Many animals make a living as actors. Some harmless animals will mimic toxic animals, in order to make potential predators think twice before attacking. On the other end of the spectrum, some predators will mimic harmless animals or objects, and use this ruse to sneak up on their prey. However, there are costs to acting if animals become typecast in a role. If predators discover that a delicious prey animal is merely pretending to be dangerous, they will learn to differentiate the truly noxious from the imitators. Similarly, if the predators always take on a single disguise, then prey animals will learn to associate the costume with danger.
Fabio Cortesi, from the University of Basel, Switzerland, and the University of Queensland, Australia, and an international group of collaborators from Australia, Canada, UK and Sweden, decided to investigate how a coral reef fish, the dusky dottyback (Pseudochromis fuscus), maximizes the benefits of being a mimic, while avoiding some of the pitfalls. Cortesi and his colleagues conducted their experiments at Lizard Island, in Australia's Great Barrier Reef, where the dusky dottyback comes in two colours: bright yellow and brown. First, the researchers confirmed that there are no genetic differences underlying the colour variation in dottybacks. When the team examined the histology of dottyback skin, they also learned that the dottybacks were able to change colour by changing the ratios of yellow to black pigment cells within the skin. Thus, the researchers had good evidence that the dottybacks change colour to imitate something in their environment. But what are dottybacks imitating, and why?
The yellow dottybacks at Lizard Island are usually found on live yellow coral with yellow damselfish (Pomacentrus spp.), while the brown dottybacks are usually found on brown coral rubble, with brown damselfish. To investigate whether the dottybacks match their colour to the background coral or to their damselfish neighbours, Cortesi and his colleagues set up artificial coral patches around Lizard Island, composed of either yellow coral or brown coral rubble, and stocked these patches with either yellow or brown damselfish. The researchers then added either yellow or brown dottybacks to the patches, creating all potential coral, damselfish and dottyback colour combinations. When the scientists returned to the patches 2 weeks later, they found that the dottybacks always matched their local damselfish neighbours and not the background coral colour.
Cortesi and his colleagues hypothesized that the dottybacks might be imitating the adult damselfish in order to prey upon the juvenile damselfish. To test this theory, the researchers brought dottybacks and damselfish into the laboratory where they could conduct more detailed behavioural observations. They found that when the dottybacks matched the colour of the local adult damselfish, the juvenile damselfish were much less wary of them and the dottybacks were able to prey easily on the unsuspecting fish. The authors noted that the juveniles should eventually figure out this disguise in the wild. However, the dottybacks are able to outsmart their prey by not being typecast in either a consistently brown or yellow role.
Although the dottybacks match their prey rather than their habitat, there is another benefit of being adaptable actors. In a final experiment, the researchers showed that the predatory coral trout (Plectropomus leopardus) strike less often at dottybacks that are matched to the coral. Thus, these little animal actors make the most of their disguise by flexibly imitating their prey and benefit from some anti-predator camouflage themselves.