That year I returned to my camp on August 27, and ten days later I excavated the buried mouse; I found only a skull and a clump of fur, along with fifteen to twenty Nicrophorus beetle grubs stacked up vertically in tight formation, consuming the last of the mouse. There were no fly maggots.
On my previous visit ten days earlier, I had set traps in the cabin to catch more mice, and I now had five dead ones in various stages of decay, from soft and stinking to dried up (and also stinking). Before setting them on the ground outside for the beetles, I tied a white string onto each carcass with a different number of knots at the end of each string so that I could identify them after they were buried. Only a few minutes after I deposited the mice, the first sextons came humming along and homing in on the scent. Seconds after they flew in, they plopped onto the ground and, with their antennae waving, made a beeline to a mouse.
In two hours one mouse had seven beetles crawling over and under it and squeaking. I was surprised to see so many beetles at one mouse, but this one was half dried up and could not serve as a brood nest, so it was not likely being strongly defended by a pair wanting to claim it. I was also amazed at the noise-making, because beetles don’t have ears; the sound is made by the friction of body parts rubbing together. Shiny greenbottle flies (botflies) came, ready to deposit eggs that would in hours hatch into hungry maggots. Occasionally, white-faced hornets on the hunt for flies flew in and patrolled close to the ground, pouncing down here and there, but their potential prey usually disappeared shortly after the hornets arrived.
Beetle pairs met at two other mouse carcasses and within an hour had buried them by excavating the earth to create a hole into which the mouse dropped. Thus was their prize removed from the competitors: ravens, botfly maggots, and other beetles. The beetles had mites all over their bodies, as though infested with parasites. But these mites are the beetles’ allies, for they kill or consume the botfly eggs that got onto the mice before they were buried.
The next summer I offered up a shrew, a fresh Blarina brevicauda, in order to again watch the undertakers. This shrew is one of the few mammals with a poisonous bite and an unpleasant odor, and even most predators who kill one then discard it. House cats commonly bring them indoors, and most people think of them as moles because they have short gray hair and a pointed nose, though not the shovel-like front feet of a mole. Shrews are among the most common animals in the north woods, but those of this species are seldom seen because, unlike most shrews, they live underground.
It was August 5, 2010. I had brought the Blarina shrew to camp late the night before and put it outside by the camp door in a clean spaghetti-sauce jar tipped sideways. The next morning, at 6 A.M., after I had had my coffee and toast and was ready to settle down for a long uninterrupted session of beetle watching, I went out for a look. Four beetles were in attendance, and in their gorgeous jet-black garb with bright orange stripes they made a pleasing picture on the shrew’s dark gray fur. They were all Nicrophorus tomentosus, a species that has short yellow fuzz on the thorax. They had already removed the shrew from the jar, and a pair of them were under it, heaving it along the ground beyond the jar lip. The other two beetles, who were much smaller, were ten centimeters distant and apparently hiding underneath the jar cap. They stayed there for at least the next hour as the larger pair continued to move the shrew.
The ground near my doorstep is solidly packed; it was not a suitable place for the beetles to bury their prize. Independently of each other, the two beetles made repeated excursions in all directions, sometimes as far as sixty centimeters away, as though searching for a place to take the carcass for burial. Then they would return to the shrew before going off in another direction. How did they find their way back each time? Did they remember the route they had taken? To find out, I intercepted a beetle and placed a spoon in front of it, guiding it to walk onto the spoon, then released the beetle two feet from the carcass. It walked directly to the carcass without any apparent search. Had it memorized the local topography? I intercepted the other beetle as it was searching sixty centimeters north of the carcass and released it a meter and a half south of the shrew. If it navigated by remembering its previous route, it should now continue to run on away from the shrew. This beetle stayed still for a few moments, then lifted off in flight and went directly back to the carcass. I now removed one of the two beetles a meter and a half away, as before, and this one preened and then flew directly back to the carcass as well. Another time I placed one of the beetles a meter away from the carcass. This one turned in many little circles as though searching before walking in a direct line back to the shrew.
