Abstract
<title>Abstract</title> <p> Surface temperature measured via infrared thermal imaging (IRT) varies with autonomic nervous system activity in endothermic homeotherms. In primates, nasal temperature decreases are interpreted as indicators of affective state, yet the specific affective component they reflect remains unclear. Previous research has linked nasal cooling to discrete emotions (e.g., “fear” or “frustration”), negatively valenced high-arousal states, or arousal independent of valence. This study tested whether nasal temperature predicts the valence of naturally occurring affect, assessed using a judgement bias task in zoo-housed bonobos <italic>(Pan paniscus)</italic> . Clarifying this relationship is critical for validating inferences about affect and welfare drawn from thermal measures. Seven bonobos were trained on an active-choice judgement bias paradigm. Task sessions were recorded with an IRT camera, and nasal temperatures were extracted from a one-second window preceding each trial. Self-directed behaviors (SDBs) were recorded during inter-trial intervals to validate the sensitivity of the task to negatively valenced affect; however, SDB rates did not credibly predict judgement bias. Nasal temperature was likewise not a credible predictor of judgement bias as a main effect or within the stimulus conditions. However, the temperature–choice relationship for two of the ambiguous stimuli diverged credibly from the conditioned stimulus. Relative to this conditioned baseline, optimistic responses to ambiguous stimuli were associated with colder nasal temperatures. This pattern may reflect positively valenced high-arousal states if animals interpreted ambiguous cues as signaling a preferred reward. Overall, nasal temperature decreases appear more consistent with arousal than with affective valence, limiting their utility as direct indicators of welfare. </p>