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On the Trails: Dung - a resource for its producers

Posted: January 4, 2013 - 1:01am
As the black bear climbed up the tree, the cinnamon bear growled and eventually dumped the contents of its gut on the bear below. That ended the face-off.  Photo by Doug Jones
Photo by Doug Jones
As the black bear climbed up the tree, the cinnamon bear growled and eventually dumped the contents of its gut on the bear below. That ended the face-off.

Long ago, my first big research project necessitated much wading around in marshes. My task was checking the nests of blackbirds: counting eggs and chicks, measuring growth of chicks, sampling food brought to chicks, and keeping track of chick mortality. The blackbirds weren’t too fond of me, so they often yelled and fussed, but what really made the job more interesting were the black terns that also nested in the marshes.

Even though I seldom came very close to the tern nests, the watchful parents were on the alert. They commonly attacked from behind — swooping down, occasionally actually hitting my head, and usually dropping fecal bombs. A few well-directed splats persuaded me to wear a hat. (Strictly speaking, bird excrement is a combination of feces and uric acid from the kidneys; birds save weight by voiding nitrogenous waste not as heavy, watery urine, but instead as lighter, semisolid uric acid.)

The Arctic terns that nest on the wetlands and near the Mendenhall Glacier would defend their young ‘uns in the same way. But they are not the only creatures to use their own fecal matter as a weapon. Howler monkeys in the American tropics hurl their dung at unwelcome intruders near their arboreal camps and feeding trees.

When common eider ducks are flushed from their nests by foxes or weasels, they defecate a noxious fluid over their eggs. The stink and perhaps an initial taste repel the would-be egg predator.

One of the rangers at the Mendenhall Glacier Visitor Center watched a bear high in a cottonwood tree void the contents of its intestines and bladder on another bear that attempted to climb up the same tree. This response may have been engendered by fear (which tends to loosen the relevant sphincter muscles), but the effect was certainly to deter the second climber.

Sometimes dung is used in construction. For instance, certain caterpillars build portable cases of fecal pellets, glued or silked together. An Australian caterpillar goes a step farther, sticking several fecal pellets together, end to end, making structural rods that support a silken tent. In fall, marmots close the entrance of their hibernation burrow with a mixture of feces and dirt. Burrowing owls are reported to pile up cow dung near the entrance to the nest burrow; this may help control the microclimate within and attract edible insects.

Dung (and urine) can be used for markers, signaling the borders of a territory or, more simply, the passing-by of identifiable individuals. A sort of “Kilroy was here.” For example, foxes may deposit a pile of dung, and then anoint it with urine and anal secretions, letting other foxes know just who was present (and probably when). Stallions with harems of mares often create groups of dung piles where they defecate repeatedly, sometimes urinating on them as well. These apparently signal other stallions that this one is the boss of the harem, and it may signal something to the mares also.

Many rodents (including beavers) and rabbits produce two kinds of feces: the usual kind, which is just deposited, and a soft, nutrient-rich kind, which they eat. It’s a way of recycling the food and extracting more nourishment. This process is called “refection.” Interestingly, that term (from the Latin for refreshing or repairing) is also used to mean a lunch or refreshment for humans (and a refectory is a dining hall). How’s that for a reflection on refection!

Just after the Old River Channel froze over this year, another retired biologist and I found a place where swans had walked around on the ice. There were remnants of green feces on the ice, but only remnants, suggesting that the swans were really hungry, because their usual green plant food was under the ice, and so they recycled their own feces.

A wonderful example is provided by the nesting habits of hornbills, large birds of Africa and Asia that nest in tree cavities. A female picks a nesting cavity, in which she will stay for several weeks and even months, while she incubates the eggs and broods the chicks. She seals herself into the tree hole by closing the opening with her feces, wood chips, and leftover food scraps, a mixture that hardens, leaving just a narrow slit through which the male feeds her and, eventually, the chicks. What a life!

Humans have often used their own waste products, euphemistically called “night soil” to fertilize agricultural fields. This practice tends to foster the distribution of certain parasites, from one human to another. Synthetic fertilizers and manure from domestic animals have replaced this source of soil nutrients in many areas.

Weapons, building material, signals, fertilizer, and food. What a versatile product!

Of course, dung turns out to be useful for creatures other than its producers, but that’s a topic for another essay.

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

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