Sexton Beetles

By LORUS J. MILNE

MANY of us can remember a happy song we sang in Sunday school, which ran: “God sees the little sparrow fall. It meets His tender view.” No one seemed at all sorry for the sparrow or concerned about what happened to it when it reached the ground. Poetically, W. H. Hudson described the same pleasant death in his Birds in Town and Village. His basic theme was the bird’s lack of apprehension — it “was never miserable.”

But what then? And what the fate of the mouse that died the same night among the leaf litter under the trees, or the old snake that died in the pasture, or ihe fish which the storm waves threw high up on the beach ?

To the scientist, the “natural" death of an animal or plant is interesting mainly as a first step in returning to the soil the raw materials of which the living body was composed. The bacteria and fungi, the insects and other animals which speed reconversion of the dead carcass into new life, are highly important in keeping these raw materials in circulation. One of the more spectacular ways in which this can be accomplished is through the activities of so-called “sexton beetles,” insect gravediggers that inter small carcasses to provide food for their offspring.

Hudson’s bird died in a winter night. Many do so in summer nights and, like Hudson’s bird, fall to the dark ground to lie motionless. A fox or a skunk may find the little body, or it may lie unmolested awhile. A large wood ant is almost sure to locate it within an hour, as the night’s jackpot in routine foraging. The ant is likely to bite out a fragment and hurry away on a long and tortuous course lo its nest, where the morsel and the news will be shared amid great excitement.

Before the ant returns with hungry relatives, a stout beetle is flying toward the dead bird, directing its flight by sensitive feelers which have picked up the faint odor of decay in the night air. By rapid stages the beetle reaches the immediate vicinity of the bird, then drops to the ground, folds its wings beneath protective covers, and forces its way through the grass root tangle to the small carcass.

Arriving at the bird, the beetle stops, touching the feathers with only its antennal knobs, perhaps rearing up slightly to rest its forefeet against the body while studying the odors. Only for a moment does the beetle hesitate. It shuffles quickly an inch or so along the side of the bird, then dexterously rolls over on its back as it crawls under the carcass. Now the beetle’s feet are against the bird’s feathers, its hard back against the rough earth. The beetle lifts the bird over itself using its strong black legs, and with an amazing display of strength actually shifts the dead body a fraction of an inch.

The bird is several inches long, the beetle about one inch; the bird weighs many times as much as the beetle. Yet the lifting is repeated again and again in rapid succession, the carcass moving farther and farther while the smooth back of the beetle slips a little against the ground. In a few moments the beetle has slid under the carcass to its farther side; then quickly emerging, it rights itself as it comes free of the bird’s dead weight.

Again the beetle examines the bird with its feelers. The initial inspection has satisfied the beetle of several facts. There is adequate meat to feed a brood of beetle young. The bird is small enough to handle, even if it must be carried some distance to a suitable burial site. It is much smaller and lighter than the dead groundhog the beetle found yesterday. Moreover the bird’s body is free from surrounding bushes and is not likely to offer insuperable resistance to being transported. It will be hard work and require frequent rest periods, but the carcass can be moved. The beetle is the first of its kind on the scene, a head start of considerable importance.

Speed and efficiency are important to the beetle, to the life chances of its unlaid eggs. Is the earth under the bird soft or hard? Are there roots in profusion or is the soil sufficiently loose to make rapid digging possible? To find the answers, the beetle pulls away the earth under its forefeet, pushes head and shoulders into the hole so formed, and starts crawling. The feet separate the earth in front of and below the insect, so that it moves ahead slowly, the soil over its back buckling upwards and breaking into fragments.

The beetle encounters a mass of roots and turns aside. A quarter inch of progress and the beetle is stopped abruptly by a half-buried rock. Again it turns. More roots. Out comes the beetle, scurrying a few inches from the bird and trying again. Still no use. The ground is very hard. Farther and farther the beetle explores, searching for a suitable burial site.

Meanwhile the ants have arrived at the bird and are busy cutting off small pieces. In this they are very inefficient, two ants frequently tugging at a fragment of skin which one of them has nipped free. Others hurry homeward with a morsel, but they go via the same devious route by which they came.

The beetle has found a soft place in the earth. It investigates and furrows nearly three inches each way and encounters no significant obstacles. The area is forty inches from the bird. Abruptly the beetle stops its plowing and returns to the carcass, traveling in an almost straight line, often pushing through tangled spots which could be avoided by a small detour. The strength of the insect is surprising. Its stout body must be filled with muscles. Arriving at the bird once more, it stops, tests the air, and again slides under the carcass, lifting and shifting it, and emerging near the head.

