26 Dung beetles interrupt a private moment
Ben Franklin was a polymath, but he couldn’t have understood much about basic physiology when he wrote “…nothing can be said to be certain, except death and taxes.” He obviously forgot about peeing and pooping, which are certain (at least for the healthy), though sometimes inconveniently so.
One peaceful morning in 1970 in the Kalahari, I was squatting and having a private moment. Part way through the event, I heard a “bzzzzzzzz,” which terminated with a “thump.” I looked to my side and saw a large scarab beetle (~ 1”) scurrying toward me (or rather toward my fresh deposit). Soon it was actively trying to pinch off a hunk of dung. Then I hear more bzzzzzzzzs and thumps – additional beetles converging around me.
I suppose I could have taken offense, but I was fascinated. I watched the beetles break off hunks dung, shape it into a rough ball, and quickly roll it off into the desert.
What happens then depends on the species (Heinrich and Bartholomew, 1979). In many cases, a male does the rolling and a female hitchhikes – a moveable feast of sorts. Eventually the male buries the dung and mates with the female, which deposits an egg on the dung. When the egg hatched, the larva would have ample supply of food. And the life cycle (or “recycle” in this case) continues.
Dung beetles weren’t common in our region of the Kalahari, but they were regular visitors at a few sites. They came in various sizes – the biggest Kalahari ones were more than an inch in length, if memory serves. Soon I noticed that big beetles rolled off large balls, whereas small beetles rolled of small ones. But there was an exception. If a big dung beetle arrived late to the feast and spotted a small dung beetle rolling a small ball away, it would steal the dung ball away from the lesser beetle.
At that time (late 1960s) optimal foraging theory was a hot topic in ecology. Theoreticians used mathematical models to predict optimal size of prey or the optimal mix of foods. One common graph plotted prey size against predator size. With a few obvious exceptions (e.g., huge whale sharks that eat tiny plankton), big predators eat large prey – hardly surprising, but that result is central to many foraging models.
That foraging literature inspired me to conceive of a future graph that would plot the size of a dung ball versus the size of the dung beetle. Gathering the data would be easy, and – weather and my digestive system permitting – I could add one or a few data points every day. And in this case, the data came to me (a “squat-and-wait” data mode?).
Almost immediately, I came up with a title for my future paper: “Scaling of coprophily in Kalahari dung beetles.” Moreover, I even had a journal in mind – Science magazine. They would reject my manuscript, of course, but I just wanted to receive the rejection letter (in those days, rejection letters came in the mail), which I would frame and hang on my office wall, thereby displaying the likely text: “Dear Professor Huey: Thank you for submitting a manuscript “Scaling of coprophily in Kalahari dung beetles…”
Alas, the manuscript never evolved. I discovered that the only person who could identify the beetles had just died. And there are many species in the Kalahari – at least 70 at one site (Davis et al., 2010).
But wait, there’s more to the story. In the early 1980s, Carolyn (my wife) made several trips to northern Botswana — paradise on earth for both of us. One of her trips coincided with the end of a multi-year drought. Intense rains and local flooding washed away the soil in some spots, exposing some curiously shaped receptacles (Fig. 1). These were hollow mud cups (3+ inches in diameter), with walls up to 1/2” thick and with a large opening at the top. Remnants of vegetation remained in the walls, but the insides were empty, clean, and smooth. Those walls had once encased a dung ball (possibly dung of an elephant) and served as a nursery for a developing dung beetle.
Why didn’t the dung casing wash away along with the surrounding soil during those torrential rains? Perhaps some of the vegetation bits added structural resistance to the walsl, or perhaps excretory products from the larva cemented the surrounding soil, increasing its resistance to water. Who knows? In any case Carolyn had the good presence of mind to collect several of the cases and to ship to them to the States, where they now serve as remembrances of dung passed. In October 2023 we gave one away to my gastroenterologist, immediately following a colonoscopy. When he sent the medical report, he added: ’…thank you for the terrific gift. I love it.”