Pallas’ Spadefoot Toad, Pelobates vespertinus (Pallas 1771) (Amphibia, Pelobatidae), the Second Amphibian Species to Tolerate Extreme Hypoxia

Pallas’ spadefoot toad is a Eurasian species of amphibian that winters for about half the year at a depth to 2 m below the level of soil freezing, since it does not survive negative temperatures. However, digging deep into the soil can cause oxygen deficiency. The minimum oxygen level sufficient for the long-term existence of Pelobates vespertinus and the lethal oxygen content in the air were revealed, and the metabolic pathways were evaluated under the conditions of normoxia and hypoxia in laboratory experiments. In addition, 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 the oxygen content in soils compared to atmospheric oxygen for more than two months, thereby remaining quite active. When the concentration is decreased by 20 times (up to 1%), a state of coma occurs, which is reversible if it lasts no longer 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 wood frog (Rana amurensis), that can tolerate the complete absence of oxygen for a long time (several months). Compared to its body weight, the spadefoot has small storage organs (fat bodies and liver) and a low content of reserve substances (lipids and glycogen) in its tissues, which are usually consumed at a low rate in normoxia at 3°C. During hypoxia, lipid utilization 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 toad 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 of wintering areas with loose soils by this species 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 this spadefoot toad (and possibly other species of the spadefoot toad genus, as well as burrowing toads) as a model for exploring adaptations to air oxygen deficiency.