Abstract
Toxic prey that signal their defences to predators using conspicuous warning signals are called 'aposematic'. Predators learn about the toxic content of aposematic prey and reduce their attacks on them. However, through regulating their toxin intake, predators will include aposematic prey in their diets when the benefits of gaining the nutrients they contain outweigh the costs of ingesting the prey's toxins. Predators face a problem when managing their toxin intake: prey sharing the same warning signal often vary in their toxicities. Given that predators should avoid uncertainty when managing their toxin intake, we tested whether European starlings (Sturnus vulgaris) preferred to eat fixed-defence prey (where all prey contained a 2% quinine solution) to mixed-defence prey (where half the prey contained a 4% quinine solution and the other half contained only water). Our results support the idea that predators should be more 'risk-averse' when foraging on variably defended prey and suggest that variation in toxicity levels could be a form of defence.
Highlights
Many species, both plants and animals, defend themselves with harmful toxins in order to reduce the chances that they are eaten [1,2]
While the probability of eating a toxic prey decreased with presentation number in a session, due to either the improving nutritional state of the predator or an increasing toxin burden [5,6,21,22], birds were less likely to eat a mixed-defence prey compared with a fixed-defence prey at any point during an experimental session
This clearly shows that birds found mixed-defence prey more aversive than fixed-defence prey, and could use preys’ visual signals to reduce their intake of prey with more variable defences. This means that predatory attacks on a toxic prey population are affected not just by the mean toxin content of a prey population [23,24,25], and by how variable it is around that mean
Summary
Both plants and animals, defend themselves with harmful toxins in order to reduce the chances that they are eaten [1,2]. The problem is that if possessing toxins is costly, for example, reducing growth or fecundity [11,12], undefended individuals will benefit from not paying those costs, while at the same time benefitting from the aposematic defence generated by more toxic individuals. This leads to increasing numbers of automimics in the population, diluting the model’s defence and increasing the costs of being conspicuous. Not all aposematic insects have inducible chemical defences that are produced upon attack [15], and alternative mechanisms to explain variability in defences in these species are required
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