Abstract :
[en] For amphibians, optimal size at metamorphosis is determined by a trade-off between opportunities for growth and risks of mortality in aquatic and terrestrial habitats, and many species show a high degree of plasticity in time and size at metamorphosis that allow them to adapt to unpredictable environments. We used as model a neotropical anuran species, the Pacific horned frog, Ceratophrys stolzmanni, that inhabits the Tumbesian dry forests of southwestern Ecuador, and lays its eggs in lentic habitats of variable size and unpredictable duration. We investigated (i) how tadpoles time their metamorphosis in order to escape a rapidly drying aquatic habitat, and (ii) the consequences of metamorphosis plasticity on the immediate and mid-term fitness of individuals. We conducted two experiments; in the first one we exposed 36 tadpoles to one of three water levels treatments: constant high, decreasing and constant low. Tadpoles that were raised in decreasing or low water level metamorphosed on average 13% and 10% faster than the ones from constant high water level, but at a lower body size and mass. Growth rates were among the highest recorded for tadpoles and individuals from the constant high water level treatment grew at a much faster rate than the others.
In a second experiment, we sampled 72 freshly metamorphosed (Gosner stage 45) C. stolzmanni individuals of assorted sizes (SVL between 23.8 and 47.9 mm, body weight between 1.2 and 11.9 g). From this spectrum of sizes, we selected the 21 largest and 22 smallest individuals and evaluated the effects of body size at metamorphosis on trophic and locomotory performance parameters. We determined their growth rate and survival over the course of an activity season (i.e., two months). Our results show that bigger size at metamorphosis is correlated with better survival chances and performance. Larger individuals were more mobile, had bigger gape width and better survival rates (95% compared to 64% for small individuals). Growth rates were not correlated with initial size, and small individuals were capable of increasing their size at a higher rate. This indicates that, although size at metamorphosis has an immediate impact on fitness, smaller individuals are able to compensate with an increased growth rate.
