Hemoglobin content in a high-altitude frog: adaptive evolution in frogs under oxygen-deficient conditions? Evolution of hemoglobin has been widely investigated in many organisms. In particular, the high altitude amphibians are famous for their ability to live in hypoxic environments for extended periods of time. In the present study, we examined the adaptive evolution of hemoglobin content in the high altitude frog Glandirana emeljanovi. This species resides at the highest altitudes on the Qinghai-Tibetan Plateau and is distributed from about 3,000 m to over 4,200 m in altitude, making it one of the most resistant amphibian species in terms of morphology and physiology against the hypoxic environment. We compared two populations in terms of hemoglobin concentrations of embryos and adults at two altitudes of 3,000 m and 3,400 m. There was a significant increase in the concentration of hemoglobin in the embryo stage with elevation at a given altitude. In adults, both the concentration of hemoglobin in hemolymph and muscle showed similar elevation pattern with elevation at a given altitude. Also, elevation from a lowland environment to a highland environment led to a significant increase in hemoglobin concentration and adult body mass, which further suggests that the increased hemoglobin concentration is the result of evolution at the genetic level. In addition, the present study provides the first evidence of adaptation to highland hypoxia in a high altitude frog species. Our results provide important insight into how animal taxa with different evolutionary history are affected by hypoxia.