Impostor Caterpillars

Impostor Caterpillars

A species of caterpillar tricks ants into treating it better than their own young.


A caterpillar's clever con game. I'm Bob Hirshon and this is Science Update.

Some fish, birds, and bugs trick other species into raising their young. Now, scientists have figured out how a European caterpillar takes this a step further: they invade ant colonies, and get treated even better than the actual ant larvae. Oxford University ecologist Jeremy Thomas says this is especially clear when food runs short.


So you actually get the bizarre situation of the ants cutting up their own small ant grubs and feeding them to these caterpillars.

Using tiny microphones and speakers, Thomas and his colleagues discovered that the caterpillars make sounds with their abdomens that mimic those of a queen ant. When the scientists played back recordings of caterpillars to worker ants, the ants showed protective behavior normally reserved for guarding the queen. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

In a recent episode of the popular TV crime series “Law and Order: Special Victims Unit,” a young woman shows up on the doorstep of an astonished couple, claiming to be their long-missing daughter. It turns out that the woman is a total stranger and a con artist, seeking attention and affection from her supposed “family.”

This situation is pretty far-fetched for humans, but in some animal species, it's common practice. Several kinds of birds and insects infiltrate other species' nests and reap the benefits of its care, at the expense of the swindled parents' actual offspring. This is a form of behavior called social parasitism: the invading species lives at the host's expense, just like a biological parasite steals food and energy from its host, except a social parasite does so through social rather than biological means.

Sometimes, a social parasite simply takes an undeserved share of the foster parents' food and protection. More often, it actively hogs the resources: for example, a bird called the cowbird lays its eggs in the nests of smaller birds, so that when the cowbird hatches, it can muscle its stepbrothers and stepsisters out of the way at feeding time. Cuckoos, another social parasite, eliminate competition altogether by hatching sooner than their host family's chicks, and instinctually pushing all the other eggs out of the nest.

The caterpillars in this study turn into lovely Alcon blue butterflies, but as you heard, they grow up under devastatingly false pretenses. Scientists already knew that although the caterpillars grow to be much larger than ant larvae, their heads resemble ant heads, and more importantly, they release chemicals that resemble those of the ants they parasitize. (Ants communicate mainly through odor-like chemical signals.)

While this explained how the ants accepted the caterpillars as one of their own, it didn't explain why the ants gave the caterpillars the royal treatment (literally, as it turns out). Unlike cuckoos or cowbirds, caterpillars didn't actively push aside their competition; the worker ants simply gave the caterpillars priority on their own. The scientists' discovery had two key components: first, the caterpillars made queen-like sounds, and second, recordings of those caterpillar sounds made the ants behave in ways they usually behave around the queen ant. So, it seems that the caterpillars use several different methods to infiltrate the ant nests and enjoy a privileged upbringing.

Social parasitism may seem like a free lunch, but it's actually very risky, kind of like spying is for people. If an impostor species is found out, it's probably curtains: for example, some songbirds recognize cuckoo and cowbird eggs and kick them out of the nests before they hatch. It's not clear why some species learn to fight social parasites while others don't. As for why this caterpillar goes to such lengths to infiltrate ant societies, Thomas says that the protection and nourishment that ant nest affords must be far better than anything butterflies could construct on their own. And since successful caterpillars live under ant care for more than a year, the con game seems to have a worthwhile payoff.

Now try and answer these questions:


  1. What is a social parasite? In what way do these caterpillars practice social parasitism?
  2. What strategies have the caterpillars developed to get accepted and pampered by the worker ants?
  3. Social parasites can successfully pass themselves off as another species' offspring, despite obvious physical differences (for example, larger size). What does this say about differences between how animals and people perceive the world? 

You may want to check out the March 6, 2009 Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: caterpillars infiltrate ant nests, loneliness and your health, how planes can fly upside down, and more.

Going Further

For Educators

Cornell University's Brood Parasites page features information about cowbirds. The article explains how researchers believe that parasitism by cowbirds may be a significant factor contributing to the declining numbers of many songbirds in North America.

Full of Deceit and Murder, Cuckoos Reveal New Disguise is about the parasitic Horsfield's hawk-cuckoo. To cadge more food from their foster parents, chicks use a colorful adaptation—yellow wing splotches—that simulate more mouths to feed. Links to other stories, photos, videos, and websites are also found on this page.

Another article, Suicidal Grasshoppers Brainwashed by Parasite Worms tells about hairworms, a biological parasite of grasshoppers, which use a chemical cocktail to brainwash their victims into committing suicide so that they can continue their life cycle.

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