Ladybirds reveal their toxicity through their colouration, new research has shown.
The insects come in a range of colours. Different species have different colourations and patterns – from the familiar red and orange, to subtler yellows and camouflaged greens and browns. These colours, a new study has shown, reveals how toxic they are.
Animals use lots of mechanisms to defend themselves, including poisonous defences. A lot of these 'toxic' animals are warn predators using colouration, but in the case of ladybirds it wasn’t known whether they were honestly signalling to predators, or even if predators heed the warnings.
"There is a theory that the colours of animals that have warning signals, are associated with their toxicity and that certain colours are more toxic than others. This is called and 'honest' signal," explains Lina María Arenas, a PhD student from the University of Cambridge.
"But we found that, in fact, ladybird colours are honest. Those that are red and orange had much stronger toxins than the brown ones, which may rely more on camouflage to defend themselves from predators."
However, it isn’t just colour that warns predators, but contrast. “Although the orange ladybirds were the most toxic, we found that they were not the the most conspicuous against a green background,” explains Arenas.
To see whether contrast was the key determiner for toxicity, the team recorded the toxicity levels of different ladybirds, and then evaluated how well they stood out from their background habitat from a bird’s point of view, using a bird vision model and image analysis.
The results showed that toxicity correlates with colour contrast against the background, as measured in degrees of the "just noticeable differences" perceived by predators. Not only does increased contrast indicate higher toxicity in terms of species, but also for individuals within a species as well.
But have the birds learnt to heed the honest advice? To test the effect of colour on predation the team engineered artificial ladybirds using plastic moulds lined with paper of different ladybird colourings.
"We found that the more conspicuous and colourful the ladybird species, the less likely it is to be attacked by birds," says Arenas.
"To do this, we used paper moulds filled with blue-tack to make them into the characteristic lady bird shape, pinned them to leaves and counted how many were attacked."
Obviously once the birds attacked the bugs they realised that they weren’t suitable for supper, but researchers could see the beak marks and therefore record the popular pickings. They found that red, orange and yellow models were attacked less than the brown and black, and that those with low contrast were attacked more.
We can now be certain that ladybirds do in fact show their true colours, and that birds decipher the danger signals.