THE GREAT IMPOSTERS Finding good day care can certainly pose a problem these days, unless, of course, you're an African widow bird. When it comes time for a female widow bird to lay her eggs, she simply locates the nest of a nearby Estrildid finch and surreptitiously drops the eggs inside. That's the last the widow bird ever sees of her offspring. But not to worry, because the Estrildid finch will take devoted care of the abandoned birds as if they were her own. And who's to tell the difference? Though adult widow birds and Estrildid finches don't look at all alike, their eggs do. Not only that, baby widow birds are dead ringers for Estrildid finch chicks, both having the same colouration and markings. They even act and sound the same, thus ensuring that the widow bird nestlings can grow up among their alien nestmates with no risk of being rejected by their foster parents. MASTERS OF DISGUISE Things aren't always as they seem, and nowhere is this more true than in nature, where dozens of animals (and plants) spend their time masquerading as others. So clever are their disguises that you've probably never known you were being fooled by spiders impersonating ants, squirrels that look like shrews, worms copying sea anemones, and roaches imitating ladybugs. There are even animals that look like themselves, which can also be a form of impersonation. The phenomenon of mimicry, as it's called by biologists, was first noted in the mid-1800s by an English naturalist, Henry W. Bates. Watching butterflies in the forests of Brazil, Bates discovered that many members of the Peridae butterfly family did not look anything like their closest relatives. Instead they bore a striking resemblance to members of the Heliconiidae butterfly family. Upon closer inspection, Bates found that there was a major advantage in mimicking the Heliconiids. Fragile, slow-moving and brightly coloured, the Heliconiids are ideal targets for insectivorous birds. Yet, birds never touch them because they taste so bad. Imagine that you're a delicious morsel of butterfly. Wouldn't it be smart to mimic the appearance of an unpalatable Heliconiid so that no bird would bother you either? That's what Bates concluded was happening in the Brazilian jungle among the Pieridae. Today, the imitation of an inedible species by an edible one is called Batesian mimicry. Since Bates' time, scientists have unmasked hundreds of cases of mimicry in nature. It hasn't always been an easy job, either, as when an animal mimics not one, but several other species. In one species of butterfly common in India and Sri Lanka, the female appears in no less than three versions. One type resembles the male while the others resemble two entirely different species of inedible butterflies. Butterflies don't "choose" to mimic other butterflies in the same way that you might pick out a costume for a masquerade ball. True, some animals, such as the chameleon, do possess the ability to change body colour and blend in the with their surroundings. But most mimicry arises through evolutionary change. A mutant appears with characteristics similar to that of a better protected animal. This extra protection offers the mutant the opportunity to reproduce unharmed, and eventually flourish alongside the original. In the world of mimics, the ant is another frequently copied animal, though not so much by other ants as by other insects and even spiders. Stoop down to inspect an ant colony, and chances are you'll find a few interlopers that aren't really ants at all but copycat spiders (or wasps or flies). One way you might distinguish between host and guest is by counting legs: Ants have six legs while spiders have eight. Look carefully and you might see a few spiders running around on six legs while holding their other two out front like ant feelers. COPYCATS Mimicry can not only be a matter of looking alike, it can also involve acting the same. In the Philippine jungle there is a nasty little bug, the bombardier beetle. When threatened by a predator, it sticks its back end in the air, like a souped-up sports car, and lets out a blast of poisonous fluid. In the same jungle lives a cricket that is a living xerox of the bombardier beetle. When approached by a predator,