These beetles appeared to be more clever than I thought possible. I would have liked to experiment further, and the mystery of their homing ability remains in my mind. But I had to stop interfering with these individuals, because at this time I was mostly concerned with their carcass handling.
Meanwhile, one of the smaller beetles that had hidden under the jar cap emerged and went directly to the carcass. Would it try to steal it away from the pair? No, its intentions were clear; the second it got there it (he!) jumped on the back of the larger of the pair and copulated with her. That took only a few seconds, and then he left immediately and hid again, this time under loose bark on the soil about thirty centimeters away. There was a lot more going on here than I had ever expected, so I kept up my watch.
At 7:15 A.M. another Nicrophorus came flying in. This one circled around for at least a minute before it finally landed at the shrew carcass. Almost immediately it mated with the resident female. Then still another flew in, and the same thing happened. The female who was getting all of this attention continued crawling around the shrew without any interruption—around and under the carcass she went, trying to bury it. I thought that the pair was “supposed” to fight the intruders off, but I saw no fighting. By this time the shrew had been moved only about thirty centimeters away from the jar, but it was still on hard-packed ground, and there was no soft soil nearby.
After the second mating, which again took only a few seconds, I picked up the visiting male with my spoon (so as not to handle him directly and startle him) and dropped him a meter and a half from the carcass, wondering if he would return to it or to her. He seemed not in the least perturbed; he just sat where I dropped him and proceeded to clean himself compulsively. He used his legs to rub his abdomen, then his head and antennae, and then he rubbed his legs against each other. Done with that, he hesitated a few moments, then took off in flight and circled around before landing on a cherry twig at least three meters distant from the shrew, where I took photographs of him. I noticed then that he was covered with mites. I had not seen mites at all while he was mating; at that time the bright orange stripes on his back stood out. Now the mites were attached all over his back, almost fully obscuring the orange so that he looked pinkish brown. The mites were camouflaging him. But where had they been just before he took flight?
To the mites, the beetles are simply a vehicle for hitchhiking to more fresh fly eggs. As soon as a beetle finds a carcass, the mites hop off and forage, and when the beetle leaves they presumably hop back on.
A friend recently told me he caught several sexton beetles in a can with some rotting meat. When he later looked in he found that two were dead and two almost dead. He saw mites “walking all over the half-dead ones as though to revive them.” One of the sextons did revive, and then all the mites on it immediately scooted under its elytra (wing covers). I wondered if the mites sensed that the beetle was leaving. This is not a preposterous idea (although I am of course not inferring conscious knowing) because the beetle would shiver before taking flight to raise its body temperature, and either the vibrations or the temperature change might signal the mites to attach themselves so they could be safely carried.
Another beetle came flying in around 8:00 A.M., apparently with the same intention as the others, and it was not just to eat carrion. First things first: an immediate mating. Still another one came at 8:15.
For a while there were five beetles at the carcass, but three eventually walked off and hid under the debris on the ground nearby. The original pair was finally alone at the carcass. I’m not sure what all of these vignettes prove, but they did cast doubt in my mind about the beetles’ reputed monogamy.
Throughout the two and a half hours I watched the beetles, I heard repeated episodes of squeaking. These sounds occurred only at the carcass, and I think they were made by the pair, because I heard the sounds as much when they were there alone as when others joined them, so the squeaking probably was some kind of communication between the two.
As the air temperatures rose through the morning, both green and blue botflies started flying in. A few red Formica slave-making ants, which had earlier in the summer moved into the roof space of my cabin, also showed up. The beetles appeared to pay no attention to these visitors, but by now it was clear that this shrew was not going to get buried. The distance to suitably soft soil was too great for the pair to move it that far. The prize would now go to the flies or the ants or would become a resource for food or sex for adult burying beetles. It was not destined to be a place and resource for rearing their young.
