk, moose, elephant seals and red-winged blackbirds, males have exaggerated traits (horns, antlers, size, coloration) that help a male convince females to accept him as a mate. So perhaps that rakish derriere is used in ritualistic combat among males squabbling over territories. Or maybe it signals superior health and the acme of masculinity to females.