October 2011
“Avoiding Attack: Design and Deception in Nature” presented by Tom Sherratt, Carleton University
Creatures evolved to survive in the natural world: a report of the October 2011 lecture, written by Joel Byrne
The lights go down, the shuffling and conversations stop and I hear myself whispering: “This is going to be another great talk.” And so it was as the Mississippi Valley Field Naturalists (MVFN) public lecture series continued October 20th with the presentation, “Avoiding Attack: Design and Deception in Nature.”
The guest speaker, Dr. Tom Sherratt from Carleton University’s Biology Department, would be comfortable being described as an ‘evolutionary biologist’, in that his “primary interest is in how natural selection has acted, and continues to act to produce organismal traits.” His research interests, both theoretical and experimental, are in evolutionary ecology. He quickly outlined the solutions animals have evolved to avoid being attacked by predators in the natural world.
The first of these, and most obvious, is to avoid detection. Fortunately for potential prey animals there are a good number of ways to disguise themselves. Crypsis, or matching your background, sure challenges a predator. So does masquerading. The science behind this was explained using the Winnie the Pooh and the honey tree story (in which Pooh tries to fool the bees by pretending to be a little black rain cloud). Disruptive patterns break up your shape. Disruptive colouration can also do the trick. Dr. Sherratt’s lab researchers went out looking for Yellow-Banded Underwing Moths and Carpet Moths on trees to see if the way they rested on the bark, vertically, horizontally, or somewhere in between reduced their rate of being attacked. They discovered that the best way to survive if you’re an Underwing is to hang on vertically and head down and this is what they do. Orienting yourself with the bark worked better for Carpet Moths. Just remember—camouflage is good; without it, you’d better find another strategy.
Develop a defense and advertise it! That is use ‘warning colours’, develop spines, get a stinger, or develop a toxin. A classic example is the poison arrow frog with its toxicity and very bright warning colours, raising the big question, why do warning signals tend to be conspicuous? And, Dr. Sherratt asked, “Do invertebrate predators pay attention to warning signals?” To answer this the lab researchers tempted dragonflies with wasps, hover flies, bees and flies attached to sticks. The dragonflies attacked one and all, not paying attention to black and yellow markings or any other warning signs. Instead they seemed to be very size-selective about their potential prey, choosing the smaller flies and bees.
But what if you, the prey, have no defenses from predators? This brings up the third solution that Dr. Sherratt illustrated with more fascinating images: Look like something defended. In other words be a mimic. If you’re an edible species, ‘the mimic’, evolve to resemble an inedible one, ‘the model’. Thus the prey species gains a degree of protection from predators by resembling an unpalatable or otherwise defended species. This is Batesian mimicry, named after Bates a contemporary of Charles Darwin. Consider hover flies, of which there are many species. They so resemble bumble bees and wasps that they have played a key role in debate about ‘perfection’ in mimicry. Another kind of mimicry, Műllerian mimicry, named after Műller, also a contemporary of Darwin, occurs when ‘an unpalatable or venomous species resembles another of the same.’ Sound familiar? These two kinds of mimicry (Batesian and Műllerian) were brought to our attention to illustrate that distinctions between different kinds of mimicry might not be that clear cut. There can be overlap, grey areas, mistakes can be made, and myths can grow. A case in point is the Viceroy Butterfly’s resemblance to the Monarch Butterfly. This was thought to be a great example of Batesian mimicry— the Viceroy a tasty (to birds) species resembling an unpalatable monarch and thus avoiding attack. But it has been discovered that the viceroy is toxic but still more edible than the monarch to some birds, Blue Jays and Red-winged Black birds, for example. Thus the viceroy butterfly is now generally classed as a Műllerian mimic of the monarch. Since both monarchs and viceroys get eaten it seems to be to their mutual benefit to stay toxic, resemble each other, and share the burden of teaching predators to not eat too much of either one. If this is too complicated a solution, a prey species can, like members of one fly family, adapt to look like wasps and pretend to actually sting would-be predators. Other kinds of mimicry were discussed including sexual mimicry accompanied by more illuminating images.
“Consider hoverflies . . .” said Dr. Sherratt during his natural history lecture. They so resemble bumble bees and wasps that they have played a key role in debate about ‘perfection’ in mimicry. This photo shows, not a wasp, but a hoverfly of the family Syrphidae. MVFN file photo courtesy Dr. Henri Goulet
Dr. Sherratt wound up his talk with a quiz. He showed an image of wasp mimics, eighteen insects of various taxa all on one page, and asked which ones were wasps. Some of the look-a-likes were bees, some flies and so on, but picking out the wasps was tough. There were only four in the whole bunch. No one guessed correctly but one or two in the audience were close. Very humbling. After the talk, Dr. Sherratt fielded questions from the crowd. There were lots of good questions—a sure sign that the talk was well received. I found a great source of further insight into these questions and more, in a book entitled Avoiding Attack-The Evolutionary Ecology of Crypsis, Warning Signals and Mimicry co-authored by our speaker Thomas Sherratt, Graeme Ruxton and Michael Speed. Dr. Sherratt asked some of the Big Questions in evolution in his talk which only wetted our appetites for more big questions. Fortunately we can get some relief from this hunger in another of his books Big Questions in Ecology and Evolution co-authored with David Wilkinson.
E.T. Seton, in his book Two Little Savages, wrote a one-sentence preface as follows: “Because I have known the torment of thirst I would dig a well where others may drink.” He was referring to his “torment of thirst” for knowledge of the natural world. Tom Sherratt gave us a hearty drink from his well. Go on-line for Dr. Sherratt’s lab, website and email. One of Dr. Sherratt’s doctoral candidates, one Tom Hossie, has a caterpillar of the day blog about caterpillars & their eyespots. One actually appears to wink!
Evolutionary Biologist Tom Sherratt (right) receives enthusiastic thanks from MVFN President Joyce Clinton at the conclusion of his MVFN talk on design and deception in nature. Photo by Pauline Donaldson
