Abstract
Predators that have learned to associate warning coloration with toxicity often continue to include aposematic prey in their diet in order to gain the nutrients and energy that they contain. As body size is widely reported to correlate with energetic content, we predicted that prey size would affect predators' decisions to eat aposematic prey. We used a well-established system of wild-caught European starlings, Sturnus vulgaris, foraging on mealworms, Tenebrio molitor, to test how the size of undefended (water-injected) and defended (quinine-injected) prey, on different coloured backgrounds, affected birds’ decisions to eat defended prey. We found that birds ate fewer defended prey, and less quinine, when undefended prey were large compared with when they were small, but that the size of the defended prey had no effect on the numbers eaten. Consequently, we found no evidence that the mass of the defended prey or the overall mass of prey ingested affected the amount of toxin that a predator was willing to ingest, and instead the mass of undefended prey eaten was more important. This is a surprising finding, challenging the assumptions of state-dependent models of aposematism and mimicry, and highlighting the need to understand better the mechanisms of predator decision making. In addition, the birds did not learn to discriminate visually between defended and undefended prey based on size, but only on the basis of colour. This suggests that colour signals may be more salient to predators than size differences, allowing Batesian mimics to benefit from aposematic models even when they differ in size.
Highlights
Predators that have learned to associate warning coloration with toxicity often continue to include aposematic prey in their diet in order to gain the nutrients and energy that they contain
We found that birds ate fewer defended prey, and less quinine, when undefended prey were large compared with when they were small, but that the size of the defended prey had no effect on the numbers eaten
We found that there was no significant difference in the numbers of undefended or defended prey eaten in any group across sessions 3e6 and no significant interaction between prey type and session (0.40 < F3, 21e27 < 0.88, 0.47 < P < 0.75; Fig. 1), but there was a significant difference in the number of undefended and defended prey eaten across sessions 3e6 in all groups (14.65 < F1, 7e9 < 50.59,
Summary
Predators that have learned to associate warning coloration with toxicity often continue to include aposematic prey in their diet in order to gain the nutrients and energy that they contain. Barnett et al 2007; Skelhorn & Rowe 2010; Halpin et al 2012) This is because educated predators benefit from eating the nutrients and energy that aposematic prey contain, when they are in a poor energetic state (Sexton et al 1966; Williamson 1980; Chai 1986; Hileman et al 1995; Barnett et al 2007, 2012). Decisions to eat aposematic prey based on the trade-off between the benefits of gaining nutrients and energy with the costs of ingesting toxins (Barnett et al 2007; Skelhorn & Rowe 2007, 2010). To our knowledge, no study has tested the effects of the energetic content of toxic prey on the foraging decisions of predators, in order to investigate how it affects selection pressures acting on aposematism and mimicry
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