Intro 1. Animals need to modulate decisions to deal with threat
Animals making decisions engage in an economic cost-benefit analysis; how do I maximize my benefits and minimize my costs of different options (Stephens and Krebs 1986)? However, when performing any activity, animals must account for the costs of predation, i.e. the chance of death (
Stephens 2007,
Lima 1990). While an animal can engage in activities to reduce this, such as hiding or being vigilant, constant vigilance produces its own costs, such as starvation or never reproducing. This forces a tradeoff in the allocation of time, attention, and other resources between the foreground task the animal is engaged in and the background threat of predation. Resource allocation is a graded response based on the priority of either outcome - a graded modulation of fearful responses - including different ways of splitting attentional resources across either time or space (Trimmer et al 2013, Mobbs 2015).
In the worst case, threat is immanent and requires direct engagement; either fight or flight. In these cases the priority of the threat takes precedence over whatever other action the animal was engaged in, and available resources are allocated to dealing with the threat. But this response depends critically on the degree of immanence; threats that are less immanent should be monitored rather than directly engaged. This gradient of threat responses means that animals allocate more or less resources to monitoring the threat based on perceived threat level (Mobbs 2015, etc). Animals can take active measures to increase their chance of detecting a predator, and will increase their vigilance based on perceived threat.
Vigilance in ecology is often operationally defined based on head movement of foragers (e.g. Treves 2000, Blanchard et al 2011, Bednekoff and Lima 1998, Shettleworth 2010). For example most birds have to alternate between scanning their environment looking for danger with their head up, and foraging for food with their heads down. In general an increase in threat will decrease the time allocated to vigilance behavior for these animals (Wahungu et al 2001, Gustafsson et al 1999, Stephens 2007).
Of course vigilance is operationally defined differently depending on the animal, as the allocation of attention in this way depends crucially on both the threat level and sensory system used. Attentional processing can sometimes be allocated without a change in overt behavior, as seen in split-attention paradigms (e.g. Broadbent 1958, Dukes and Kamil 2000). If attention can be allocated without behavior changes, as might be possible for auditory processing, then attention might not need to be allocated across time as a distinct vigilance task. However the same is not true for visual processing in many animals including humans, since high resolution needed for predator identification is limited to the fovea. For visual tasks this produces a time tradeoff, where visual attention cannot be split at one time and must be split across time (Nobre 2001, Hoppe and Rothkopf 2016).