Researchers at Trinity College may have discovered how brightly coloured insect species survive extinction at the claws of predators, writes Dick Ahlstrom
Zoologists from Trinity College Dublin may have cracked a long-standing evolutionary conundrum: how do certain insects, frogs and fish became brightly coloured as a way to warn predators that they are inedible? Their findings turned existing theories on their head and provided new insights into animal behaviour.
"The research is about how animals deal with novel food," explains Dr Nicola Marples, a lecturer in animal behaviour in Trinity's department of zoology. "That is important because it relates to how insects evolved colour patterns because insects are novel foods for birds." It is all about "co- evolution between predator and prey", she says.
Advertising your toxicity to predators by strutting about in extremely bright colours is a risky survival strategy, but it clearly works for some species. But how did these species first acquire their bright colours, given that they become exceptionally visible to hungry predators looking for a fast food dinner?
There is an inherent evolutionary difficulty associated with this, says Marples. "Because there are two traits there, colour and toxicity, and evolution happens slowly, the chance of both traits evolving at once is practically nil," she explains.
"The traditional theory held that you had to have the toxicity first," says Marples. The assumption was that the bright colours would come along later. "The breakthrough we made is you can have the colour patterns first and then evolve the toxicity."
She and colleagues Dr David Kelly from Queen's University Belfast and Dr Robert Thomas of Cardiff University joined forces to devise and conduct experiments on this puzzle using feeding trials involving wild bird populations. They published their findings in a recent issue of the journal Evolution and in Trends in Ecology & Evolution.
They set up feeding trials with robins in the wild, but other birds soon joined the experiments including blue tits, great tits, black birds and chaffinches. The researchers discovered that whatever the species, they split between "dietary conservatism" and being "adventurous eaters".
About half of the birds were conservative, only eating foods that were familiar to them. "The down side of that is the other half of the birds see an insect and say 'I will eat this'," adds Marples
The team baked small pieces of coloured pastry and trained birds to take these from feeding tables. Initially only one colour of pastry was put out, but once the birds became familiar with this food a new colour was introduced to the feeding tray.
Initially 20 pieces of pastry "prey" were put out, all of one colour, for example brown. Once the birds became used to this prey one piece of red pastry was introduced along with 19 brown. The researchers waited until 10 prey had been eaten and then checked to see which prey "species" had survived on the table.
These were allowed to "breed" back up to the full 20 again in proportion to how many survivors had been left. This was repeated until one species or colour had died out and the other colour took over completely.
They found that survival was heavily dependent on whether the table was visited by conservative or adventurous eaters. Adventurous eaters cleared out the familiar prey but also the novel food with equal gusto. Picky eaters were much slower to gobble up the "strange" food, however, and tended to leave this alone.
"You now have a warning signal saying 'I am new' and that is enough to deter the birds from eating it," says Marples.
This conservative streak emerged in about a third of the tables, with the new prey colour surviving and increasing in number until it had driven the familiar colour to extinction. This showed that it was possible for a novel colour to appear and survive despite standing out from the crowd.
"We did this with lots of colour combinations," says Marples. It didn't matter what combinations were used, a large fraction of the novel prey survived to out-breed the familiar prey.