Scat-Firing Caterpillars Elude Predators
Several species of caterpillars have developed an interesting system for waste disposal;
they fire their fecal pellets a distance of up to 40 times their body length away from their homes, at a speed of 4.2 feet (1.3 meters) per second. The equivalent distance for a 6-foot-tall (1.8 meter) human would be around 240 feet (73 meters).
Scientists have long speculated on the evolutionary factors that would favor the development of this extraordinary behavior.
"While studies of foraging have been a cornerstone of ecological research, analogous issues related to defecation have received much less attention," said Martha Weiss, an ecologist at Georgetown University in Washington, D.C.
A lot of animals distance themselves from their waste, usually for reasons of hygiene. Some nestling birds, for example, package waste into mucilage-coated sacs ready for convenient disposal by the adults in the nest. Other animals are known to use scat for surprising purposes; larval tortoise beetles pile fecal shields on their backs to protect them from predators. Some caterpillar species climb onto silk strings decorated with fecal pellets, which are known as frass. The frass helps protect them from ants.
In Weiss's research of scat-launching caterpillars, she found the first experimental evidence that the adaptation serves to protect the caterpillar larvae from wasps and other predators.
Scat-Throwing Launch Pad
Skipper butterfly caterpillars are able to fire frass pellets by pumping up blood pressure directly under an anal "launching pad" on which extruded fecal pellets rest.
"It's the equivalent of the mechanism involved in flicking a pea," said Stanley Caveney, a biologist at the University of Western Ontario in London, Canada.
Caveney discovered the "scatapulting" mechanism, leaving scientists with questions about its purpose.
One explanation favored by scientists was that the caterpillars are just good housekeepers and keep their silk-stitched leaf shelters spotless for hygienic purposes.
Weiss's research provides the first experimental evidence for the predator avoidance idea.
"Until now, there has been no direct evidence to suggest the idea that fecal firing behavior in caterpillars helps them to avoid being caught by [predators]," Caveney said.
To test the link between wasp attack and frass accumulation, Weiss introduced the leaf shelters of the silver-spotted skipper caterpillar (Epargyreus clarus) into captive paper wasp (Polistes fuscatus) colonies. The roomy leaf pockets contained either hidden frass pellets or similar-looking black glass beads.
She found that the wasps that visited leaf shelters spent more than 70 percent of their time on those containing frass.
When caterpillar larvae were added to the leaf shelters, the evidence was equally as dramatic. During 5-minute trials, 14 of the larvae housed with frass were devoured by wasps. In contrast, only three were eaten in the shelters containing the black beads.
Related experiments showed that accumulation of frass in their leaf shelters did not affect the caterpillars in terms of crowding or disease. Skipper caterpillars in close contact with 30-day accumulations of frass were no less likely to make it to a healthy adulthood than those in frass-free homes.
The findings are reported in the April issue of the journal Ecology Letters.
Sniff Test
Predatory wasps appear to be attracted to the odor of caterpillar frass, said Weiss. "Evolutionarily, it seems that frass ejection helps to protect larvae from predation by natural enemies," she said. "If they retained frass in their shelters they would be more likely to be killed by wasps or other enemies."
The findings are "novel, interesting, and significant," said Caveney. "[The study] shows convincingly that frass may be used as a homing signal to locate prey."
Predator avoidance is a significant factor driving the evolutionary development of silver-spotted skipper caterpillars. Earlier experiments conducted by Weiss have shown that up to 30 percent of caterpillars in some wild populations can be killed by predators over just a few days.
In fact the risk of being found by predators is so great that many distantly related caterpillar lineages have independently developed the ability to fire frass, said Weiss.
That leaves just one question: Why are pellets ejected so far, so fast?
"A shot distance of a few centimeters or so would generally be sufficient to propel the pellet off the leaflet surface," said Weiss. "Perhaps it's just a by-product of caterpillar physiology."