Vigilance (behavioural ecology)
Vigilance, in the field of behavioural ecology, refers to an animal's monitoring of its surroundings in order to heighten awareness of predator presence.
[1] Vigilance and feeding (both searching for and handling food) are generally mutually exclusive activities, leading to foragers facing a trade-off between energy intake and safety from predation.
[2] Optimality models can be used to predict foraging decisions of an animal based on costs (predation risk, starvation) and benefits (safety, food), which are also affected by physiology such as hunger levels.
[3] Small food items are consumed immediately to maximise energy intake, as they require little handling time so the predation risk is low.
Similarly, if hungry animals have a higher chance of dying from starvation than from predation, it is more beneficial to sacrifice vigilance to fulfill their energy requirements.
When three-spined sticklebacks (Gasterosteus aculeatus) are deprived of food, they prefer to feed in locations with a high density of water fleas.
[6] Similarly, juncos that have been deprived of food exhibit lower levels of vigilance, instead focusing on rapid feeding, which is a behaviour incompatible with scanning.
If the predation risk is so great that an animal must maintain a level of vigilance that drastically inhibits feeding, it may opt for an alternative.
For example, the bluegill sunfish (Lepomis macrochirus) has the choice of foraging on plankton in the safety of reeds or on benthic invertebrates which are a better quality food source.
[8] When a predator (the largemouth bass) is present, smaller sunfish spend the majority of their time foraging in the reeds despite this choice reducing their food intake and seasonal growth rate.
[8] Nocturnal animals alter the timing of their foraging based on the level of light – avoiding feeding when the moonlight is bright as this is when predation risk is highest.
[11] As individuals in smaller flocks have a greater need to be vigilant (see more in Vigilance in groups), large pieces of food are more beneficial as they require a longer handling time that can be simultaneously spent scanning, whereas birds feeding on small pieces must intermittently stop foraging to scan their environment.
Wood pigeons (Columba palumbus) in large flocks are more able to escape predation by goshawks because they are able to spot them and fly away more quickly than they would individually.
[18] Lower individual levels of vigilance with increasing group size has been observed in many bird, mammal and fish species.
[1] Sentry duty is particularly important for species whose foraging activity is incompatible with vigilance, or who feed in areas where they are highly exposed to predators.
For example, dwarf mongooses (Helogale parvula) dig up arthropods from the ground to eat; an activity which requires fixation of both their vision and olfaction on the prey.
[22] Often, the sentry makes quiet calls that function as a ‘watchman’s song’ to reassure the rest of the group that an individual is on guard.
[23] In response to a vocalising sentry, pied babblers (Turdoides bicolor) decrease their own vigilance, spread out further from the group and forage in more exposed patches resulting in a higher intake of biomass.
Cheetahs (Acinonyx jubatus) select less vigilant Thomson's gazelles (Eudorcas thomsonii) before initiating a chase and target them for attack.
