In the burial process the surface of the ball becomes coated with the dominant clayey soil. It is further reshaped underground and fortified with saliva, creating a rough shell. The female guards and maintains the ball until the larva has eaten away most of the dung and has pupated inside the hollow thus created. The female then leaves. The fresh pupa that remains underground is at first soft and milky white, but one can see the imprints of legs and other body parts, reminiscent of an Egyptian mummy wrapped in cotton gauze. After the rains come again and soften the soil, perhaps a year after the egg was laid, the adult beetle emerges from its “mummy case” by bursting through the shell of dung and saliva and burrowing up out of the earth. It probably waits until nightfall to fly over the veldt in search of a fresh dung odor plume.
I wondered why elephant dung (and many other kinds) is such a prized resource. Isn’t dung a waste product? Why would elephants not extract all the nutrients from the vegetation they ingest? The current hypothesis is that the food is shunted through their gut so quickly that they only skim off the cream, so to speak. But if thousands of beetles find sufficient nourishment in a half-liter of dung, it would seem that many nutrients must be left. Do elephants not have strong selective pressure for energy efficiency? That can’t be true, since their very large size and energy demands should create a high selective pressure to extract every ounce of energy from everything they eat. I realized, however, that they achieve their efficiency in digestion by relying on symbiotic microorganisms in their gut, which have the enzymes that break down the roughage they ingest. But the corollary is that these magical symbionts are to the elephants what cows are to us. Their “owners,” who herd the microorganisms in their gut, must practice restraint. That is, they can’t evolve enzymes that would kill their symbionts in order to digest them for a quick profit. They must let them live, and indeed elephants may have some unknown mechanisms that block them from digesting their “cows.” Also, these commensals must have evolved mechanisms that keep them from being digested by their hosts. As a result, some of these organisms end up whole in the feces. Once shed, however, they are a potential source of protein for those who have no economic restraint on killing the geese that laid the golden eggs for the elephants. And so I suspect that the dung beetles enjoy a free ride, thanks to the gut symbionts that the elephants need but can’t keep completely.
Dung is a valuable resource to many animals, and a system for recycling it probably evolved soon after it became available on the landscape. Cretaceous deposits in the Two Medicine Formation in Montana have shown that dung-processing beetles had already evolved relatively “modern” burrowing behaviors, and there were many such beetles at that time: they dug burrows apparently to remove the dung they collected from the intense competition aboveground. Given the many dung-producing animals—elephants, buffaloes, antelopes, giraffes, warthogs, baboons, lions, hyenas, jackals, leopards, hippopotami, humans, and rhinos—there is plenty of it and lots of variety. As animals drop dung, they, with the aid of the recyclers, produce soil and often spread and plant seeds of plants that, like the microbial symbionts, are resistant to digestion and can be transported alive in the guts of animals. Elephants, for example, are fond of fruit, and they spread great numbers of seeds from the many fruiting plants they eat. Some plants rely entirely on elephants for their seed dispersal, much as others rely entirely on certain species of wasps or bees for the pollination of their flowers and thus their propagation. A recent study has shown that Asian elephants spread seeds over a distance of one to six kilometers; Congo forest elephants spread seeds as far as fifty-seven kilometers.
As I examined my collection of dung beetles from Africa and started to draw them, I was impressed by the perfection of their unique shapes and sizes. I first drew the largest, Heliocopris dilloni, which lives exclusively off elephant dung. When these beetles come flying in to an elephant dung pile, they crash-land, then fold their large wings and tuck them under their burgundy-brown wing covers. They lumber along slowly, looking like muscular weight men on a track team. But these miniature bulldozers can tunnel straight down into the ground. For digging they have a four-pronged bulldozer-blade extension on the front of their flattened head, and their front tibia have shovel-like lateral extensions to push loosened soil out to the sides. Their hind legs are short, thick, and packed with muscles, and the ends of their hind tibia act as “treads” to harness all that power. Instead of coming to a single point, as in most beetles, the tibia of H. dilloni present a roughly square surface with several backward-pointing spines, to provide traction as the beetle pushes head-first through the ground. After digging a tunnel under the elephant dung pile, the male meets a female there and then stays near the top of the tunnel to bring loose dung down to her. She then makes the dung ball that will serve as her oviposition site and as the food for their larva. In some tunneling species the male brings down enough dung for the female to make several balls, and thus several offspring are produced in one nesting.
In contrast to the bulldozing H. dilloni are the rollers. These beetles come in all sizes, but all have the ability to remove dung from where it falls and take it away to a suitable place for burial, where they have a chance to keep it to themselves. Of this group, the S. laevistriatus are like deep-chested, thin-legged distance runners. When they come flying in at 30 kilometers per hour, they hit the ground running fast. As I have mentioned, they come at dusk, just barely before the crowd—or cloud—of thousands of others, most of which are tiny beetles that bury themselves directly in the dung. The S. laevistriatus have to be fast, not only because the competition for dung is intense, but because they need time to package it and haul it off. They are in danger while aboveground at a dung pile, because mongooses and birds such as hornbills and guinea fowl check out dung piles for insects to eat. I often saw mongoose tracks and the remains of larger beetles—the soft abdomen had been taken and the rest left. Dung piles are scenes of opportunity much like large animal carcasses, but they are laced with a high dose of risk.
