Organisms can live at high altitude, either on land, in water, or while flying. Decreased oxygen availability and decreased temperature make life at such altitudes challenging, though many species have been successfully adapted via considerable physiological changes. As opposed to short-term acclimatisation (immediate physiological response to changing environment), high-altitude adaptation means irreversible, evolved physiological responses to high-altitude environments, associated with heritable behavioural and genetic changes. Among vertebrates, only few mammals (such as yaks, ibexes, Tibetan gazelles, vicunas, llamas, mountain goats, etc.) and certain birds are known to have completely adapted to high-altitude environments.
Human populations such as some Tibetans, South Americans and Ethiopians live in the otherwise uninhabitable high mountains of the Himalayas, Andes and Ethiopian Highlands respectively. The adaptation of humans to high altitude is an example of natural selection in action.
High-altitude adaptations provide examples of convergent evolution, with adaptations occurring simultaneously on three continents. Tibetan humans and Tibetan domestic dogs share a genetic mutation in EPAS1, but it has not been seen in Andean humans.
Invertebrates
Tardigrades live over the entire world, including the high Himalayas. Tardigrades are also able to survive temperatures of close to absolute zero (), temperatures as high as , radiation that would kill other animals, and almost a decade without water. Since 2007, tardigrades have also returned alive from studies in which they have been exposed to the vacuum of outer space in low Earth orbit.
Other invertebrates with high-altitude habitats are Euophrys omnisuperstes, a spider that lives in the Himalaya range at altitudes of up to ; it feeds on stray insects that are blown up the mountain by the wind. The springtail Hypogastrura nivicola (one of several insects called snow fleas) also lives in the Himalayas. It is active in the dead of winter, its blood containing a compound similar to antifreeze. Some allow themselves to become dehydrated instead, preventing the formation of ice crystals within their body.
Insects can fly and kite at very high altitude. Flies are common in the Himalaya up to . Bumble bees were discovered on Mount Everest at more than above sea level. In subsequent tests, bumblebees were still able to fly in a flight chamber which recreated the thinner air of .
Ballooning is a term used for the mechanical kiting that many spiders, especially small species such as Erigone atra, as well as certain mites and some caterpillars use to disperse through the air. Some spiders have been detected in atmospheric data balloons collecting air samples at slightly less than 5 km (16000 ft) above sea level. It is the most common way for spiders to pioneer isolated islands and mountaintops.
Fish
Fish at high altitudes have a lower metabolic rate, as has been shown in highland westslope cutthroat trout when compared to introduced lowland rainbow trout in the Oldman River basin. There is also a general trend of smaller body sizes and lower species richness at high altitudes observed in aquatic invertebrates, likely due to lower oxygen partial pressures. These factors may decrease productivity in high altitude habitats, meaning there will be less energy available for consumption, growth, and activity, which provides an advantage to fish with lower metabolic demands.
The naked carp from Lake Qinghai, like other members of the carp family, can use gill remodelling to increase oxygen uptake in hypoxic environments. The response of naked carp to cold and low-oxygen conditions seem to be at least partly mediated by hypoxia-inducible factor 1 (HIF-1). It is unclear whether this is a common characteristic in other high altitude dwelling fish or if gill remodelling and HIF-1 use for cold adaptation are limited to carp.