The Australian gidgee skink Egernia stokesii is unusual among lizards in that it maintains stable social aggregations of related individuals. Experiments were conducted to investigate whether lizards living in groups gained benefits from the collective detection of approaching threats. In captive colonies, lizards living in a group detected an approaching threat earlier than lizards alone, and lizards basking in a group spent more time in a non-vigilant, eyes-closed, state than lizards alone. These results imply that individuals in this species gain from the enhanced vigilance associated with group membership. Since many juveniles remain within their natal groups, group vigilance may enhance inclusive fitness.