The design of anesthetic protocols for frogs is commonly hindered by lack of information. Results from fishes and rodents do not always apply to frogs, and the literature in anurans is concentrated on a few species. We report on the response of treefrogs (Hyla chrysoscelis and H. versicolor) to tricaine methanesulfonate. Body mass did not differ significantly between the species or between sexes. In the first exposure of a frog to TMS, variation in induction time was best explained by species (H. chrysoscelis resisted longer) and body mass (larger animals resisted longer). Multiple exposures revealed a strong effect of individual variation on induction time and a significant increase of induction time with number of previous anesthesia events within the same day. Recovery time was mostly explained by individual variation, but it increased with total time in anesthetic and decreased with induction time. It also increased with number of days since the last series of anesthesias and decreased with number of previous uses of the anesthetic bath. This is one of the first studies of anesthesia in hylids and also one of the first assessments of the factors that influence the variability of the response to anesthesia within a species. 1. Introduction Anesthesia of frogs has been conducted mostly in scientific studies focused on physiology and more recently in taxonomic and ecological studies to allow for painless manipulation or euthanasia [1–5]. A variety of drugs and modes of administration have been used and comparative studies have revealed a great deal of variation in response among species [6, 7]. A well-informed choice of anesthetic and protocol leads to an anesthesia that does not harm the animal, maintains it sedated for the necessary amount of time, and is easy to handle [8]. Data on anesthesia in fishes and rodents can be applied to frogs only to a limited extent. Like fishes, frogs respond to anesthetics in a bath, but while fishes primarily absorb the drug through the gills, adult frogs lack such structures and absorb the drug through their permeable skin [9, 10]. In relation to mammals, amphibians metabolize and eliminate drugs at much slower rates because of their ectothermic metabolism [11]. Comparative studies are therefore necessary to assess the responses of amphibians across the range of available anesthetics and also across taxa. This will establish a basis for appropriate choices of anesthetics and protocols. The anesthetics most commonly employed in frogs include benzocaine [12], tricaine methanesulfonate [7, 13], eugenol (clove oil) [14,
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