What can vigilance tell us about fear?

Animal vigilance is concerned with the monitoring of potential threats caused by predators and conspecifics. Researchers have argued that threats are part of a landscape of fear tracking the level of risk posed by predators and conspecifics. Vigilance, which is expected to vary with the level of risk, could thus be used as a measure of fear. Here, I explore the relationship between vigilance and fear caused by predators and conspecifics. The joint occurrence of vigilance and other physiological responses to fear, such as increased heart rate and stress hormone release, would bolster the idea that vigilance can be a useful marker of fear. While there is some support for a positive relationship between vigilance and physiological correlates of fear, a common theme in much of the empirical research is that vigilance and physiological correlates of fear are often uncoupled. Uncoupling can arise for several reasons. In particular, vigilance is not always a sensitive or specific marker of the internal state of vigilance. Vigilance might occur in animals who do not appear overtly vigilant or conversely an animal might appear vigilant without necessarily maintaining a state of vigilance. Animals in a fearful state might also be unable to allocate time to vigilance if they are too hungry. Vigilant animals might not show physiological responses associated with fear if they become desensitized to threats. For all these reasons, inferring fear from vigilance is fraught with ambiguity