Larval over-wintering: plasticity in the timing of life-history events in the common frog

In ectothermic animals living in temperate regions, winter is a critical time. During early ontogeny, the stage or size an individual reaches at the onset of winter can have a significant effect on its survival and other life-history characteristics. Generally, temperate amphibian larvae complete metamorphosis in the summer and spend their first winter as juveniles. However, some anuran amphibians show variation in this aspect of their life history, with some individuals remaining as aquatic larvae during winter. We investigated growth and development during the larval period of Rana temporaria at a field where over-wintering has been recorded. In the laboratory, we investigated whether larval over-wintering could be induced at temperatures equivalent to those recorded in the field pond; this was done at both high and low food availability. Larvae within the field pond showed a bimodal distribution in developmental stage as early as July, when temperatures were still increasing, separating individuals that would metamorphose and over-winter as juveniles from those that would spend the winter as larvae. Individuals that over-wintered as larvae remained at a relatively undeveloped larval stage throughout the summer and autumn. The decision on whether to over-winter as a larva appears to be made relatively early in the season. Those individuals that adopt this slow developmental trajectory may benefit in being able to metamorphose at a larger size in Spring. In the laboratory, mean temperature and food availability affected development and growth, but did not affect the life-history stage at which individuals over-wintered. Our results suggest that there are other factors, in addition to temperature and food availability that contribute to the observed plasticity in development pattern and over-wintering strategy.