What factors allow opportunistic nocturnal activity in a primarily diurnal desert lizard (Ctenotus pantherinus)? Chris E. Gordon, Christopher R. Dickman, Michael B. Thompson ⁎ Institute of Wildlife Research, School of Biological Sciences, Heydon-Laurence Building (A08), University of Sydney, NSW 2006, Australia abstract article info Article history: Received 15 June 2009 Received in revised form 8 February 2010 Accepted 11 February 2010 Available online 17 February 2010 Keywords: Arid zone Diel activity Mean selected temperature Metabolic rate Prey sensory perception Skink Termite Most animals show strong 24-h patterns of activity, usually being diurnal or nocturnal. An Australian desert skink, Ctenotus pantherinus, is unusual in being active day and night when all other Ctenotus species are diurnal, making it an excellent model to explore factors that promote night-time activity. We tested whether C. pantherinus 1) selects cooler temperatures than diurnal skinks, 2) shows no difference in mean selected temperature between day and night, 3) has the same metabolic rate during the day and night, 4) selects termites over other prey types, 5) can detect prey using only auditory or olfactory senses, and 6) experiences lower predation risk at night than during the day. C. pantherinus shows many features of diurnal skink species, with a high mean selected temperature (36.1 ± 1.6 °C) that is the same night and day, and a 32% lower metabolic rate at night than during the day. C. pantherinus selects termite prey over other insects and can detect prey using only auditory and olfactory senses; models of C. pantherinus experienced less predation at night than during the day. Preference for termites and reduced predation risk at night favour opportunistic nocturnal activity in this predominantly diurnal lizard and may contribute to its wide geographic distribution in arid Australia. © 2010 Elsevier Inc. All rights reserved. 1. Introduction Animals generally show predictable 24-h patterns of activity, being strictly diurnal, nocturnal or crepuscular (Takahashi et al., 2001). Circadian rhythms allow animals to anticipate imminent changes in their social and physical environment and to prepare appropriate responses (Aronson et al., 1993). The ecological consequences of such rhythms may be profound. For example, differences in the daily time of activity allow species to avoid interspecific competition and to partition shared resources, resulting in increased local species diversity (Carothers and Jaksic, 1984; Werner and Anholt, 1993; Pinter-Wollman et al., 2006). Shifts in daily activity also can occur between trophic levels, with predators and prey influencing each others' activity (Halle, 1993; Kramer and Birney, 2001). Circadian rhythms are mostly ubiquitous and important for in- dividuals and ecological communities. Once established, the rhythms are often constrained within lineages (Daan, 1981) and are costly or difficult to change (Kronfeld-Schor and Dayan, 2003). Species that can be either nocturnal or diurnal offer unique opportunities to identify factors that influence diel activity. For example, some species of fish can be active either by day or night, and shift their daily activity in response to the presence of competitors, predators, nutritional status, or environmental factors such as temperature and light intensity (Fraser and Metcalfe, 1997; Metcalfe and Steele, 2001). Foraging ants and termites change their daily activity according to air humidity and temperature (Abensperg-Traun, 1994; Reid, 1995). Our aim is to identify factors that determine daily activity in an Australian desert lizard, the panther skink, Ctenotus pantherinus (Peters) in the Simpson Desert. C. pantherinus is a Spinifex specialist that is unique within its genus of otherwise diurnal lizards in being active during the day and at night. Night activity occurs throughout the night from sunset till after midnight (Pianka, 1986; Gordon et al., 2009). It has the largest geographical range of any species of Ctenotus (Cogger, 2002), and is one of the most frequently captured skinks in field surveys (Downey and Dickman, 1993; James, 1994; Rotsaert, 2008). Thus, C. pantherinus is an unusually good model to test hy- potheses about mechanisms that shape diel activity patterns. Specifically, we test hypotheses about thermal physiology, prey preferences and detection, and predator avoidance in C. pantherinus. Both diurnal and nocturnal reptiles often select cooler tempera- tures at night than during the day (Rismiller and Heldmaier, 1982; Innocenti et al., 1993; Refinetti and Susalka, 1997; Ellis et al., 2006). Selection of cool temperatures at night by diurnal lizards represents a voluntary hypothermia that reduces energy costs (Regal, 1967). In contrast, selection of cool temperatures by nocturnal lizards at night may allow exploitation of resources of food and space unavailable to diurnal species (Werner and Anholt, 1993), or reflect a common evolutionary constraint imposed by the correlation of high sunlight levels with high temperature (Refinetti and Susalka, 1997, Autumn et al., 1999a,b). Similarly, metabolic rate generally is lower at night Comparative Biochemistry and Physiology, Part A 156 (2010) 255–261 ⁎ Corresponding author. School of Biological Sciences, Heydon-Laurence Building (A08), University of Sydney, NSW 2006, Australia. Tel.: + 61 2 9351 3989; fax: + 61 2 9351 4119. E-mail address: mike.thompson@bio.usyd.edu.au (M.B. Thompson). 1095-6433/$ – see front matter © 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.cbpa.2010.02.007 Contents lists available at ScienceDirect Comparative Biochemistry and Physiology, Part A journal homepage: www.elsevier.com/locate/cbpa