African dung scarab beetles I collected at elephant dung during research in Tsavo National Park in Kenya, along with two beetles from South Africa, Kheper nigroaneus (top right) and an unidentified dung beetle species I collected in Kruger Park (center right). The large beetle at center is the elephant dung beetle Heliocopris dilloni, and to its left is Scarabaeus laevistriatus. The tiny one at lower left lives in the dung pile. Many of the others roll the dung away, and H. dilloni buries its ball directly beneath the pile. The larger beetles are black or brown; some of the smaller ones are metallic green or blue.
IN AFRICA IT is hard not to be reminded of our beginnings. Almost at random, as I watched for dung and beetles on the ground, I found chipped stone, and one six-inch rock with many rough chips flaked off that was almost certainly an Acheulian hand ax (see my drawing on page 50) dating back perhaps a million and a half years. I picked it up, and I still hold it in awe today, not quite believing that it is what it appears to be, a hand ax probably used to cut into animal prey or carrion. The scene on the wall of a nearby rock shelter reminded me that hominids, who evolved here, may have faced a situation at carcasses analogous to what the beetles face at elephant dung balls. It depicted a series of thin stick figures in full running stride—in form and function they were “distance runners” chasing down an antelope. Would these early men have grabbed what they could from the carcass as quickly as possible and then run? Or was their speed more critical in catching a live animal?
Fresh elephant dung balls are relatively solid, and when a Scarabaeus laevistriatus lands, its first priority is to run all over the pile in search of an already made ball. Stealing saves the time and energy necessary in making his own ball. But as in any competition involving physical strength and agility, body (muscle) temperature as well as body size are often deciding factors. Most fights between beetles that we observed in Africa lasted only a few seconds, and losers and winners were easily differentiated; the loser was the one who was flipped off t
he ball, and the winner the one who rolled it away. Immediately after each fight, we weighed the contestants and took their body temperatures with an electronic thermometer. To our surprise, the winners were not necessarily the largest beetles; they were those with the highest muscle temperature, several degrees centigrade higher than our own. These were the ones with the fastest leg speed, which in beetles is directly related to muscle temperature.
The ground under our feet was red clay, and I used water from our water bottle to make clay balls. The beetles ignored them until I dipped them into fresh elephant dung, after which they fought over them as readily as for the “real,” beetle-made balls. This worked so well that Bart and I staged many fights without having to wait for fresh balls to be made, to the point that we were not always able to intercept the clay balls before they were rolled away.
Although the beetle with the higher body temperature is favored to win a contest over a dung ball, that heat comes at a price. The beetles can shiver while they make their ball, but if they shivered for the whole half-hour it takes, they could burn through their energy reserves. They are like long-distance runners who can sprint at the beginning of a race but much less readily at the end, when their energy reserves are depleted. Thus the beetles seek out others with balls as soon as they land, when they are still fresh, warmed up by their flight metabolism and thus better able to win contests. To avoid having their balls taken, the ball makers must try to stay hot, continuing to shiver to protect their investment. Not all of them could do so.
When there were no already made balls to be had, the S. laevistriatus rollers quickly got to work tearing bits of dung out of the pile with their pitchforklike front tibia and then patting the dung into a ball, their front feet all the while moving in a whir. They were able to move fast because they had arrived hot-bodied from their flight; the exercise created and stored heat. Some of the large beetles had body temperatures up to 113 degrees F, about 15 degrees higher than ours and that of most other mammals. Hot beetles worked fast and usually could finish a ball of about baseball size in five to ten minutes; then they would start to roll it away.
Their long slender legs moved fast as they ran and rolled the ball, and if they were still at about 108 degrees F, they could run at an average speed of 11.4 meters per minute on level ground. If their temperature was 90 degrees F, however, their running speed was only 4.8 meters per minute.
We ourselves were running out of time and resources in our fieldwork, and after we returned home I was left with a question. Under less competition, would the S. laevistriatus work at a more leisurely pace and not bother to maintain a high body temperature? We had found that one species that worked in the daytime, when there is little competition, did work more slowly and had lower body temperature. In part to answer my question, I went with my graduate students Brent Ybarrondo and James Marden to southern Africa. Unfortunately, we found no S. laevistriatus during the several weeks that we were in Botswana, the Union of South Africa, and Zimbabwe.
James instead studied a species of swift-running tenebrionid beetle in which the males run on all six legs behind females on the ground. Brent and I examined Kheper nigroaeneus, a dung-ball roller in Kruger National Park. Like most other dung-ball rollers, the parents in this species have only one offspring per brood ball, which the female tends underground for around twelve weeks. Like the nocturnal S. laevistriatus, these diurnal beetles fought at dung piles, and again the winners were the hotter ones. But if there was stiff competition at the fresh dung source, the beetles either left to search for another, less crowded dung pile or they made only a small ball, thus reducing their ball-making time and the chances of having their ball stolen. The drawback of this strategy, however, was that the little balls could not provide enough food to serve as brood balls; they were used only as food for the adults. (Because the female expects to rear an offspring only at a large ball, she probably chooses the ball rather than the ball maker.)
