March 23, 2012
Researchers Look Into Hoverfly Disguises
Mimicry in the animal kingdom is a useful tool that many insects employ to make themselves appear more fierce in order to escape from becoming a meal. However, researchers have been puzzled as to how some of the worst mimickers still seem to escape certain demise.
Researchers at Carleton University in Ottawa, Canada set out to find the nature of mimicry in one of these imperfect impersonators: the hoverfly.
Some hoverflies have evolved to be nearly indistinguishable from bees and wasps, while others have evolved only a crude resemblance, the researchers wrote in the latest issue of the journal Nature, in an article entitles: “A comparative analysis of the evolution of imperfect mimicry.”
The researchers found that larger hoverfly species seem to be better mimics of stinging insects than their smaller relatives -- perhaps because larger insects are more attractive to predators. The team believes these larger insects had more to gain by perfecting their mimicry skills.
They also suggest that in smaller hoverflies, mimicry perfection isn´t essential.
“Mimicry of harmless species pretending to be dangerous ones in order to avoid being eaten is one of the best celebrated examples of the outcome of evolution by natural selection,” Professor Tom Sherratt, lead researcher of the study, told Pallab Ghosh of BBC News. “Good examples of mimicry are highlighted in biology text books, but many mimics are poor and their emergence remains something of a puzzle.”
Mimicry is common in many animals, including snakes, spiders and butterflies, which have evolved to look like other species to escape predation. But it has remained a big mystery why even the worst copy-cats seem to be just as abundant as the good ones.
In Charles Darwin´s theory of natural selection, one would surmise that it would be better for all copy-cats to closely resemble the species they are trying to impersonate.
The researchers explained that under Darwin´s theory, hoverflies that mimicked the sounds of wasps would over time evolve to perfect that skill. And those that poorly mimicked those sounds would hence all be eaten and die out.
But the Canadian researchers suggested why this has not happened.
An explanation as to why some species might not achieve this perfected mimicry is the “eye of the beholder” theory. The researchers theorize that poor mimics represent a mixture of unappetizing species and so, although they do not perfectly resemble any one of them, they do represent the worst possible combination of them.
To probe that mystery further, the researchers studied 31 different species of hoverfly, some of which looked very close to the bees and wasps they try to resemble, while others not so much.
They first quantified how close each species resembled bees and wasps by showing photographs to people and asking them to give each species a score out of 10.
The researchers then combined the results with an objective score obtained by comparing measurements of the body parts of each species and their bee or wasp counterpart to reach an overall score for similarity.
They found that the larger the hoverfly was, the more closely it resembled the real bee or wasp. And the smaller species were not very good mimickers at all.
“If you are a small hoverfly then birds are not going to be very interested in you,” Sherratt explained to Ghosh. “You are a relatively unprofitable meal and so the selection on mimicry is relatively weak.”
“All you need to do is vaguely look like a wasp, and a bird will be sufficiently deterred to leave you alone because it's just not worth taking the risk if it turned you were a wasp because the benefit is that much smaller,” he added. “But if you are a nice fat juicy hoverfly, you are a substantial meal to a bird, and in those cases you might experience even stronger selection to resemble something like a wasp or bee and therefore gain protection from predators.”
The scientist´s next plan to study a species of caterpillar that uses mimicry to impersonate a particular type of snake. They also want to see if mimicry extends to even larger species of animals.
“Mimicry provides some of the most exquisite examples of the power of natural selection,” said Sherratt. “There is a famous saying that ℠Nothing makes sense in biology except in the light of evolution.´ It is just as important to investigate examples of seemingly poor adaptation than sitting back and enjoying the celebrated examples of adaptation we see in textbooks.”