PALLAS’ SPADEFOOT, <i>PELOBATES VESPERTINUS</i> (PALLAS 1771) (AMPHIBIA, PELOBATIDAE), THE SECOND AMPHIBIAN SPECIES TO TOLERATE EXTREME HYPOXIA

Abstract

Pallas’ spadefoot is a Eurasian species of amphibian that winters for about half a year at a depth to 200 cm, always below the level of freezing, since it does not survive negative temperatures. However, significantly digging into the soil can cause oxygen deficiency. The minimum sufficient for a long-term existence of Pelobates vespertinus and the lethal oxygen content in the air were revealed, as well as metabolic pathways were evaluated under the conditions of normoxia and hypoxia in laboratory experiments. Beside this, the seasonal dynamics of temperatures and oxygen concentrations in sandy loam soils at different depths were determined in a typical wintering biotope of the species (the Oka River valley). Animals have been found to be capable of withstanding a tenfold decrease in oxygen content in soils compared to atmospheric oxygen for more than two months, thereby remaining quite active. When the concentration is decrease 20 times (up to 1%), a state of coma occurs, which is reversible if it lasts not more than a day. This shows a high degree of resistance of the species to stress resulting from reoxygenation and staying in an environment with variable oxygen levels. Thus, P. vespertinus is the second amphibian species following the Siberian frog (Rana amurensis), which can tolerate its complete absence for a long time (several months). Compared to body weight, the spadefoot has small storage organs (fat bodies and liver) and a low content of reserve substances (lipids and glycogen) in tissues, which are usually consumed at a low rate in normoxia at 3°C. During hypoxia, lipid utilisation is suspended and glycogen consumption is increased, indicating a shift from aerobic to predominantly anaerobic metabolism. The cumulative effect of reduced metabolism due to low temperature (3°C) and the activation of glycolysis due to the lack of oxygen provides a high level of resistance to hypoxia, distinguishing Pallas’ spadefoot from other swarming amphibian species studied. The oxygen content in the ground air of typical places at wintering depths (in sandy massifs) is shown to coincide with the atmospheric oxygen. The ubiquitous choice by the species of wintering areas with loose soils is assumed to be associated not with their excellent aeration, but rather with the ease of digging. The results obtained seem to be promising for using the study spadefoot (and possibly other species of the spadefoot genus, as well as burrowing toads) as model for exploring the adaptations to air oxygen deficiency.