WHY do some animals become cannibals, even though they risk lethal illness as
a result? A new study of caterpillars suggests a surprising
answer—cannibalism might help some animals avoid hungry predators.
Many animals—mostly fish, amphibians and insects—eat their own
species, especially when food is scarce. Cannibals improve their chances of
thriving by stamping out some of the competition.
But sometimes it’s unclear why animals resort to cannibalism. For instance,
the army worm Spodoptera frugiperda, a caterpillar that lives
on maize whorls in Central America, often eats its kin even if other food is
available. And lab experiments by Jason Chapman and Dave Goulson at Southampton
University have shown that the cannibals fare badly after lunching on their
neighbours, partly because it is relatively easy to contract infections from
their own species. “They probably also pick up physical injuries because the
victim retaliates,” says Chapman, now at the Institute of Arable Crops Research
in Rothamsted, Hertfordshire.
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So why do these army worms eat each other? Chapman suspects that the
cannibals, by depleting their own communities, might make themselves less
vulnerable to predators such as earwigs. A smaller community on a maize whorl
may attract fewer predators that hang around for less time. And sparse army worm
populations do less damage to plant foliage, leaving more hiding places for the
remaining caterpillars, he says.
To test the idea, Chapman and his colleagues measured the densities of army
worms and their predators on maize whorls in a field in southern Mexico. Sure
enough, the most damaged whorls—which presumably had the highest larval
density—also had the highest densities of predators, hinting that reduced
predator attack is one benefit of cannibalism. “In a lot of herbivorous insects,
this could well be the reason,” says Chapman.
“It’s certainly an interesting suggestion, but I’d like to see it taken a lot
further,” says Laurence Hurst, an evolutionary geneticist at the University of
Bath. He says the study doesn’t examine whether army worms eat their neighbours
simply to avoid suffering the same fate themselves: “If there are two of you on
a leaf, it might be that if you don’t cannibalise your neighbour, you’ll be
cannibalised yourself.”
Chapman, whose results will appear in Behavioral Ecology and
Sociobiology, agrees that more work is needed to rule out other
possibilities. He hopes to measure rates of predator attacks in different army
worm communities directly and then look for the same trends in other insects.