This blog is part of our colourful countdown to the holiday season where we’re celebrating the diversity and beauty of the natural world. Click here to read the rest of the colour countdown series.
Ossi Nokelainen of Jyväskylä University sheds light on the evolutionary puzzle of conspicuous colouration and what it means for predators – and their prey.
Have you ever wondered about why so many animals advertise themselves using bright, conspicuous or dazzling colours? Indeed, some do this to signal about their defensive qualities. The brilliant examples include Dendrobatidae poison frogs, Heliconius butterflies and Arctiid moths. The phenomenon of signalling about anti-predator defences is called aposematism and it consists of at least two lines of defence: a signal that is meant to be conspicuous and distinctive to would-be predators, and a defence that makes the prey animal unprofitable and less desirable as prey to pursue. Aposematic defences are multimodal as they elicit responses through multiple sensory modalities (i.e., vision, sound, smell, touch, taste). Perhaps the most explored combination of anti-predator defences is colour signalling combined with chemical defence, which is based on predator education in detecting, associating, learning and memorising the unpleasant prey encounters.
Although aposematic organisms are commonly expected to evolve a locally similar warning coloration, there is a stunning range of phenotypic variation across aposematic animals. This is considered somewhat paradoxical, because certainly if it’s easier for predators to learn one set of warning signals rather than many of them, this should drive evolution of locally similar warning signals. But perhaps the dilemma is overstated. While the above reasoning has firm theoretical and experimental grounds, in natural populations there is a wide array of conditions that influence the functional ecology of adaptive coloration.
The conditions for visual signalling are set by lighting and viewing background, which both influence how conspicuous the visual signals may be in the habitat. How prominent these warning colour patterns are also depends on who views them, which means that receivers’ perception matters. While vision modelling can provide insights on how visual anatomy of the predators influence the capability of detecting brightness and colour information, we still have only a rough idea about the subtle cognitive processes that prime the colour-based behaviour of animals in different environments. So, although the concept of colour is intuitive (everybody knows what is meant by colours), its significance to predators is difficult to quantify as the sensation of colours is built in the brain and these memories cannot be easily accessed by measuring colour metrics.
In our own research, we investigated context-dependent effects in blue tit (Cyanistes caeruleus) predatory behaviour. We tested if they change their preferred (or avoided) prey depending on the background context and light environment. As a starting point, we looked at how quickly blue tits found their prey (artificial prey were used as visual stimuli) when viewed against a yellow background or grey background. As might be expected, the blue tits found the prey faster against the yellow background, where the colour cues were present.
Next, we tested whether in controlled conditions, only changing the lighting may cause birds to change their colour-based behaviour towards the aposematic, colour polymorphic tiger moth (Arctia plantaginis), which has either a white or a yellow male colour morph. We used two types of lighting, replicating shaded forest lighting, and unshaded bright light. In this situation, we were specifically looking at the trade off of the aposematic colouration versus predating on the most visible moth. If we are to follow what we learned from our previous experiments of detectability, we would expect the birds to attack first those moths that stand out the most (the white moths in the shaded light and the yellow moths in the bright light), however if we are to follow the aposematic line of thought, we would predict that those moths that stand out brightest should be least predated on.
We found that birds attacked the yellow morph significantly more than the white morph in forested light environment and vice versa in bright light. This indicates that the light environment is the determining factor for the bird’s predatory behaviour as the warning signal of prey promotes a different reaction of predators under different lighting conditions, which might explain the polymorphisms in the aposematic colouration of the tiger moth.
We still have more to learn about how signals work in variable natural environments. The next steps in the study of aposematism are intriguing. These may include exploration of the cognitive mechanisms of predators, how different predator communities may change the selection for warning signals, or how natural viewing conditions may affect how the signal is being separated from the ‘noise’ of the visual environment.
Ossi Nokelainen, Jyväskylä University, Finland