I watched the beetles try to move the shrew for the rest of the day to document the developing story as fully as I could. By noon, when temperatures in the shade had risen to 86 degrees F, as many as eight beetles—all N. tomentosus—were at the shrew carcass at the same time. I had watched nearly continuously and observed fourteen additional matings. On the other hand, I had seen only two tussles, each lasting only seconds—no real fighting. The pair eventually moved the shrew about a meter farther before they stopped trying; it was still on packed soil. Though botflies were abundant, I saw no eggs. I observed one large muscid fly deposit a live larva but saw no developing maggots.
By noontime the beetles had finally chewed a hole through the belly skin of the shrew. At least two entered the carcass and were presumably feeding there or else trying to find a way out, as the skin heaved here and there. From then on the carcass did not budge from the spot. By 3:00 P.M. there were no beetles left, but that night, at 8:45 P.M., I found two back under it.
The next day I opened the carcass and found seemingly fresh meat but not one maggot, despite the dozens of both green and blue botflies that had been on it the day before, along with the beetles. I moved the shrew onto soft soil, and for the rest of that day there were never more than two beetles on the carcass, which the pair finally buried.
MICE AND SHREWS are of manageable size for these undertakers, but what would happen, I wondered, if they got something far too big, such as a super-sized “mouse”—a fresh road-killed gray squirrel? In the first two hours after I laid the cut-open squirrel on the ground next to my cabin, it attracted five N. tomentosus. The next day there were up to eighteen of these beetles on the carcass at a time, with one to four of them “calling” (standing still with hind end up in the air dispensing their come-hither scent to attract others). More beetles were flying in while others were leaving. I saw no evidence of pairs, though; most of the beetles were feeding on the meat, and there was a lot of random-seeming mating.
The next day the temperature dropped from 75 degrees F to 55. There had been very few botflies, and now all of the beetles left. Most walked from a half meter to two meters away from the squirrel carcass and buried themselves under the leaves and in the soil. A raven came and took away the carcass with its still almost fresh meat, putting an end to my attempt to find out what the beetles might do.
I decided to let a rooster stand in for the squirrel so I could get a more detailed picture. I laid a fully feathered dead bantam rooster belly-down (but not cut open) on the ground in the woods. Temperatures were in the high 80s, and when I checked the rooster the next day, it had already attracted hundreds of green botflies, whose maggots could devour the whole animal in days. The rooster’s feathers were coated with hundreds if not thousands of white botfly eggs. When I turned the bird on its back, more than a dozen Nicrophorus beetles scattered in all directions. The dry leaves surrounding the rooster fizzed like an opened champagne bottle. All the sextons were running fast and burying themselves under the loose leaves. I was so startled to see them I didn’t catch a single one.
At this carcass I noted several species of Nicrophorus. I had not known how much variation in coloration and size there is between species, and I wondered if I should examine all of the beetles in detail to find out what species they were. Should I wait to see if they would fight it out until only one pair of each species remained? Or would all the beetles somehow “cooperate” and fight off the imminent takeover by the flies? I decided to just wait and see what would happen.
By the next day there were even more Nicrophorus beetles on the rooster carcass. Curiously, there were no more botflies, although temperatures continued to hold in the 80s. Even more curious, all of the botfly eggs that had been deposited on the feathers now seemed shriveled; they had apparently died. There was not a maggot in sight. The skin of the rooster looked pristine, as though it had been sterilized. I was seeing the result of the sexton beetles’ battle with the flies, and here the beetles had won. The numerous (about two dozen) beetles at this large carcass had probably reduced the competition from maggots, which usually overwhelm a carcass and claim all or most of it. With so many beetles, though, each one paid a price, because when two or more pairs are breeding communally at a carcass, the larger individuals are stimulated to increase their ovarian development, while the smaller beetles’ development is delayed. I was glad that I had let the drama play out, yielding such a clear and dramatic result.
