Daily and seasonal features in the hind limb withdrawal latency of tegus (Salvator merianae) and green iguanas (Iguana iguana)

Abstract

This study evaluated the effects of daytime and season on the limb withdrawal latency time (LWLT) of lizards. Captive-raised, juvenile tegus and green iguanas (n = 6 each) were housed indoors, where a temperature gradient (27–40 °C) and a constant photoperiod (12-h cycle) were provided. Body temperatures and LWLT were measured at 00:00 h, 06:00 h, 12:00 h and 18:00 h over winter, spring, summer and fall. Both lizards had body temperatures lower than 30 °C at 00:00 h/06:00 h and higher than 30 °C at 12:00 h/18:00 h. In spring, tegus had higher body temperatures than in fall at 12:00 h (34.2 ± 1.4 °C versus 30.3 ± 1.9 °C) and in winter at 18:00 h (33.6 ± 1.2 °C versus 30.4 ± 1.0 °C); iguanas presented increased body temperatures at 00:00 h in summer (28.1 °C [28.0–28.2]) compared to winter (25.5 °C [25.3–25.7]). The LWLT of tegus was elevated at 06:00 h compared to 12:00 h and 18:00 h in winter (12.0 ± 3.8 s versus 7.6 ± 1.8 s and 7.0 ± 3.2 s, respectively), spring (12.0 ± 2.7 s versus 7.4 ± 3.1 s and 7.7 ± 1.3 s, respectively) and fall (14.8 ± 2.3 s versus 10.2 ± 1.2 s and 8.9 ± 1.9 s, respectively). Iguanas showed season-related fluctuations in LWLT, with higher values at 18:00 h in winter (15.5 ± 6.1 s) and fall (14.8 ± 2.6 s) than in summer (9.2 ± 1.9 s), as well as higher at 00:00 h (16.6 ± 5.4 s) and 12:00 h (15.4 ± 3.9 s) in spring than in summer (11.0 ± 2.9 s and 9.3 ± 1.3 s, respectively). Body temperature was poorly correlated with LWLT in tegus (r_s = - 0.292; p = 0.003) and iguanas (r_s = - 0.143; p = 0.162). Researchers should consider circadian and circannual rhythms as potential biases in antinociception-focused lizard studies.