Ballistic defecation: Hiding, not hygiene

Evading predators may be the big factor driving certain caterpillars to shoot their waste pellets great distances.

Caterpillars of the silver-spotted skipper (Epargyreus clarus) can fire their feces as far as 153 centimeters, reports Martha R. Weiss of Georgetown University in Washington, D.C. Accounts from other researchers record tales of shots by caterpillars of other species of up to a meter.

To compare three theories of what evolutionary force drove the development of such firepower, Weiss set up challenges for groups of silver-spotted skippers, a species in which a caterpillar builds a series of shelters out of curled leaves and silk lines as it grows.

One hypothesis proposed that blasting the waste far away lowers the risk of disease. To test this, Weiss let feces build up in little containers housing some of her caterpillars. As they developed, she found no obvious difference between them and caterpillars in pristine containers.

Another hypothesis proposed that the waste ejection keeps sewage buildup from crowding the caterpillars out of their homes. Yet when Weiss forced caterpillars to build new shelters more often than normal, she saw a significant drop in caterpillar welfare only in the most extreme version of this test–when evicted caterpillars constructed 32 shelters, instead of the usual 9, as they grew and developed.

In contrast, Weiss found support for the hypothesis that waste draws a predatory wasp to the caterpillars. For example, wasps killed 14 out of 17 caterpillars on leaves where she had put waste pellets but only 3 of 17 caterpillars on leaves she had adorned with glass beads. In the April Ecology Letters, Weiss reports that predators are the most likely force behind pellet ballistics.


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Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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