Variation in Diel Activity Across Life Stages in the Túngara Frog

The circadian rhythm is a 24-hour cycle that regulates physiological and behavioral processes. Amphibians exhibit complex natural histories with circadian rhythms that can vary among life stages. The túngara frog (Engystomops pustulosus) is a small terrestrial anuran native to Central America, with an aquatic larval stage and a terrestrial adult stage. In this study we examine the diel activity patterns of tadpoles, froglets and adults to reveal any changes to diurnality along their lives. Enclosures containing groups of captively raised frogs were photographed twice with a five-minute interval at each hour of the day. An index of movement was calculated for each group and hour and this procedure was repeated for enclosures containing adults, juveniles, froglets (metamorphosed juveniles) or tadpoles. A preliminary analysis shows that froglets and adults shared a bimodal distribution of activity with peaks soon after sunset and sunrise. Adult frogs were predominantly active soon after sunset, however, while froglets were more active after sunrise. The shift to nocturnality with age in túngara frogs appears to involve swapping of activity between the two peaks of a bimodal pattern fixed in time as opposed to gradually shifting the time of peak activity in a unimodal pattern. Further analysis of the diel activity patterns among the remaining life stages should reveal the full range and mode of change of diel activity in the túngara frog.