Projectile Pooping

Research Blogging / by Dave Munger /

When it comes to eliminating wastes, some animals are overachievers. Silver-spotted skipper caterpillars and Adelie penguins both can fling poo to startling lengths. But how, and why?

Credit: Flickr user mescon

Not long ago, we had a new mailbox installed in front of our house. For a few hours, it was shiny, bright, and attractive. But within a day, the local mockingbirds had discovered that it was a great place to perch, and it was soon covered with bird droppings. So much for home improvement!

But if we had been lucky enough to have silver-spotted skipper caterpillars or Adelie penguins perching on our mailbox, it would have stayed poo-free (Of course, North Carolina mailboxes are not part of the usual habitat for either species, but I can dream, can’t I?). Adelies and silver-spotted skippers share a common behavior: Instead of defecating where they stand, they launch their poo a considerable distance: up to 1.4 meters away from their nests in the case of the caterpillars.

Naturally, the first question Victor Benno Meyer-Rochow and Jozsef Gal had about this behavior in Adelies is “what kind of pressure do these birds generate when launching?” As the UK undergraduate who blogs as “SamW” noted, Adelie penguins are a protected species, so researchers aren’t able to perform the invasive and potentially harmful observations required to directly measure the forces. Instead, Meyer-Rochow and Gal examined photographs. When Adelies poop, they stand on the edge of their nests, their rear ends facing outward, and expel the fecal matter as far as possible. The researchers estimated the penguins’ range at about 40 cm—not as far as the caterpillars, but still considerably better than the average human can do. Unlike humans, Adelies have a single “vent” for all bodily excretions (and sex)—the cloaca. Based on the size of the cloaca and the viscosity of penguin poo, Meyer-Rochow and Gal calculated the internal pressure at poo-launching at as high as about 600 grams per square centimeter (compare this to about 100 grams per square centimeter for humans). As SamW puts it, “no matter how hard you try to poop, a penguin can do it harder.” The research was published in 2003 in Polar Biology.

Why do these animals like to shoot the sh—um, excreta? While researchers can guess that penguins launch their poo to keep feathers clean, they don’t actually know for sure. With silver-spotted skippers, however, one intrepid scientist has gone to great lengths to understand exactly why they project their poo (or “frass,” entomologists’ preferred term for insect excrement). Meera Lee Sethi, a freelance writer, discussed the work of Georgetown University biologist Martha Weiss in a story for Inkling Magazine last week. Weiss had three hypotheses about why the caterpillars expelled their frass: The frass could be unhygienic; the sheer volume of frass could crowd a caterpillar out of its home; or frass might attract predators.

To test the first hypothesis, Weiss collected butterflies from campus and raised some of their offspring in clean cages, while allowing frass to accumulate in the others’ homes. Even though the “dirty” cages showed signs of poor sanitation, with fungus growing on the frass within days, there was no discernible difference between either group of offspring’s development. So apparently the frass isn’t ejected for hygienic purposes.

In a separate study, Weiss had shown that she could induce the caterpillars to abandon their nests and build new ones by stuffing them half-full with frass, so clearly caterpillars don’t like crowding. But does this process actually come at a biological cost to the caterpillars? Weiss forced caterpillars to build new nests at different rates: every one, two, three, or four days, while again measuring their growth rate. The difference between the groups was barely discernible, so frass isn’t getting tossed to relieve crowding or reduce the caterpillars’ homebuilding burden. This left only the third hypothesis: Could caterpillar frass attract predators?

Paper wasps commonly prey on caterpillars, so Weiss took empty caterpillar nests and attached either frass or similar-looking glass beads to them, then placed them in a cage with paper wasps. The wasps were attracted to the frass-bearing nests, but not the bead-bearing nests. So Weiss’ careful research finally had its payoff: caterpillars shoot their poo at least partially to throw predators off their trail! The study was published in Ecology Letters in 2003.

This explanation is unlikely to hold for penguins, though. Adelies breed in large colonies, readily perceived by predators. Indeed, the profuse quantities of poo produced by Adelies enable scientists to view their colonies from space!

Over the decades, poo has been an amazingly fruitful way for researchers to learn more about animal behavior. For more on poo, try searching for related terms on ResearchBlogging.org.

Dave Munger is editor of ResearchBlogging.org, where you can find thousands of blog posts on this and myriad other topics. Each week, he writes about recent posts on peer-reviewed research from across the blogosphere. See previous Research Blogging columns »

 

Originally published August 25, 2010

Tags biology structure theory

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