Tolerance to Low pH in a Population of Moor Frogs, Rana arvalis, from an Acid and a Neutral Environment: A Possible Case of Rapid Evolutionary Response to Acidification

Abstract

A possible evolutionary process towards increased adaptation to acid conditions was examined in a population of moor frogs, Rana arvalis Nilsson, exposed to critically low pH levels for approximately 15 generations. Rana arvalis eggs from the acid locality studied were not significantly larger than those from a neutral locality, but egg size variation was significantly smaller. No embryonic or early larval survival variables were correlated with egg size, and neither size at, nor time to metamorphosis was correlated with egg size. In acid water, embryonic and larval survival variables were all significantly higher in progeny from the acid locality, and larvae from this population metamorphosed earlier and at a larger size than larvae originating from a neutral locality. All premetamorphic stages of Rana arvalis from the acid locality had a higher acid tolerance than the population from a neutral environment. Thus, an adaptation to acid conditions may have taken place in the moor frog population exposed to increasingly low pH breeding water during a period of 30 to 40 yr. Limb deformations at low pH and high growth rate could not be verified in froglets of this species.