No butterflies were harmed by this research
With a species using cryptic resemblance [camouflage] for its protection, the very existence of neighbours involves a danger to the individual, since the discovery of one by a predator will be a step in teaching it to recognize the crypsis. With an aposematic [bad-tasting, warning-coloration] species, on the other hand, the existence of neighbours is an asset, since they may well serve to teach an inexperienced predator the warning pattern. -- William Hamilton, 1964This week's paper describes research that could have been a winning science fair project. "Does colour polymorphism enhance survival of prey populations?", published online by Lena Wennersten and Anders Forsman in Proceedings of the Royal Society, helps answer an interesting evolutionary question, using materials available in many kitchens.
It has been suggested that variability within a species (polymorphism) in color may reduce losses to predators. One reason is that predators may not have enough mental capacity to search for two or more different-looking prey at the same time. According to Robert Gegear, who gave a seminar here recently, this is why bees often focus on one species of flower at a time, passing over species they might visit on other occasions.
If it is true that groups of individuals varying in color are less likely to be eaten (at least completely), that would raise all sorts of interesting questions. Suppose red individuals are more likely to be eaten but distract attention from members of their species that are green? Who wants to volunteer to be red? But this paper is interesting, not for its contributions to theory, but for their method and their results.
They made "artificial pastry prey", cylinders of dough resembling caterpillars, in different colors: red, yellow, brown and green. They placed groups of 12 "prey", perhaps similar to family groups, in woodland trees, distributed over 1-2 square meters, and then recorded how long it took for wild birds to find and eat them. Some groups were all the same color, whereas others had four different colors.
Over the course of the experiment, 79% of the 2976 prey "disappeared or bore evident beak marks." All-green groups survived longest, on average. Mixed (polymorphic) groups were next. Within polymorphic groups, green "prey" survived much longer than the other colors. So there didn't seem to be any individual or group-level advantage to being any color other than green, and no group advantage to having a mixture of colors.
The authors argue against the possibility that groups with only cryptic coloration (brown and green, say as opposed to the red, yellow, brown and green they used) might survive longer than one-color groups chosen from the same range of colors. I would like to see actual data on this, however. If there's a student out there looking for a great science fair project, here's your chance!