Behavioral Effects of Thyroxine in a Lizard (Ameiva undulata: Teiidae)

Abstract

We investigated the effects of supplemental thyroxine (T₄) and thyroid deficiency on evoked and spontaneous behavioral responses in a lizard (Ameiva undulata). We quantified spontaneous behavior as distance walked without external stimulation and evoked behavior as organismal excitability during the first minute of forced activity on a treadmill, as running time to exhaustion during graded-intensity exercise, and as the outcome of staged competition for prey. Plasma T₄ was reduced to nondetectable levels by surgical thyroidectomy and was increased from 6.4 ± 0.59 to 22.0 ± 3.33 ng/mL (mean ± SE; P < 0.001) by intraperitoneal T₄-pellet implantation. Standard metabolic rate (SMR) was reduced by thyroid deficiency (P = 0.008) but was not stimulated by T₄ supplementation (P = 0.665). Excitability was increased by T₄ (P < 0.025) but was unaffected by thyroid deficiency. Spontaneous behavior was replicable between duplicate trials (r² = 0.47, P < 0.001) but did not differ among hormone treatment groups (P = 0.780) or among excitability categories (P = 0.881). Neither running endurance (P = 0.646) nor the outcome of staged competition for prey (0.05 < P < 0.10) were significantly affected by hormone treatment, and endurance was not correlated with spontaneous behavior (P = 0.148). However, running endurance was significantly lower in lizards of high than low excitability (P = 0.018), and winners in competition trials were usually more excitable than losers (P = 0.057). Physiologically realistic increments in plasma T₄ can increase evoked organismal excitability without being reflected in spontaneous behavior and without stimulating SMR. Excitability is reflected in rapid fatigue during activity and in success during competition for prey. Thyroxine can induce significant behavioral responses without an obligate energetic cost.