Stimulation of terrestrial-substrate preferences and locomotor activity in newly transformed tiger salamanders (Ambystoma tigrinum) by exogenous or endogenous thyroxine

Recently transformed adult tiger salamanders (Ambystoma tigrinum) that had never resided on land were treated with increasing, alternate-day injections of thyroxine (experiment 1) or single injections of particular dosages of this hormone (experiment 2). In all cases control salamanders received equivalent volumes of saline on a similar schedule. In both experiments, subjects treated with thyroxine displayed significant levels of terrestrial-substrate preference in a moisture-gradient apparatus. Salinetreated controls never emerged from the water. In experiment 2, animals that received single thyroxine treatments also displayed heightened levels of land-directed locomotor activity. In experiment 3, blood plasma was collected from salamanders displaying spontaneous land preference and others displaying water preference. Radioimmunoassay for thyroxine indicated that salamanders that had moved to, and resided upon, a terrestrial substrate some time subsequent to metamorphosis had significantly higher plasma levels of this hormone than did animals preferring to remain submerged. These results indicate (1) that thyroxine stimulates, and possibly maintains, emergence from water and terrestrial preference in newly transformed tiger salamanders and (2) that this hormone stimulates a locomotor activity component associated with the movement of formerly aquatic adults to the terrestrial habitat.