Ants! The beetle pushes its way through them, scattering and throwing them into confusion, temporarily driving some of them away. The beetle hurries around the bird and sets out for the plowed area with remarkable accuracy. Part way there it stops, changes direction sharply, and going off a small distance, does a little more exploring for a burial site. None is suitable. Off the beetle goes again to its plowed area, traveling quickly and as though by compass. After a little more loosening of the soil, back it goes to the bird. There are more ants now, but they scatter as the stout beetle scrambles through them. The beetle ignores the ants and stops to examine the bird.

The beetle goes under the bird as before, entering on the side nearest the plowed area. Lifting, heaving, struggling, the beetle moves the carcass half an inch toward the burial site before sliding out on the far side of the body. Quickly the beetle turns over, back up against the bird, and scrambles under the body again to emerge where it first went in. The smooth back of the beetle does not shift the carcass at all. Again the beetle rolls over and the body is shifted by powerful legs another half inch before the insect. slips out on the far side. Four times more the beetle shuttles back and forth, and the bird is two and a half inches nearer the burial site.

It is very strenuous effort. The ants are leaving; the beetle is too disturbing to them. The bird does not stay still; it moves along the ground, even rolling over occasionally. It jerks as though alive again. The beetle is very industrious, pausing to rest only at long intervals. Suddenly another beetle arrives. In a moment the two have met under the carcass. The new arrival is of the same sex as the “owner" of the carcass. Out come both beetles, the first arrival angrily pursuing the latecomer, both squeaking in high-pitched complaining tones. There is a roughand-tumble fight at the side of the bird. The second beetle breaks away, runs a short distance, and takes to flight. The first beetle scrambles around the carcass and returns to work.

A few minutes later another beetle arrives. This time it is of a different sex and no objection is raised. At last a mate has arrived, but the lack of courtship is peculiar. Apparently it is the mate’s assistance that is appreciated, rather than any inherent charm. Although the newcomer is much smaller in size, twelve legs are better than six, and the bird is moved along much faster. The first beetle leaves the body to run over to the plowed area. It is less than thirty inches away now. In three hours the beetles will have the bird at the chosen burial site.

Even with two sexton beetles cooperating, the movement of the carcass is not without difficulties. Grass clumps may be parted or by-passed, but once in a while some stem or stick bars the way. Sometimes a loop is formed by a long-stalked leaf such as clover, and the carcass becomes wedged into the loop. A beetle will investigate, put its head and shoulders under the loop, its feet in the feathers of the bird, and strain ahead. Usually such forceful treatment stretches the loop enough so that, when relaxed, it will clear the carcass. If it does not, the beetles may loosen the earth under the body to let it sink a little and come free, nr one may settle down on the restraining strand to chew it to the breaking point. Meanwhile the mate struggles to move the carcass, or “rests” by running over to do a bit more plowing in the burial site.

By daylight the bird is being let into the ground. The beetles shuttle under it, pushing out the loosened earth on all sides, sinking the carcass into the hole. As they scramble around we can see that each is mostly black, with orange-red marks on the wing covers. One beetle is slightly more than an inch long, while its mate is about three quarters as large. Both are heavily built, with strong black legs. They hurry continually and coöperate with wonderful efficiency. In an hour only the bird’s tail feathers show above ground, and the few early-rising flies which were attracted to the vicinity leave for more available carrion.

By noon all of the bird has vanished, and the beetles with it. The sun has dried out the earth they loosened so that it is only with difficulty that we can see where they let the carcass into the ground. They are still working on it, sinking it lower by shifting the earth from below it to above it. When the bird’s body is two inches below the surface of the ground, they will have a rounded chamber hollowed out around it. There they will remove all feathers and work the body into a ball. The feathers will be pulled away into near-by earth where they are out of the chamber.

It is amazing to think of two small insects carrying a bird as large as a full-grown robin, taking it as much as ten feet from where it fell, and working it into the ground unlil two inches of earth separate the bird from the outside air. Yet twenty-four hours is sufficient for this burial and for the preparation of the denuded body in its earthen cell.

At this stage the male beetle may leave, or both may remain to attend to their progeny. The adults prepare a short tunnel away from the main chamber, and at intervals along its walls the female lays her fifteen eggs.

While these are incubating, the parents work on the ball of food they have buried. They keep it free of excess moisture, yet do not allow it to dry out. Any fly maggots, other insects, or earthworms which appear must be disposed of by the parent beetles before they endanger the food supply. If some form of embalming is carried on, we do not know just what is done. We do know that bacterial action is restrained and that the mass of tissue turns quickly to a gray paste which lasts for a month or more in this condition if the adults continue in attendance.

The beetles now feed on this food and, when their eggs hatch, nourish the young by regurgitating partly digested material. The young are agile little grubs at first, but they grow rapidly on the constantly available food, becoming fat and relatively helpless. When the young reach full size, the parent beetles lead them away through the soil several inches from the shrunken remnant of the food mass, and there leave them to pupate. In a few weeks they will have transformed into adult sexton beetles, ready to search out their own dead bird, mouse, or beached fish.