4.1 From synchronization to coordination
With spectral analysis of vigilance behaviour that takes time sequences of vigilant and non-vigilant state into consideration, we reported the first evidence of group vigilance shift from synchronization to independence and to coordination. As expected from theory, coordination is evolutionarily unstable (Rodríguez-Gironés and Vásquez, 2002; Sirot and Touzalin, 2009), less important time-expenditure (Ward, 1985), sentinels are at great risk of getting depredated (Ridley et al., 2013), unless significant benefit could be met to enhance survive and foraging odds (Santema and Clutton-Brock, 2013; Brandl and Bellwood, 2015) or to help coping with frequent disturbances (Ge et al., 2011) of family members within small groups (Wickler, 1985; Rasa, 1986; Rodríguez-Gironés and Vásquez, 2002). Although coordinated vigilance is a kind of time-waste behaviour, it leaves fewer alert gaps for foraging groups (Pulliam, 1973; Sirot and Touzalin, 2009).