Foraging in groups allows collective predator detection in a mammal species without alarm calls

Prey can use alarm calls to indicate the presence of a predator in social groups. However, how does it work in species that do not produce such signals? We investigated how detection of a threat occurred in groups of kangaroos. We showed that detectors, through their vigilant posture, provided information about the threat to other close group members and reactions proceeded sequentially like a domino effect. Collective detection thus appears not to be restricted to social species that exhibit conspicuous alarm signals.Although collective detection plays a key role in determining individual survival, few studies have carefully examined the collective process of detection. We investigated collective detection in the eastern grey kangaroo (Macropus giganteus), a species that forages in temporary groups and rarely produces auditory alarm signals on detection. In experimental trials, we exposed one group member to a predation threat (a python, Morelia spilota) that its companions could only detect indirectly by observing the reaction of the detector. We videotaped these and control trials in which individuals were not exposed to the python and focal females simply used vigilance routinely. Our aims were to 1) examine whether collective detection occurred and, if so, 2) investigate the temporal pattern of the information transfer among individuals. The latencies between the focal females first scans and those of their 4 neighbors were shorter in the exposed than those in the control groups. The latencies between successive individuals scans were on average shorter in the exposed groups and at the beginning of the reaction chain, and interindividual distances acted to constrain information transfer. More individuals became vigilant in the exposed than in the control groups. Thus, detection of the snake by focal females provided information about a potential threat to other close group members and reactions to this initial detection proceeded sequentially like a domino effect. Collective detection thus is not restricted to social species that exhibit conspicuous alarm signals.