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An individual-based profitability spectrum for understanding interactions between predators and their prey

There is confusion in the animal behaviour literature over the use of the terms ‘toxicity’ and ‘unpalatability’, which are commonly used interchangeably when describing the function of chemical compounds in prey, although these terms describe very different functions. Toxic chemicals cause fitness-reducing harm, whereas unpalatability provides aversive taste but no reduction in fitness. Furthermore, chemical defences are only one aspect of prey profitability. We argue that if predators are maximizing fitness, all prey can be described in terms of their costs and benefits to predators across all currencies, giving each prey item a positive or negative position on a ‘profitability spectrum’. Adaptively foraging predators should be selected to eat only prey with a positive profitability. The context of each predator–prey encounter also alters the profitability of the prey. Given that profitability is a function of the current state of both the predator and the prey individuals, we explain why it should be considered to be an attribute of a particular encounter, in contrast to its present usage as an attribute of a prey species. This individual-centred perspective requires researchers to investigate, through both theoretical models and empirical studies, the complex conditions in which predators and prey meet in real life.