Effects of Thyroxine on Standard and Total Metabolic Rates in the Lizard Sceloporus undulatus

Abstract

This study investigated the effects of chronic thyroxine (T4) release via implanted pellets on standard and total daily metabolic rates in captive and field-active lizards Sceloporus undulatus. Plasma T4 was elevated to 42.2 ± 19.96 ng mL⁻¹ (X ±1 SE) in T4-pellet-implanted lizards, compared with 8.56 ± 1.36ng mL⁻¹ in lizards receiving placebo pellets (P = 0. 047). However, plasma T3 was reduced by T4 treatment (0.9 ± 0.20 ng mL⁻¹ [hormone] against 2.4 ± 0.92 ng mL⁻¹ [placebo], P = 0 071). Similarly, among field-active lizards, plasma T4 was elevated by T4 treatment (36.6 ± 5.71 ng mL⁻¹ [hormone] against 10.2 ± 1.33 ng mL⁻¹ [placebo], P < 0.001), and plasma T3 was lowered (1.6 ± 0.15 ng mL⁻¹ [hormone] against 2.5 ± 0.52 ng mL⁻¹ [placebo], P = 0. 046). Standard metabolic rates (SMR) were 59% higher infield-active T4-treated lizards compared with field-active placebo-treated lizards (P < 0. 001). Thyroxine treatment did not significantly elevate SMR of captive lizards (P = 0.079). Standard metabolic rate was significantly higher in field-active lizards than in captive, T4-treated lizards (P = 0.008). Total daily metabolic rates (TDMR) of T4-treated field-active lizards were 16% greater than TDMR of placebo-treated field-active lizards (P = 0.046; one-tailedt-test). Total daily metabolic rates of captive lizards were not altered by T4 treatment (P = 0. 769). These results confirm that T4 has greater physiological activity infield-active lizards than in captive lizards. These results also indicate that the entire increment in TDMR of field-active lizards can be attributed to the increment in SMR and suggest that T4 has no effect on overall activity levels.