To determine how many beetle species were using the carcass, I needed to catch them all. I left the beetle horde to reassemble under the rooster for a couple of hours, and when I came back I carefully removed all of the leaf litter and loose soil around the carcass to limit the beetles’ escape routes. Then, keeping an open jar at the ready to dump the beetles in, I turned the rooster over and started grabbing. After I caught all the running beetles I could see, I started digging. Those that had made it under cover were not always easy to see, because the second defense of these beetles who bury the dead is to fake death; they curl up with their legs extended, just like a dead specimen. They lie on their side or back so that you see only their dark underside against dark soil; the bright orange patches on their backs are not visible. Despite their evasive maneuvers, I managed to get a haul of thirty-nine Nicrophorus beetles, which turned out to be of four different species.
I followed the fate of the rooster carcass for another five days, continuing to collect beetles, for a total of seventy sextons (fifty-eight N. tomentosus, nine N. orbicollis, two N. defodiens, and one N. sayi). The N. tomentosus, by far the majority, were the same species I had observed in pairs on the many mouse, shrew, and chipmunk carcasses I had set out that month. There had been no burial of this rooster, but neither had the flies taken it; it remained maggot-free. On the sixth night it was finally taken away by a large animal, probably a skunk or a raccoon.
THERE WERE MORE mysteries here than I could ever solve, making these results all the more exciting. But as always, some of the most surprising revelations from my observations had nothing to do with what I was originally looking for. In this case something unusual caught my eye when I dumped some of the captured beetles into a second jar.
As I have mentioned, the first escape strategy by these beetles when a “predator” comes to the carcass they are on is to run and hide and then burrow into the soil. I’ve described how they “play possum,” feigning death, but when I picked them up they quickly gave up on that defense and bit me instead. In the jar, with these three options unavailable, some beetles tried a different tactic: flying out. I looked closely at a beetle in my jar, admiring its black back with bright orange markings—not only because the markings were beautiful but also because they can identify the species. I was shocked to see the bright orange and black turn, in a flash before my eyes, to brilliant lemon
yellow the instant the beetle took flight! How could that be?
Like many insects living today, the ancestors of beetles had two pairs of wings. Now, however, they have only one pair. The original first pair has become modified into a two-part hard shell, the elytra, or wing covers, which are useless in flight but serve as a coat of armor covering the wings when they are not in use. The elytra are often decorative. The beetle’s membranous wings are usually at least twice as long as the animal’s body, but when not in use they are stored under the elytra, folded up like sheets put away in a drawer. In most kinds of beetles, the wing covers are usually spread passively to the sides during flight or simply folded over the back. In either case, an observer sees no color change. But I had seen a dramatic color change. Or was I seeing things?
I realized I had never seen any orange on the beetles flying near carcasses. I had seen yellow, which I had assumed was because the thorax of N. tomentosus is covered with yellow fuzz. Now I wondered if I had overlooked the orange and black because of the beetles’ fast, erratic flight. I looked again: yellow only. I decided the camera might see what I could not, so I took some wild shots of flying beetles. I managed to get several fuzzy pictures, which were enough to confirm my hunch: the backs of the beetles in flight were yellow!
I then examined both live and dead beetles, trying to flip their elytra to the sides, as they would be in flight, to expose the top of the abdomen, which I found was black. But as I tried to lift the elytra of both living and dead beetles, they spontaneously rotated, twisting up with the outer edges turning inward. Then, when I moved the elytra back, they locked into position to cover the abdomen so that the formerly outer surface was now inward. In other words, the elytra, unlike those in any other beetles, as far as we know, lock over the back with the dorsal side down and the previously hidden ventral side up. And this previously hidden ventral side of the wing covers is . . . lemon yellow! Thus the “secret” of the beetles’ color change is that this yellow underside is exposed when the beetle flies. In all other beetles in the world (to my knowledge), the upper elytral surfaces remain up during both rest and flight.
Life Everlasting Page 2