Journal of Thermal Biology 31 (2006) 268–273 Time of testing affects locomotor performance in nocturnal versus diurnal snakes John Llewelyn, Richard Shine à , Jonathan K. Webb School of Biological Sciences A08, University of Sydney, NSW 2006, Australia Received 17 August 2005; accepted 26 October 2005 Abstract Studies of the thermal dependence of locomotor performance in ectotherms have provided extensive data on species differences, but often have neglected the time of day at which the test organism is usually active. To compare performance abilities among species that are active at different times of day, do we need to measure performance at the time of day that each species is normally active, or can we adopt the (logistically more convenient) alternative of testing all taxa at the same time (i.e. during daylight hours)? We scored swimming speeds of six species of Australian elapid snakes, incorporating both diurnal and nocturnal species, at a range of temperatures encompassing the usual conditions experienced during activity, and at night as well as by day. Nocturnal species swam faster by night than by day, whereas the reverse was true for diurnal taxa. The magnitude of species differences in speed depended on test temperatures as well as time of day. Thus, interspecific comparisons of locomotor abilities need to consider not only the differing activity temperatures normally experienced by species active at different times of day, but also circadian rhythms in performance. r 2005 Elsevier Ltd. All rights reserved. Keywords: Behaviour; Circadian; Elapid; Methodology; Reptile 1. Introduction For many different kinds of biological studies, we need to quantify an organism’s ability to perform specific activities; robust quantification of such abilities then allows us to compare individuals within a population, or compare among populations or species (Gans, 1974; Garland and Losos, 1994). For example, such data allow us to explore questions such as the determinants (e.g. genetic, develop- mental) of variation in performance, or the consequences (e.g. for microevolution, or the outcome of predator–prey encounters) of such variation. Locomotor speeds are perhaps the most frequently measured aspects of organis- mal performance, reflecting their ease of measurement and their plausible (and sometimes, empirically demonstrated) relationship with fitness (van Berkum and Tsuji, 1987; van Berkum et al., 1989; Jayne and Bennett, 1990). Interspecific comparisons of locomotor abilities have been used to examine many issues, including the predicted match between a species’ morphology, performance and its habitat use, mediated through locomotor ability (e.g. Losos, 1990a, b; Losos et al., 1993; Van Damme et al., 1997; Van Damme and Vanhooydonck, 2001) and the expected coevolution between activity body temperatures and thermal optima for performance (Huey and Hertz, 1982, 1984a, b; Huey et al., 1984; Huey and Bennett, 1986). Clearly, such comparisons require robust methods that provide valid measures of speed. One potentially important issue that appears not to have been addressed involves the time of day when such tests are conducted. Testing during daylight hours has undeniable logistic advantages, but means that some species are tested at times of day when they would not normally be active. Some studies that have quantified locomotor speeds of nocturnally active species have relied upon measuring locomotor speeds during daylight hours only (e.g. Autumn et al., 1999; Kearney et al., 2005). Indeed, the authors of the latter studies used bright fibre-optic lighting to encourage nocturnal lizards to run towards a dark shelter-site. Other researchers have ARTICLE IN PRESS www.elsevier.com/locate/jtherbio 0306-4565/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jtherbio.2005.10.005 à Corresponding author. Tel.: +612 9351 3772; fax: +612 9351 5609. E-mail address: rics@bio.usyd.edu.au (R. Shine).