A recently
released article in Scientific Reports
details an extreme amphibian adaptation. In a study entitled “The Siberian wood
frog survives for months underwater without oxygen,” Berman and colleagues explore
hypoxic resistance in the Siberian wood frog, Rana amurensis. Contrary to popular belief, Berman et al. (2019) demonstrates
that the distribution of R. amurensis
in northern Asia is not necessarily limited by oxygen depletion in waterbodies
that do not freeze. Like other cold-tolerant ranid species, R. amurensis can survive in cold
climates like Siberia’s by spending the winter months (overwintering) underwater
where it does not freeze. Since R.
amurensis can also overwinter on land, many scientists believed that this species
was more cold tolerant that others, allowing it to occupy a large geographical range
in northern Asia. However, other studies have shown that R. amurensis does not tolerate cold temperatures better than other
ranid species. Perhaps the secret to its large distribution then, as this study
investigates, is its ability to survive in severely hypoxic, if not anoxic,
conditions. Berman et al. (2019) demonstrates that waterbodies in this species’
habitat range are indeed extremely hypoxic, reporting oxygen levels of 0.2-2.1
mg/L. Further, the research team exposed R.
amurensis individuals to highly reduced oxygen levels of 0.1-0.2 mg/L in sealed
containers. Frogs in these containers were able to survive up to 97 days, with reduction
in activity only occurring toward the end of the experiment. Two individuals
were also transferred from one of these containers to another with the same reduced
oxygen level, but also with a highly reduced volume of water on day 54. These individuals
survived for an additional 24-25 days. These results suggest individuals of R. amurensis can highly reduce their
oxygen consumption while still remaining active and can potentially switch to
anaerobic respiration.
Many aquatic turtles also
demonstrate hypoxic resistance, but such a characteristic in Amphibia is little
explored. This paper reports the first known case of extreme hypoxic resistance
in amphibians that overwinter underwater. Such research lends itself to a number
of subsequent questions. Do any other amphibian species exhibit extreme hypoxic
resistance? What physiological mechanism allows R. amurensis to survive in these conditions? Certainly these questions
and more merit further research into the extreme adaptations of amphibians. Nonetheless,
this study sheds light on the many amphibian anomalies scientists have yet to
discover and explore.
Berman, D. I., Bulakhova,
N. A., & Meshcheryakova, E. N. (2019). The Siberian wood frog survives for
months underwater without oxygen. Scientific
Reports, 9, https://doi.org/10.1038/s41598-018-31974-6
2 comments:
When I lived in Alaska, I was surprised and saddened when I found a frog that seemed frozen solid and dead in the backyard under the snow. A frog, in Alaska? Later I learned that Rana sylvatica, the wood frog, does indeed live even north of the Arctic Circle, and goes to physiological extremes generating extremely high glucose and urea levels to raise the intracellular osmolality. While intra- and extra-vascular fluid spaces froze, the high osmolality prevents cells themselves from freezing and bursting. In this article https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4331536/ blood urea levels up to 186.9 ± 12.4 μmol ml-1 were tolerated. I don't know what a "normal" urea level in a frog is, but in humans it is <20. While R. amurensis has evolved incredible hypoxia tolerance to survive in low oxygen waters, R. sylvatica overwinters under the leaf litter, freezing up to 60% of its tissues, stopping breathing and heartbeat. Amphibean adaptations are amazing.
Very "cool" article! Interesting contrast to the freeze tolerance of L. sylvatica!
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