Posted on 04/02/2003 11:20:01 AM PST by SteveH
News in Science
2/4/2003
Caterpillars fling faeces afar to fool foes
[This is the print version of story http://www.abc.net.au/science/news/stories/s822336.htm]
[photo]
Above: a skipper caterpillar lays down a silk guy-wire for its leaf shelter. Below: adult skippers make some more (Pics: M Weiss, C Williams)
Caterpillars shoot their faeces more than a metre from their homes to protect themselves from predators, an American ecologist has discovered.
Dr Martha Weiss of Georgetown University in Washington, wondered if animals gain an evolutionary advantage by developing sophisticated strategies to manage their waste.
In the current edition of Ecology Letters, Weiss reports on experiments with hygiene, crowding, and predator attack to explain why some caterpillars use an explosive method of waste disposal.
She studied the silver-spotted skipper, Epargyreus clarus, a common butterfly throughout much of the continental United States. Its caterpillar larvae have evolved two interesting behaviours: they construct shelters by folding leaves and tying them down with silk, and ballistically ejecting their faecal pellets great distances away from the shelters.
The skipper caterpillar extends a hard latch-like plate on its anus out of the leaf shelter, to forcefully launch grape-seed sized faecal pellets with a rapid squeeze of blood pressure. On average, caterpillars can shoot these pellets a distance of 19 times their body length.
Older and larger caterpillars can launch 24 g pellets nearly 40 times their body length, at speeds of 1.3 metres per second; the longest distance recorded by Weiss was 153 cm.
Based on the caterpillar's size, this remarkable feat is the human equivalent of spitting a seed about 70 metres.
Weiss hypothesised that accumulating faeces in a shelter would expose the caterpillars to microbial and fungal infections, and ejecting them would maintain hygiene. However, she found no significant difference between the survival of caterpillars raised in 'dirty' environments, or in 'clean' conditions where the faeces were removed daily.
Weiss' second hypothesis was that excess waste would crowd caterpillars out of their shelters. To build new shelters more often would mean significant costs in energy and materials, and increased vulnerability to predators while building. But even when half the shelter space was filled, and caterpillars were forced to reconstruct shelters four times more frequently, there was no significant difference in the proportions that survived.
A Polistes fuscatus wasp eats a caterpillar, tracking it by the smell of its faeces (Pic: M. Weiss)
Under natural conditions, predation is a major cause of caterpillar mortality. So Weiss tested her third hunch: that caterpillars remove faeces to eliminate the 'smelly' chemical signals that might alert predators to their presence.
Weiss introduced the caterpillar's natural enemy, the predatory Polistes fuscatus wasp, offering indivduals a choice between caterpillars sheltering with faecal pellets around them, and caterpillars with glass beads of similar size and colour to the pellets that were replaced.
Within five minutes, 80% of caterpillars surrounded by faeces were killed. In contrast, the caterpillars with the glass beads survived predation unscathed.
Weiss believes that the defecation behaviour of skipper caterpillars enables individuals to distance themselves from the chemical cues that reveal their presence to the enemy. This is likely to be the driving force behind the evolution of their faecal ejection, she said, more important than hygiene or crowding factors.
"If they retained faeces in their shelters, they would be more likely to be killed by wasps or other enemies," Weiss told ABC Science Online. "It's interesting that the trait has evolved independently in many taxa [groups of species]."
Weiss notes that faecal ejection also occurs in at least 17 families of butterflies and moths. While not exclusively exhibited by shelter-dwellers, it is usually seen in species with 'high site fidelity' - implying an underlying selection pressure. Previous work by Weiss shows that shelter-dwelling behaviour also protects against predators.
Defecation behaviours shaped by natural selection can be found throughout the animal world, she said: "Some larval tortoise beetles pile 'faecal shields' on their backs for protection from enemies. Other caterpillars climb strands of silk decorated with faeces that hang from leaves, to be safe from ants. Nestling birds in some species package their waste in a mucus bag that is carried off by the parent birds."
The study of ecology and evolutionary biology has been largely focused on the feeding strategies of animals; Weiss said it may be time for scientists to also look at waste disposal as a potentially promising new field which she dubbed 'evolutionary faecology'.
"Faecology refers to the study of defecation behaviour in a range of ecological contexts," she said in an interview. "The evolutionary part adds a historical perspective."
Mark Horstman - ABC Science Online
And you give me insects?
I'm sorry, the journalists and politicians already have that field covered.
I've always wanted to know how the wasp tracks the caterpillar...
sw
Ah yes. . . a few habaneras; some black & tans, I know that feeling.
Isn't that what Democrats do as well?
Shoot, I thought this was another Rachel Corrie versus the D-9 story.
silver-spotted skipper caterpillars fling faeces afar to fool foes
ten times, real fast
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