'Disgust' helps warning signals evolve – as long as others are watching.
If predators must eat prey to learn warning signals, how can this successful defence ever evolve? Social transmission provides an answer.
The paper in Nature Ecology & Evolution is here: http://go.nature.com/2CAFrC3
If you’ve attended a talk or lecture on warning signals, then you’ve probably seen the photo that inspired this paper – a blue jay is tricked into eating a toxic monarch butterfly, with disastrous consequences for both the butterfly and the bird. My first thought when seeing this picture was: yuck!
The image is a powerful lesson why predators avoid prey with warning signals: they associate the signal with the unprofitability of the prey. But there in lies the problem – this defence only works if predators recognise the signal. If every predator needs to eat the prey to learn, how do any new warning signals that arise survive and outcompete alternatives?
If I were another blue jay, I would be pretty deterred from trying to eat a monarch myself. Seeing this image led me to ask: Could watching the unfortunate consequences of another predator learning increase avoidance by others? If it did, this might be an important piece of the puzzle to resolve how warning signals evolve – fewer prey would need to be eaten for all predators to learn.
A coffee with a visiting professor to my department, Johanna Mappes (University of Jyväskylä, Finland), provided the opportunity to test my hypothesis. She thought it was unlikely, but worth a shot – although social information is well-known to influence where and what animals should eat, there are fewer studies demonstrating that it can influence avoidance learning.
After securing funding through a NERC (Natural Environment Research Council UK) research fellowship, I travelled to Finland to spend a winter in the darkness. Literally. Not only are the hours of daylight extremely short at that latitude, but our experiments relied on my assistants and I sitting in the dark to observe the birds through one-way glass.
To recreate the initial appearance of a mutant warning signal, we used the ‘novel world’. Johanna designed and has fine-tuned the protocol over many years at the Konnevesi Research Station: the subzero temperatures make it simple to catch wild great tits in feeding traps, and the birds even put on weight while in captivity for our experiments thanks to the expert care of the technician!
The next step was to figure out how we could manipulate social information and have control over what observers were seeing. A conversation in the corridor back at Cambridge helped solve the problem - Neetlje Boogert (now at the University of Exeter) suggested I consider video playback. With that, we were ready to start our experiment to test if social information can reduce the cost of conspicuousness for novel aposematic prey.
We found that birds with social information were much more likely to avoid the conspicuous prey than naïve control birds, even across two more days of tests without additional video playback. This meant that the birds were gathering social information about the warning signal from observing one disgusting experience, and that the loss of one prey in the population reduced predation overall. It looked like this might be enough to help new aposematic prey types to spread, but we needed more evidence to be confident.
Again Finland made this possible – Johanna was a director of a Centre of Excellence in Biological Interactions (funded by the Academy of Finland) with Hanna Kokko (University of Zürich). Luckily for us, Hanna was also intrigued by our idea and offered to explore the problem mathematically. Her work supported our conclusions from the experiment: aposematic prey have a hard time evolving unless predators learn by observing the foraging mistakes of others.
Like many birds, great tits show distaste by wiping their beak and shaking their head - is this one reason why our “disgust” expression also evolved? I’ve used this trick to teach my daughter what should be avoided, although it’s hard to know if this is a case of nature, or being nurtured by my work!
There have been many other hypotheses proposed for explaining how warning signals evolve, from spatial distributions of prey to evolved biases against novel foods. Experiments, however, have usually produced ambiguous results. Our study shows that there is another component to this story that may well end up bringing the puzzle pieces together. Considering the social environment of the predators is helping make something paradoxical, possible.