Moving Day and Night: Highly Labile Diel Activity Patterns in a Tropical Snake Rickard Abom 1 , Kris Bell, Lauren Hodgson, and Lin Schwarzkopf School of Marine and Tropical Biology, James Cook University, Townsville, QLD 4811, Australia ABSTRACT Most animals have well established diel activity patterns (e.g., diurnal, crepuscular, or nocturnal), and changes in behavior from diurnal to nocturnal are rare in single species. We radio tracked 50 keelback snakes in a single population, locating them up to four times a day, over five periods of the year in the Australian dry tropics to describe temporal variation in diel movement patterns. Snake body temper- atures were also recorded to determine the relationship between activity patterns and body temperatures. Season influenced diel activity patterns significantly. Keelbacks were more likely to move, and moved further in the daytime in the mid-dry (June–July), and late dry (Aug–Sep) seasons. In the mid-dry season, 87 percent of movements were diurnal, whereas in the mid-wet (Feb–March) season, although snakes were much more likely to move, only 43 percent of movements were diurnal. In the late dry season, snakes were slightly more likely to move at night than at any other time of day, and so at this time of the year, snakes could be classified as noctur- nal. Thus, overall increased movements in the mid-wet season (austral summer) were associated with more crepuscular and nocturnal movement. There was a significant relationship between individual snake body temperatures and movement rates in all seasons. Changes in movement patterns may be related to body temperature, and this diurnal species becomes cathemeral in the tropics in summer, when it is possible to maintain high body temperatures both day and night. Key words : diel activity; dry tropics; environmental temperatures; keelback snake; seasonal wetland; Tropidonophis mairii. DIEL ACTIVITY PATTERNS IN ANIMALS ARE OFTEN WELL ESTABLISHED WITHIN SPECIES, and most animals have set activity times, so that, in general, species can be classified as diurnal, crepuscular (active at dawn and dusk), or nocturnal (Metcalfe et al. 1998, Kronfeld-Schor & Dayan 2003). Diel activity patterns are typi- cally so strongly fixed that characters such as eye morphology, communication methods, and body coloration evolve to func- tion most efficiently at the time of day when individuals are most likely to be active (Snyder & Weathers 1975, Endler 1993, Rydell & Speakman 1995, Ernst & Zug 1996, Brischoux et al. 2010). A few species show variation in diel activity, and in such ani- mals, diel activity patterns can be influenced by variations in the timing of food availability, predator activity, and interspecific competition (Carothers & Jaksic 1984, Metcalfe et al. 1998, Kronfeld-Schor et al. 2001, Fraser et al. 2004). One major factor affecting activity patterns are fluctuations in environmental tem- peratures (e.g., basking times of turtles, lizards, and snakes vary with diurnal temperatures as these temperatures vary with season; Frankenberg 1978, Gibbons & Semlitsch 1987, Schwaner 1989, Andrews 1994, Brito 2003). Even in species that show variations in activity, however, such variations are typically small. Usually, crepuscular species can become somewhat more diurnal (Ashby 1972), or the reverse (Vieria et al. 2010), often as temperatures or annual light cycles become unsuitable for activity at the ‘normal’ time of day. Few species engage in both diurnal and nocturnal activity, or shift between the two (Fraser et al. 1993). On the other hand, two temperate snake species, Thamnophis radix (Heckrotte 1962) and Crotalus atrox (Landreth 1973), season- ally shift diel movements patterns in relation to fluctuating envi- ronmental temperatures; snakes move diurnally in the cooler winter period and nocturnally in the hotter summer period. Keelback snakes (Tropidonophis mairii) occur in tropical north- ern Australia and have been reported in the literature as diurnal (Lyon 1973, Wilson & Swan 2004), and as crepuscular to noctur- nal (Shine 1991). These conflicting reports suggested that their diel activity patterns may vary widely. We measured the activity patterns of keelback snakes by radio tracking adults at five different periods throughout the year, and located individuals up to four times daily. Because snakes are ectotherms, and therefore likely to be strongly influenced by tem- perature, we recorded individual body temperatures and environ- mental temperatures, and examined the relationship between temperature, diel activity, and movement patterns of tracked snakes among seasons. We focused on temperature effects on diel activity, because other factors that influence diel activity patterns in other animals, such as timing of food availability, predation and competition, were unlikely to influence diel activity patterns in these snakes, at least not as strongly as body temperature might (Brown & Shine 2002). Keelbacks typically consume frogs, fish, and tadpoles, which are nocturnal (frogs), or always available (fish and tadpoles) (Greer 1997, Wilson & Swan 2004). Large changes in timing of food availability were unlikely, therefore, to have caused variation in diurnal cycles of activity in these snakes. Their predators are mostly birds, including both diurnal and nocturnal species active all year (hawks and owls, pers. obs. of predation during radio tracking). Predator pressure was, therefore, also unlikely to cause Received 22 June 2011; revision accepted 31 October 2011. 1 Corresponding author; e-mail: rickard.abom@my.jcu.edu.au 554 ª 2012 The Author(s) Journal compilation ª 2012 by The Association for Tropical Biology and Conservation BIOTROPICA 44(4): 554–559 2012 10.1111/j.1744-7429.2012.00853.x