African Widow Bird Essay, Research Paper
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, the cricket will also prop up its behind — a tactic sufficient to scare off the enemy, even though no toxic liquid squirts out. Going one step further than that is a native of the United States, the longicorn beetle, which resembles the unappetizing soft-shelled beetle. Not content to merely look alike, the longicorn beetle will sometimes attack a soft-shelled beetle and devour part of its insides. By ingesting the soft-shelled beetle’s bad-tasting body fluid, the longicorn beetle gives itself a terrible taste, too! Protection is by no means the only advantage that mimicry offers. Foster care can be another reward, as proven by the African widow bird. And then there’s the old wolf-in-sheep’s-clothing trick, which biologists call aggressive mimicry. The master practitioner of aggressive mimicry is the ocean-going anglerfish. Looking like a stone overgrown with algae, the anglerfish disguises itself among the rocks and slime on the ocean bottom. Protruding from its mouth is a small appendage, or lure, with all the features of a fat, juicy pink worm. The anglerfish lacks powerful teeth so it can’t take a tight grip on its prey. Instead, it waits motionless until a small fish shows interest in the lure, and then wiggles the lure in front of the fish’s mouth. When the small fish is just about to snap at the lure, the angler swallows violently, sucking the fish down its hatch. Diner instantly becomes dinner. SEXUAL IMITATORS Of all the many impostures found in nature, probably the sneakiest are those of the sexual mimics: males who imitate females to gain an advantage at mating time. Here in Ontario we have a sexual mimic, the bluegill fish. Male bluegills come in two types: the standard male and the satellite male, which looks just like a female bluegill. In preparation for mating, the standard male bluegill performs the job of building the nest, where he bides his time until a female enters it to spawn. Satellite fish don’t build nests, choosing instead to hover around the nest of a standard male until the moment when a pregnant female enters. The satellite fish follows her into the nest, deceiving the nestbuilder into believing that he is now in the presence of two females. The three fish swim around together, and when the female drops her eggs, both males release a cloud of sperm. Some of the eggs are fertilized by the resident male, some by the satellite male, thus passing on passing on different sets of male genes to a new generation of bluegills. Another case of sexual mimicry has recently been uncovered in Manitoba among the red-sided garter snakes. The little town of Inwood, Manitoba and the surrounding countryside is garter snake heaven, where you can find the largest snake colonies on Earth. Every spring, the red-sided garter snake engages in a curious mating ritual. Soon after spring thaw, the males emerge first from their winter cave and hover nearby. The females then slither out a few at a time, each one exuding a special “perfume” which signals to the fellows that she’s ready to mate. At first whiff of this lovely odour, a mass of frenetic males immediately besieges the female, wrapping her up in a “mating ball” of 10, 20 or sometimes as many as 100 writhing males, all hoping to get lucky. Scientists have now discovered that some male red-sided garters give off the same perfume as the female, and they do this while intertwined in the mating ball. Male and female red-sided garters look exactly alike, so the male with the female scent can effectively distract many of the males from the real female, giving the imposter a better shot at getting close to the female and impregnating her. Males passing as females, fish as bait, beetles as ants — amidst all this confusion, it still sometimes pays to just be yourself, which could certainly be the motto of the amazing hair-streak butterfly family. Decorating the hair-streak’s lower hind wings are spots that look like eyes, and out-growths that look like antennae, creating the illusion that the butterfly has a second head. Whenever the hair-streak alights, it jerks its dummy antennae up and down while keeping its real antennae immobile. Presumably, this dummy head exists to distract predators. If so, we finally have the first scientific proof that two heads are better than one.