Liisa Hämäläinen, a Ph.D. candidate at the University of Cambridge, discusses how predators’ previous experience with toxins influences social information use in this insight.

About the paper

What is the background behind your paper?

Aposematism is an antipredator strategy where prey use conspicuous warning signals (typically bright colouration) to warn predators about their unprofitability, such as their toxicity or chemical defence. However, this only works if predators know the signal, and until they do, visible aposematic prey are an easy target for naïve predators. So, how is it that warning signals are so common in nature?

Until recently, we have tended to assume that predators learn about prey defences only by direct interaction with prey. Recently (see Thorogood et al. 2018 Nature Ecology and Evolution), however, we have started to incorporate broader sources of information for predator learning. Predators can actually gather information about prey quality from multiple sources and learn about novel defences by watching how others forage or respond to bad-tasting prey (social information). This could reduce attacks on aposematic prey and help us to solve the evolutionary puzzle of warning signals. However, this situation is complicated by the existence of palatable mimics in prey populations – these prey resemble aposematic model species but do not invest in chemical defences and consequently do not taste bad. If predators also see others eating palatable mimics, this could make them more likely to attack aposematic prey. In our paper, we investigated how previous experience with defended prey influences a predator’s decision to use these different sources of social information.

What’s your paper about?

Our aim was to test whether predators pay more attention to the negative foraging experience of others when they have themselves recently encountered defended prey (their “toxin load” is increased). We used wild-caught great tits as predators in captive learning experiments. This provides an excellent study system: social information can be manipulated by showing birds videos of the foraging behaviour of another individual (“demonstrator”), and the foraging choices are then investigated using “novel world” environment, where birds encounter novel prey items with artificial symbols, ensuring that they are naïve to all prey types.

We first manipulated the birds’ experience with toxins by giving them two bitter-tasting chloroquine-injected mealworms or two palatable water-injected mealworms as a control. Great tits then saw a video of a demonstrator consuming a novel aposematic prey or a control video (prey items only). Birds typically respond to distasteful prey by performing vigorous beak wiping and head shaking, and seeing this “disgust response” of the demonstrator can provide observers information about prey unpalatability. After the video playback, we tested observers’ foraging choices when they encountered two types of novel prey: conspicuous and unpalatable aposematic prey (a square symbol) and cryptic and palatable prey (a cross symbol). We found that birds that had watched a demonstrator eating a distasteful aposematic prey chose to attack fewer prey items of the same type (squares). However, contrary to what we had predicted, this did not depend on their previous experience with toxins, suggesting that social information is valuable to all naïve predator, regardless of whether they have a “bad taste in their mouth”.

Next, we tested if now-educated birds would sample again the previously unpalatable prey if they saw another individual eating a similar-looking undefended prey (palatable mimic). This time half of the birds watched a video of a demonstrator eating a prey with a square symbol without showing any disgust responses. However, we found that the videos had no effect on the foraging choices: all birds were hesitant to sample previously defended prey even when all prey items were now palatable. This suggests that recent personal experience with defended prey can override conflicting social information. Therefore, while social information about aposematic prey can increase predator avoidance, model-mimic dynamics are unlikely to be affected when predators have recent personal experience of the model’s defence.

How is your paper new or different from other work in this area?

Aposematism and model-mimic dynamics have received wide experimental and theoretical interest since aposematism was first described 150 years ago. However, most of these studies have focused on individual learning of predators. Together with other recent work (e.g. Landová et al. 2017 Animal Cognition; Thorogood et al. 2018 Nature Ecology and Evolution), our paper provides experimental evidence that predators also can use social information when learning about aposematic prey. Social learning has now been demonstrated in many different contexts and across a wide range of taxa, however, we are only just beginning to investigate the potential evolutionary consequences of social transmission of information. Our work demonstrates that social information about prey unprofitability can reduce predation pressure on novel aposematic prey and therefore influence selection pressures for prey defences and signalling.

About the author

What are you currently working on?

I am currently finishing my PhD at the Department of Zoology in the University of Cambridge. During my PhD, I have studied how predators use social information about aposematic prey and their palatable mimics, how this varies within and between species, and what consequences this has for predator-prey coevolution. I have investigated this using blue tits and great tits as my model predators. In addition to experiments with birds in captivity, I am currently investigating information use in a wild predator population, where individuals have more opportunities to use different information sources.

What is the best thing about being an ecologist?

One of the best things of being an ecologist is the diversity of the job: you can combine data collection and experimental work with data analysis, reading, writing and presenting your results. It is exciting to plan new experiments and think about the world through senses of birds – there is always something new to learn and interesting questions to ask!

Read the plain language summary for Liisa’s paper here or read the paper in full here.