In pied babblers, sentinels detect predators at a greater rate than other group members, suggesting that sentinel behaviour functions as an effective anti-predator activity in this species, similar to findings for both meerkats (Suricata suricatta
) and Florida scrub-jays (Aphelocoma coerulescens
; McGowan & Woolfenden 1989
; Manser 1999
). Our research also confirms that this behaviour is adjusted according to the benefits of investment: overall, groups invest more in sentinel activity when predation risk is high. This is, to our knowledge, one of the first studies to experimentally manipulate the benefits of sentinel activity according to predation risk in a cooperative species, and our results agree with those of Eggers et al. (2005)
, that facultative adjustment of behaviour can occur quickly in response to changes in predation risk, allowing species to quickly adapt to current conditions.
It is interesting to note that the increase in sentinel activity following the snake presentation was higher than that for the playback experiments, despite very similar rates of sentinel activity prior to the manipulations. This may represent a graded response to environmental information: in the case of the model presentation, individuals were actually able to see
the predator, resulting in a greater increase in sentinel behaviour when the increase in risk is more certain. Our playback results also confirm that pied babblers use cues provided by heterospecifics to provide them with information about the likely presence of a predator, as observed in other species (Rainey et al. 2004
; Magrath et al. 2007
), and this is, to our knowledge, the first study to show that such indirect information regarding the likely presence of a predator affects sentinel activity.
Previous work on sentinel activity has suggested that state dependency explains the patterns of behaviour observed (Wright et al. 2001
). In Bednekoff's (1997
) theoretical model, he suggested that acting as a sentinel is the safest place for an individual to be once it is satiated, and several feeding experiments have shown that individuals do guard more following satiation (Clutton-Brock et al. 1999
; Wright et al. 2001
; Bednekoff 2003
). However, our experiments demonstrate that the immediate benefits of sentinel activity (in terms of detection and monitoring of predators) can also affect patterns of sentinel behaviour, similar to vigilance patterns observed in non-cooperative species (Beletsky 1989
, reviewed in Lima 2009
). If individuals became sentinels simply because they were satiated, then we would expect changes in predation risk to have little or no effect on sentinel activity. By contrast, our results suggest a strong effect, and we suggest that further empirical research considers both the costs and benefits of sentinel activity to determine the primary factors affecting investment in this behaviour in cooperative societies.