Looking alike helps harmful insects survive
23 June 2010, by Tamera Jones
Birds learn to avoid different species of foul-tasting or harmful insects faster if those insects use similar warning signals rather than completely different signals, say scientists.
The Monarch butterfly and the Viceroy butterfly clearing demonstrating Müllerian mimicry.
This means it's no evolutionary coincidence that different bee species all look alike. Their black and yellow stripes serve to warn predators not to eat them.
The new research shows that looking the same means that birds learn to avoid them sooner than if they each had completely different patterns or colours.
'Insects like bees and wasps both have a nasty sting that makes predators like birds reject them,' explains Dr Hannah Rowland from the University of Liverpool, who led the research.
'But we wanted to know if similar warning signals used by two different species of insects benefit both species – do predators learn to avoid them quicker so that both species survive better?' adds Rowland.
Scientists have long known that some harmless insects copy wasps and bees and other harmful insects to avoid being eaten. Hoverflies, for example, are harmless, but look like wasps so birds tend to avoid them.
'We wanted to know if similar warning signals used by two different species of insects benefit both species.'
Dr Hannah Rowland, the University of Liverpool
But in 1878, German zoologist Fritz Müller suggested that it would benefit even harmful species to look alike, because predators would learn that black and yellow stripes, for example, are a signal to be avoided.
Although it may seem obvious, until now, researchers have struggled to find experimental evidence to support this idea. This is partly because it's been notoriously difficult to design studies that test Müller's theory accurately.
'In other experimental studies, the warning signals between the different types of prey used were too similar. Predators couldn't necessarily distinguish between them,' says Rowland.
In a report published in Behavioral Ecology, Rowland and her colleagues describe in detail how they designed a complex experiment to try to answer these questions.
Wild birds help Dr Hannah Rowland demonstrate the power of Mullerian mimicry
To see if mimicry benefits two different, but similar-looking species, the researchers trained wild great tits (Parus major) to forage for so-called artificial prey items – almond glued between two pieces of paper with three different markings. A cross signified edible prey, while either a black square or a black circle meant the prey was harmful.
The researchers soaked pieces of almond marked by a black box or a black circle in quinine to make the almond bitter-tasting. 'Birds reject bitter tasting food, as it signifies poison,' explains Rowland.
'Because we wanted to test the survival benefits of signal similarity, we had to design artificial prey that would be treated as visually distinct,' she adds.
The researchers then tested three different experimental conditions. In the first they presented the great tits with only artificial prey printed with either a black box or a black circle with an equal number of edible prey (marked with a cross). 'This treatment is like giving the birds just bees to eat,' says Rowland. Here the birds ate 34 per cent of the artificial prey.
In the second condition, the great tits were allowed to forage for two distinct types of inedible prey – marked with a black box and a black circle – as well as an equal number of edible prey. This is equivalent to letting the birds choose between a wasp or a bee. In this situation, the great tits ate on average 22 per cent of what was offered to them.
In the last condition they gave the birds the same quantity of just one type of inedible prey as in condition two - either all black box or all black circle. As in the first two tests, the scientists threw an equal amount of edible prey into the mix. As a comparison, this is like giving the birds either two species of wasps or two species of bee.
The researchers found that in the last condition, the birds only ate 18 per cent of the prey.
'We clearly and unambiguously show that if two harmful species look alike, each is protected against predation. This is because the costs of teaching a predator to avoid them are shared between both species,' says Rowland. This is precisely as Müller originally suggested.
'Our results go some way to explain why different species which have their own defences – like bitter-tasting butterflies or poisonous frogs – tend to look similar,' adds Rowland.
Hannah M. Rowland, Tom Hoogesteger, Graeme D. Ruxton, Michael P. Speed and Johanna Mappes, A tale of 2 signals: signal mimicry between aposematic species enhances predator avoidance learning, Behavioral Ecology, June 4 2010, doi:10.1093/beheco/arq071
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