Abstract
Responses to ambiguous and aversive stimuli (e.g. via tests of judgment bias and measures of startle amplitude) can indicate mammals’ affective states. We hypothesised that such findings generalize to birds, and that these two responses co-vary (since both involve stimulus evaluation). To validate startle reflexes (involuntary responses to sudden aversive stimuli) and responses in a judgment bias task as indicators of avian affective state, we differentially housed hens with or without preferred enrichments assumed to improve mood (in a crossover design). To control for personality, we first measured hens’ baseline exploration levels. To infer judgment bias, control and enriched hens were trained to discriminate between white and dark grey cues (associated with reward and punishment, respectively), and then probed with intermediate shades of grey. For startle reflexes, forceplates assessed responses to a light flash. Judgment bias was only partially validated: Exploratory hens showed more ‘optimism’ when enriched, but Non-exploratory hens did not. Across all birds, however, startle amplitudes were dramatically reduced by enrichment (albeit more strongly in Exploratory subjects): the first evidence that avian startle is affectively modulated. Startle and judgment biases did not co-vary, suggesting different underlying mechanisms. Of the two measures, startle reflexes thus seem most sensitive to avian affective state.
Highlights
Assessing animals’ affective states, and thence their welfare, requires well-validated indicators that reliably track affective valence, i.e. the positive and/or negative dimension of emotions and moods (e.g.1,2)
Subjects’ responses to ambiguous stimuli are measured as indications of their ‘optimism’ or ‘pessimism’, such that their tendencies to interpret an ambiguous cue as a DS+ or DS− indicate positive or negative affective states respectively
Like humans in negative affective states, rats subjected to stressful housing conditions responded more ‘pessimistically’ to these ambiguous stimuli, treating such tones as if more likely to be a DS− than DS+
Summary
Assessing animals’ affective states, and thence their welfare, requires well-validated indicators that reliably track affective valence, i.e. the positive and/or negative dimension of emotions and moods (e.g.1,2). Tests for judgment bias are not always sensitive to humans’ affective state Reasons for such false negative results include that findings can be influenced by the type of task used (e.g.12,13,15), and by subjects’ developmental stage[14]. To be valuable in animal welfare assessment, any new judgment bias task must first be validated as an indicator of that species’ affective state, rather than assumed to be one This was one of our aims for hens. In humans, positive affect manipulations do not always successfully reduce startle reflexes (e.g.20); anticipating rewarding events may even potentiate startle (e.g.32) Such effects may reflect how much readiness for physical action each state induces[39]. Assessing the impact of affective state changes on the avian startle reflex, using laying hens as a model, was our second aim
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