Abstract
Few of the amphibian species that occur in the Subarctic and in mountains are adapted to low sub-zero temperatures; most of these species overwinter underwater. It is believed that the distribution of the species that overwinter underwater can be limited by the low oxygen levels in waterbodies covered with ice. We show that the colonisation of the coldest areas of Northern Asia (to 71°N) by the Siberian wood frog (Rana amurensis) was facilitated by a unique adaptation, the ability to survive extreme hypoxia — and probably anoxia — in waterbodies during overwintering. The oxygen content in the overwintering waterbodies that we have studied in different parts of the range of this species fell to 0.2–0.7 mg/L without causing any large-scale mortality among the frogs. In laboratory experiments the R. amurensis survived for up to 97 days in hermetically sealed containers with water that contained less than 0.2 mg/L oxygen at temperatures of 2–3 °C, retaining the ability to respond to external stimuli. An earlier study of a broad range of frog species has shown that very few of them can survive even brief (up to 5–7 days) exposure to oxygen-free water. The revealed adaptation to prolonged extreme hypoxia is the first known case of this kind among amphibians overwintering in water.
Highlights
The adaptive strategies of northern amphibians, which allow them to survive conditions atypical of the amphibian class, have long attracted the attention of researchers[1,2,3,4,5,6]
Since there are data indicating that R. amurensis can overwinter underwater and on land, for a long time it was mistakenly assumed that this species, flourishing in areas with extremely cold winters, should be one of the most cold-tolerant among amphibians[9]
The range of R. amurensis includes large areas where oxygen is depleted in winter in most waterbodies[13,15,23,24]
Summary
The adaptive strategies of northern amphibians, which allow them to survive conditions atypical of the amphibian class, have long attracted the attention of researchers[1,2,3,4,5,6]. Measurements of oxygen levels in overwintering waterbodies of Rana amurensis both in northern and southern parts of its range have unequivocally shown that this species does overwinter underwater at extremely low concentrations of oxygen (Table 1).
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