Abstract
Aposematic prey warn predators of their toxicity using conspicuous signals. However, predators regularly include aposematic prey in their diets, particularly when they are in a poor energetic state and in need of nutrients. We investigated whether or not an environmental factor, ambient temperature, could change the energetic state of predators and lead to an increased intake of prey that they know to contain toxins. We found that European starlings, Sturnus vulgaris, increased their consumption of mealworm, Tenebrio molitor, prey containing quinine (a mild toxin) when the ambient temperature was reduced below their thermoneutral zone from 20 °C to 6 °C. The birds differed in their sensitivity to changes in ambient temperature, with heavier birds increasing the number of toxic prey they ate more rapidly with decreasing temperature compared to birds with lower body mass. This could have been the result of their requiring more nutrients at lower temperatures or being better able to detoxify quinine. Taken together, our results suggest that conspicuous coloration may be more costly at lower temperatures, and that aposematic prey may need to invest more in chemical defences as temperatures decline. Our study also provides novel insights into what factors affect birds' decisions to eat toxic prey, and demonstrates that selection pressures acting on prey defences can vary with changing temperature across days, seasons, climes, and potentially in response to climate change.
Highlights
Aposematic prey warn predators of their toxicity using conspicuous signals
We found that European starlings, Sturnus vulgaris, increased their consumption of mealworm, Tenebrio molitor, prey containing quinine when the ambient temperature was reduced below their thermoneutral zone from 20 C to 6 C
The birds differed in their sensitivity to changes in ambient temperature, with heavier birds increasing the number of toxic prey they ate more rapidly with decreasing temperature compared to birds with lower body mass
Summary
Aposematic prey warn predators of their toxicity using conspicuous signals. predators regularly include aposematic prey in their diets, when they are in a poor energetic state and in need of nutrients. There is good evidence to support this idea, from studies showing that predators eat more toxic prey when they have been food restricted (Swynnerton 1915; Sexton et al 1966; Gelperin 1968; Williamson 1980; Chai 1996; Barnett et al 2007, 2012), or when the number or size of alternative palatable prey in the environment is small (Lindström et al 2004; Halpin et al 2013) Under such conditions, the benefits to acquiring nutrients from defended prey will be increased relative to the costs of ingesting their toxins. All endotherms, including birds, have a thermoneutral zone (TNZ), which is a range of ambient temperatures at which the metabolic cost of maintaining body temperature
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