Effects of sublethal lead exposure on the behaviours of green frog (rana clamitans), bullfrog (rana catesbiana) and American toad (bufo americanus) tadpoles

Abstract

We examined the effects of sublethal exposure to lead (Pb) on avoidance‐preference responses, locomotor activity, discriminate learning, and retention (memory) of green frog (Rana clami‐tans) and bullfrog (Rana catesbeiana) tadpoles, and on avoidance‐preference responses of American toad (Bufo americanus) tadpoles. Sublethal exposure to Pb inhibited both acquisition and retention of discriminate avoidance learning in green frog and bullfrog tadpoles. Green frog and bullfrog tadpoles used in learning acquisition tests were exposed to 0 or 750 μg Pbl‐1for 144h; those used in retention tests were similarly exposed for 120 h. Lead‐exposed and control animals were conditioned to associate illumination (Conditioned Stimulus) with shock (Unconditioned Stimulus) in an aquatic shocking apparatus. Learning acquisition tests showed greater mean response times and fewer avoidances for Pb‐exposed tadpoles. Retention tests also showed greater mean response times, but no avoidances for Pb‐exposed animals. Sublethal exposure to 750 μg Pbl‐1 thus adversely affects acquisition learning and retention. Green frog and bullfrog tadpoles used in avoidance‐preference and locomotor activity bioassays and American toad tadpoles used in avoidance‐preference bioassays were exposed to 0,500,750 or 1000 μg Pbl‐1 for 144 h. Results indicated no preference for or avoidance of Pb by tadpoles and no significant effects of Pb on gross locomotor activity. Pre‐exposure of the tadpoles to these concentrations of Pb did not induce an increased sensitivity of the animals to subsequent encounters with Pb. Analyses using the folded F‐test, however, indicated a significant increase in the variability of locomotor activity. The significantly greater variability in activity of green frog and bullfrog tadpoles in the 625,750 and 1000μgPbl‐1 exposure groups indicates that variability of response may be a more sensitive indicator of sublethal Pb toxicosis than gross locomotor activity. Avoidance‐preference responses and spontaneous locomotor activity are less sensitive indices of sublethal Pb toxicosis of green frog and bullfrog tadpoles than is discriminate avoidance learning.