Great tits that exhibit disgust when they eat a horrible meal may inadvertently be driving their prey to evolve.
Many prey animals have evolved deterrents so that predators will be reluctant to eat them: they may have an unpalatable taste or even be poisonous. They often signal this with bright and contrasting colours, like the yellow and black stripes of a wasp.
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But it’s not clear how these “aposematic” signals evolved. “Predators need to first learn the signal before they avoid it, but until they do these prey are very easy to detect and kill,” says of the University of Cambridge, UK.
Thorogood and her colleagues have tested “a forgotten hypothesis”, she says. “If predators learn by observing another predator’s mistake, this might protect the prey population just long enough for them to reproduce, multiply, and evolve.”
Marked envelopes
The team gave a choice of two new foods: slivers of almond, some of which had been dosed with chloroquine phosphate. That is “the same chemical that gives tonic water its characteristic taste, but which many birds find distasteful,” Thorogood says.
The almonds were placed in small envelopes, and the bad-tasting ones marked with a prominent black square. In total, 30 birds learned to open the envelopes and eat the almonds inside.
Next, 15 birds watched a video of a “demonstrator” great tit eating a foul-tasting almond from a marked envelope. The bird spat out the almond immediately, then vigorously wiped its beak on the perch. This is a well-known sign of disgust in great tits.
The other 15 birds simply saw video footage of both types of envelope, with nothing to warn them away from the bad almonds.
Learning by watching
Both sets of birds subsequently learned to avoid the square envelopes, but the 15 “trained” birds ate fewer.
In other words, the great tits learned to avoid the foul-tasting “prey” by watching each other. By eating mainly the tasty ones, the birds allowed the proportion of foul-tasting prey to expand at the expense of the tasty ones.
“The conspicuous prey were 40 per cent more likely to survive when predators were socially-informed [compared to] naïve,” says Thorogood.
It shows the power of social learning, says at the University of Colorado Boulder. “The signal is cheap – the disgust response is not intentional – and can be broadcast from one bird to many.”
Nature Ecology & Evolution
