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On the Trails: Using animal excretions as resources

Posted: January 11, 2013 - 1:00am
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A dung beetle has made a nice sphere of dung and will roll it to a suitable place for burying; its larvae will live on the dung while they are maturing.  Photo by Bob Armstrong
Photo by Bob Armstrong
A dung beetle has made a nice sphere of dung and will roll it to a suitable place for burying; its larvae will live on the dung while they are maturing.

Some animals use their own dung as a resource, for defense, or construction, or food (see Empire, Jan. 5, 2012). Animal excretions can be used as a resource by other organisms, as well.

Some organisms could even be called fecal specialists.

Certain mosses depend on vertebrate feces (and sometimes bones) for a place to live. These mosses produce sticky spores that adhere to flies that are attracted by the fecal aroma and the prospect of getting lunch from the partially digested material. After visiting one dung pile, the flies move on to another. Thus the spores get carried to a new dung pile where the moss can grow. Also, at least one species of lichen favors sites with rocky outcrops and branches that provide good perches, where birds land and deposit their excretions.

Ivory gulls in the far north commonly eat polar bear dung. And just recently, a friend and I watched a passel of crows apparently consuming gull droppings and poking through material the gulls had regurgitated. Yum!

Dung beetles favor the excrement of herbivores and, to a lesser extent, omnivores; most of these beetles belong to the group known as scarabs. The beetles have scoop-shaped heads that, along with the antennae, are used to make balls of dung. The best-known dung beetles roll their balls of dung, often weighing much more than the beetles themselves, to a place where the beetles can bury them. These balls represent such an important resource that beetles sometimes fight over them. Other dung beetles just bury a wad of dung where they find it, or burrow into a dung pile. The dung is used as food for adult beetles and as both food and habitat for beetle larvae. The dung-rollers were conspicuous features of Egyptian mythology, and images of scarab beetles appear frequently in articles made for tourists in Egypt.

Dung beetles have now been imported to many places in the world, because they help recycle nutrients into the soil. In addition to improving soil fertility, they may decrease greenhouse gas emissions and improve hygiene by decreasing the above-ground accumulation of fecal matter.

Bird excrement is partly fecal matter and partly uric acid, excreted at the same time. A typical bird dropping is a white splat with dark lump in the middle. Some creatures take advantage of a common tendency to avoid eating droppings: Some spiders, adult moths, and butterfly larvae have come to look like bird droppings, and this gives them some protection from predators.

When I travel outside of Southeast, I sometimes see clouds of butterflies, all sipping liquid from a puddle. This can be a beautiful thing to see — colorful wings held vertically like little sails, sometimes dozens of them on a single puddle. This behavior is, of course, called puddling. The liquid in this case is mammalian urine, and the butterflies are getting much needed minerals — an important supplement to a diet of sugary nectar.

The biggest users of animal excretions are microbes of various sorts, uncountable trillions of them, but that is a topic far too big for this space. Arguably the next biggest users of animal excretions are humans. Humans have used dried bison, or cattle, or camel dung for fuel. Animal manure is sometimes incorporated as a source of fiber in building adobe bricks and walls. Urine can be used as a mordant for setting natural dyes. For example, Tlingits and other northwest coastal people used urine as a mordant to set the red colors obtained from alder bark. And I seem to recall reading that some Natives in the High Arctic used to ferment a pouchful of dead birds, feathers and all, in a soup of human urine, and later eat the delectable result.

With the rise of agriculture came a gradual understanding that fields would produce more if they were fertilized. So began the practice of adding manure of domestic animals to fields, or allowing such animals to graze over a field, leaving their excretions behind to fertilize the next crop. On a small scale, the guano of cave-roosting bats has been used for fertilizer.

An important source of animal fertilizer was bird guano, collected from bird colonies. Many islands and coastlines around the world provide nesting sites for millions of seabirds, whose guano sometimes accumulates in enormous quantities. The Inca people of Peru knew the value of this natural fertilizer and quarried bird guano from some off-shore islands where thousands of sea birds nest; they transported it from shore to field on the backs of llamas. But Europeans were slow to adopt the use of bird guano. It wasn’t until the mid nineteenth century that poor crop yields and the need for more intensive farming made guano popular among farmers. Then guano mining and shipping became big business, shipped by the ton from Peru to England, for example. The United States got into the act too, claiming ownership of many guano islands in the Caribbean. Eventually, guano was replaced by nitrates mined from the ground (some of which may have been ancient, centuries-old, guano) and then by synthetic fertilizers made in factories.

A famous and productive ecologist once wrote a 554 (!!!) page treatise on guano, summarizing what was known, in 1950, about the distribution of guano deposits around the world and their chemical composition. I suspect it will be a long time before that achievement will be matched.

• Mary F. Willson is a retired professor of ecology.

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