Fussy eaters explain bright beasties – TCD researchers solve evolutionary conundrum

Posted on: 17 November 2005

An evolutionary conundrum that has been puzzling biologists for many years has been solved by fussy eaters, according to researchers in TCD’s Department of Zoology. This research is published in two high profile evolutionary journals, Evolution and Trends in Ecology & Evolution and has been presented at the annual conference of the European Society for Evolutionary Biology in Krakow, Poland.

Brightly coloured animals attract the attention of predators so are in danger of getting eaten. If they are poisonous, the predators soon learn to avoid them, and learn to link that colour pattern with being inedible. Animals signalling their inedibility like this are termed aposematic, and are found in a wide range of animal groups including insects, amphibians and fish. However, there is a cost to this signalling system – when the colour pattern is first evolving, none of the predators yet know the meaning of the signal, so it would be expected that the new signaller gets attacked and killed before it can get the signalling system going. So there is a big problem with evolving new signals, and an even bigger problem way back in prehistory, when this system was first being invented.
Scientists have for years tried to find solutions to the paradox of aposematic animals. Now researchers at Trinity College Dublin and their collaborators in Queen’s University Belfast and Cardiff University have come up with an explanation. Many predators don’t like trying out new types of food, so any animal which comes up with a new signal, and so looks different from the others of its species, will not be eaten as long as the predators can find some other food with which they are more familiar.
“In experiments where small pieces of pastry were used as food for wild birds coming to bird tables, the birds were first taught to expect pastry to be a certain colour. Once they had learned this, a new colour was introduced to the feeding tray. To our surprise, instead of eating the new colour as soon as they saw it, the birds avoided it for many days, even though it was perfectly edible,” explained Dr. Nicola Marples, lecturer in animal behaviour, Deptartment of Zoology.

In an experiment, the population of pastry “prey” were treated by the researchers as if they were real prey animals, and allowed to “breed” back up to the total population size (20 pastry animals) each day, in proportion to how many survivors were the familiar colour, and how many were the new colour. The birds were then allowed to eat from the bird table again, the population bred up to 20 again, and so on, until one colour died out and the other colour took over completely. The results of this experiment showed that, at some bird tables the new colour was eaten immediately -just as expected, but at about 1/3 of the tables, the new prey colour survived and increased in number until it had driven the familiar colour to extinction. This showed that it is possible for new colours to survive and spread if their local predators are picky about their food.