Predators like these Great tits (Parus major) eat a wide variety of insects, but some of those insects are so unpleasant to eat that birds tend to avoid them. How does this trait evolve in prey animals when its maintenance and origin depend on the predators learning by eating them? (Image Credit: Shirley Clarke, CC BY-SA 3.0).

Social information use about novel aposematic prey is not influenced by a predator’s previous experience with toxins (2019) Hämäläinen et al., Functional Ecology, https://dx.doi.org/10.1111/1365-2435.13395

The Crux

Many animals in nature have evolved a defense strategy known as aposematism, meaning that they display warning colors or patterns that tells predators that they are not worth eating due to their toxicity. Predators can learn to avoid aposematic prey by either sampling different prey animals and learning for themselves, or they can watch other predators eat different prey species and, depending on the reaction of that predator, learn what may or may not be good to eat.

The paradox of the evolution of this aposematic trait is that toxic prey species are not only highly visible and easily noticed by predators, but they must be attacked in order for predators to learn that they shouldn’t eat them, meaning that these prey species may not even survive long enough for them to enjoy the benefits of predator avoidance. The question then becomes are aposematic prey able to persist in nature because predator learn to avoid them? The authors of today’s paper wanted to investigate how predators that have learned to avoid toxic prey will watch and learn from other predators eating new, possibly toxic prey.

What They Did

The predators used in this experiment were Great tits, small birds native to Europe. For the prey, the authors used small pieces of almond inside of paper packets to make two different prey types, palatable and aposematic. For the palatable prey, the packets had small crosses drawn on them (making them harder to see), and for the aposematic prey the packets had squares drawn on them (more conspicious). The aposematic almond pieces were soaked in a chemical solution, making them toxic and foul-tasting to the birds, while the palatable pieces were soaked in water as a control.

Half of the birds had their toxin levels manipulated by feeding them mealworms injected with the same chemical solution that the nuts were soaked in, while the control birds were fed plain mealworms. This was done in order to test if birds that had already consumed toxic food were more hesitant to eat new, possibly toxic food. In addition, half of the birds fed toxic mealworms and half of the birds fed regular mealworms were shown videos of other birds eating from the control and toxic almond packets, in order to teach them which packets had palatable food and which contained the toxic food. This tested the birds’ ability to watch and learn about their new prey from other predators. The other half of the birds watched a video that simply showed the packets, without any information on their toxicity.

Did You Know: Mimics in nature One thing that is constant in nature is the trade-off. You can’t be good at something without paying the price for it, either with costly resource investment to be good, or by being bad at something else. Some animals that live near aposematic individuals have evolved to look like these toxic animals, despite not being toxic themselves. This means that they enjoy the protection of looking toxic, without having to produce the costly toxins that make their truly toxic counterparts so unpalatable. This can, however, backfire for both the toxic species and the mimic. If a predator learns that there are mimics in a population of prey species, then the predator is more willing to not only eat the mimics, but it is also more willing to risk eating a toxic species in the off chance that it is, in fact, an edible prey item.

What They Found

Regardless of which video the birds had seen prior to the feeding trial, most of the birds attempted to eat a visible, toxic food packet as their first choice. Birds that had been fed toxic mealworms, yet had not been shown the video of which packets were toxic, were more hesitant to eat any of the packets, most likely because their bodies were in worse shape than the control birds due to the toxic food they had already eaten.

Interestingly, the toxin level of the birds did not influence their choice of prey when foraging. The only factor that significantly affected prey choice was whether or not the bird had seen other birds eating toxic food before.

Problems

The premise of this study is that predators learn to recognize unpalatable, toxic prey and will then avoid it. Great tits eat seeds in addition to insects, and I am not an expert in this field or with this system, but the authors’ use of almonds inside of paper packets with markings drawn on them seems too far removed from a natural scenario to make the results meaningful. Scientists make use of all kinds of objects and tools to get the job done (I mean, this group used a sex toy to make spiders think they were being attacked), but using paper packets instead of mealworms (which the authors had access to) doesn’t seem too biologically realistic.

So What?

The authors predicted that previous experience with toxic food would affect the likelihood of birds eating new, potentially toxic food. However, previous experience did not have an effect, and instead the best predictor of predator behavior was watching and learning from other predators.

Although toxin load did not have an effect on the predator’s decision making process, it is possible that other factors may have played a part. In this experiment the birds were offered plenty of food, toxic and otherwise, but in environments with few prey species, predators may choose to continue eating toxic food, as they would have no other choice.