The Herpetological Journal

pp.21-28

Authors: Jon Loman

Abstract: Do moor frog (Rana arvalis) and common frog (R. temporaria) tadpoles increase developmental rate if there is a risk of their pond drying up before metamorphosis? To study this, I performed an experiment designed to mimic natural conditions in many drying ponds. The number of tadpoles per tank was constant during the experiment but the water level was lowered in experimental tanks so that crowding increased. Experimental tadpoles grew and developed more slowly than control tadpoles that were in constant water volume. Also, metamorphosis was delayed (i .e. a smaller proportion had metamorphosed when the experiment was concluded on 1 August) and the metamorphs were smaller. I conclude that, due to crowding, the tadpoles in this experiment were not able to speed up development rate adaptively. Performance of the tadpoles in the experiment was compared to that of R. temporaria tadpoles in the field. These lived in a pond where desiccation resulted in division of the water body into a small pool and a large pool. The small pool dried out completely before the rest of the pond. Tadpoles in this pool were smaller and had relatively smaller hind legs, suggesting slower development. This pattern confirms the result of the experiment, supporting my suggestion that the experimental set-up mimicked many natural situations. Of particular interest is the fact that other studies - carried out both in the same geographical area and elsewhere - have shown R. temporaria to have the ability to respond adaptively to pond drying. The fact that it did not do so in this particular experiment, as well as in the field pond studied here, shows that care must be exercised when extrapolating from one study to the properties of a species. Different conditions, both in the field and in experiments, may well give different responses.

Keywords: amphibian development, competition, frog tadpoles, drying ponds