of both their vision and olfaction on the prey. 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. 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. Guarding may appear to be an altruistic behaviour as an individual on sentry duty is unable to feed, may be more exposed to predators, and may attract the predators attention when they make an alarm call. However meerkats on sentry duty are at no greater risk of predation as they are generally the first to detect predators (e.g. jackals, eagle species) and flee to safety. Meerkats also only go on guard once they are satiated so if no other individual is on sentry duty, guarding may be the most beneficial behaviour as the individual has no requirement for feeding and can benefit from early predator detection. Cheating In a large group, it is possible for an individual to cheat by scanning less than other members of the group without having any negative effect on the collective vigilance of the group. Cheating would seem to be the most beneficial strategy as the individual still benefits from the collective detection of the group while being able to feed more than other individuals. However, cheating is not a stable strategy because if all members of the group did it, then there would be no collective vigilance. Adopting a very high level of vigilance in a group is also not a stable strategy as an individual who adopts a lower level of vigilance would have a greater advantage. The evolutionarily stable (ESS) scanning rate is the rate that if all group members adopted, an individual that scanned more or less frequently would have a lower chance of survival. Maintaining individual vigilance may be a more beneficial strategy if vigilant animals gain some sort of advantage. Non-vigilant individuals are often the last to flee to safety, as groups generally flee in succession from the individual who spots the predator, individuals who were vigilant when the first animal departs, and finally the non-vigilant individuals. Predators may also target less vigilant prey as they are likely to detect the predator later and therefore respond more slowly. Cheetahs (Acinonyx jubatus) select