In my collection was one beetle that I found puzzling. It had a large, generally flattened body and superficially looked like Pachylomera femoralis, but unlike that species, which has been reported to roll balls, this one had none of the anatomical tools for ball making, namely, flanges on the front tibia and front of the head. Instead, it had hugely developed front femurs studded with sharp spikes on the front. It looked like a “degenerate” P. femoralis, and from body shape and structure I wondered if it might, as in bumblebees, be a species that had evolved by parasitizing a closely related host. From its shape I suspect that it may have evolved from being a dung roller to one that enters the tunnels of other species, then uses its hugely muscular front feet to pry the competition off their brood balls, the way S. laevistriatus pries competitors off freshly made balls.
IN THEIR GREAT diversity of specializations to harvest a similar resource, the dung beetles are an evolutionary laboratory. The first dung-eating beetles had the resource to themselves; all had an equal chance, and not much speed and skill were required. After competition made the resource more difficult to get and keep, specialists had an advantage. Getting there first was part of the winning combination to get dung from the dung-pile bonanza.
Most beetles are warm-weather flyers. As with the sexton beetles, which in Maine and Vermont are common only late in the summer and when it is warm, the dung beetles are highly seasonal and seem to be largely restricted to the tropics. I have seen only two dung scarabs in Maine, and both were on carrion. One might guess that they went extinct along with a main food source, the dung of bison, but cows took the place of those herbivores, and there are no dung scarabs at the droppings of cows, deer, or moose; in contrast, the dung of the equivalent bovids and antelopes in Africa is used up almost immediately by dung scarabs in the rainy season. In northern Europe, where the ancestral bovids have been exterminated and replaced by cows, there are no swarms of dung scarabs at dung pats—at least I saw not one scarab during two weeks in August 2011 when I worked as a cowherd in the Swiss Alps. Australia presents still another scenario. Here there is a tropical climate but no resident bovids until very recent times, when cows were introduced by Europeans.
The dung beetles’ work has ecological significance. It fertilizes and aerates the soil and retards the spread of pathogens and disease organisms. But different types of dung beetles are adapted to handle specific kinds of dung in different seasons and habitats. Their specific roles in the ecosystem are difficult to determine, because we can’t experiment by removing them. However, an almost continent-wide “experiment” from Australia answered many questions. The Australian entomologist and ecologist George Bornemissza, who was born in Hungary, collected beetles as a boy in that country. After obtaining his PhD at the University of Innsbruck, Austria, he emigrated and joined the zoology department at the University of Western Australia. One of the first major differences he noted between his native Europe and Australia was the large number of dung pats covering the ground where cattle were grazing, which he had not seen in Europe, where the dung decayed because of the moist climate in the north and because of beetles in the south. He realized that the native Australian beetles were not adapted to handle the dung of cattle. He therefore proposed to import beetles who were up to the job, and he started what would be a twenty-year work, the Australian Dung Beetle Project, for which he would receive the Medal of the Order of Australia in 2001.
Bornemissza, supported by the Commonwealth Scientific and Industrial Research Organization (CSIRO), searched in thirty-two countries for the most suitable dung beetle species to handle the cowpat problem in Australia. It was indeed a serious problem for two main reasons. First, the cowpats had a tendency to dry out and remain on the ground, in time reducing grazing land. Second, they were an ideal breeding ground for the very pesky bush fly, Musca vetustissima. If Bornemissza could find beetles that were able to stand the Australian climate and that would recycle the dung, it would solve two great problems in one stroke.
Introducing exotic species is always potentiall
y dangerous, and all the beetles Bornemissza wanted to try out had to be raised in quarantine to make sure they were not carrying parasites that might become pests. Fifty-five dung beetle species were introduced into Australia, and they have become a proven success; they are now saving pastureland by increasing the health of the soil. The final report of the Queensland Dung Beetle Project concluded that the beetles are “undoubtedly worth many millions of dollars per year.” The pesky bush flies have been reduced to such an extent that the “Australian salute,” a flick of the hand used by generations of Australians to shoo away the fly, has become a “dwindling gesture.”
Bornemissza has now moved to Tasmania, where he and Karyl Michaels have focused on the impacts of clear-cutting and burning of logging slash, which has led to a large decrease in the beetles that reproduce in decaying wood. One of those wood-boring species, now endangered, has been named after him, the stag beetle Hoplogonus bornemisszai.
THE DUNG BEETLES, despite their diversity and ability to handle almost any kind of dung, are like many undertaker beetles in that they handle only the fresh article. Once the dung dries out, as it usually does in the tropical dry season, the beetles stop work. They are active mostly during the wet season, when they can burrow into the soil. Their young develop underground in the dry season, and the signal for their reemergence is the seasonal rains, which soften the soil. All the dung deposited in the meantime would be left on the ground—were it not for termites.
Life Everlasting Page 13