J. Zool., Lond. (2004) 263, 95–99 C  2004 The Zoological Society of London Printed in the United Kingdom DOI:10.1017/S0952836904004923 Enhanced vigilance in groups in Egernia stokesii, a lizard with stable social aggregations Elvira J. Lanham and C. Michael Bull* School of Biological Sciences, Flinders University of South Australia, GPO Box 2100, Adelaide, SA 5001, Australia (Accepted 6 November 2003) Abstract 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. Key words: lizards, Egernia, sociality, vigilance INTRODUCTION Many vertebrate species spend much of their time in social groups of varying size and permanency. One advantage of this behaviour is that group membership can reduce individual risk from predators by a dilution effect, by predator confusion or by enhanced vigilance resulting from collective detection of approaching predators (Bednekoff & Lima, 1998). Enhanced vigilance in social groups has been widely studied in mammals (Bednekoff & Ritter, 1994; Frid, 1997; Manser, 1999) and birds (Lima, 1995; Slotow & Rothstein, 1995; Sadedin & Elgar, 1998; Lima & Bednekoff, 1999). Commonly it is reported that individuals in groups spend a smaller proportion of their own time vigilant, but by responding to the flight response of neighbours, they react to approaching predators earlier than solitary individuals (Hilton, Cresswell & Ruxton, 1999; Cresswell, Hilton & Ruxton, 2000). Thus, in groups, individuals have more time for other activities such as foraging. Collective detection of predators has rarely been investigated in lizards, because very few previously studied lizard species form social aggregations (Duffield & Bull, 2002). However, female sleepy lizards Tiliqua rugosa detected an approaching threat earlier when they were feeding with a male partner than when feeding alone (Bull & Pamula, 1998), and pairs of the skink Egernia cunninghami detected approaching threats sooner than individual lizards (Eifler, 2001). This paper describes *All correspondence to: C. M. Bull. E-mail: michael.bull@flinders.edu.au investigations of Egernia stokesii, a lizard species that lives in long-term stable social aggregations of up to 17 individuals (Duffield & Bull, 2002). In early studies of vigilance, birds and mammals that were exposed to predators were assumed to partition their time into periods of vigilant behaviour and periods of non-vigilant behaviour (Pulliam, Pyke & Caracao, 1982; Lima, 1987). For instance, feeding birds were considered to be either looking up and scanning for predators, or looking down at their food, and not scanning. More recently, Lima & Bednekoff (1999) showed that feeding birds can still detect approaching predators, although less efficiently than non-feeding individuals. They suggested that researchers should refer to states of high and low vigilance, and should recognize that a cost of high vigilance is reduced feeding rate. Lizards spend less of their active time feeding than do mammals and birds. However, for lizards, heliothermic basking has a high predation risk. Some lizards close one or both eyes while basking (Mathews & Amlaner, 2000). This is possibly to minimize water loss from the moist surface of the eye (Kavanau, 1997) and may be an important water saving behaviour in arid environments. Basking lizards are at risk from predatory snakes or carnivorous mammals on the ground (Downes, 2002), and from raptors and other predatory birds from the air (Schwarzkopf & Shine, 1992; Blomberg & Shine, 2000). Moving lizards can use olfaction to detect and avoid sites frequently used by ambushing terrestrial predators (Downes & Shine, 1998), but vision is probably more important for a basking lizard to detect an approaching predator. Basking lizards respond to danger by moving rapidly to a nearby refuge (Cooper, 1999).