Organisms at high altitude

Alpine chough in flight

Organisms can live at high altitude, either on land, or while flying. Decreased oxygen availability and decreased temperature make life at high altitude challenging. Despite these environmental conditions, many species have been successfully adapted at high altitudes. Animals have developed physiological adaptations to enhance oxygen uptake and delivery to tissues which can be used to sustain metabolism. The strategies used by animals to adapt to high altitude depend on their morphology and phylogeny.

Land organisms

Plants

There is a moss that grows at 6480 m (21260 ft) on Mount Everest.^[1] It may be the highest altitude plant species.^[1]

Invertebrates

Tardigrades occur over the entire world, including the high Himalayas.^[2] Tardigrades are also able to survive temperatures of close to absolute zero (−273 °C (−459 °F)),^[3] temperatures as high as 151 °C (304 °F), 1,000 times more radiation than other animals,^[4] and almost a decade without water.^[5] 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.^[6][7]

Fish

Naked carp in Lake Qinghai at 3,205 m (10,515 ft)

Fish at high altitudes have a lower metabolic rate, as has been shown in highland westslope cutthroat trout compared to introduced lowland rainbow trout in the Oldman River basin.^[8] 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.^[9][10][11] 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.^[8]

The naked carp from Lake Qinghai, like other members of the carp family, can use gill remodelling to increase oxygen uptake in hypoxia.^[12] 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).^[13] 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.

Mammals

The Himalayan pika lives at altitudes up to 4,200 m (13,800 ft)

Small mammals at high altitude face the challenges of maintaining body heat in cold temperatures, due to their large volume-to-surface area ratio. As oxygen is used as a source of metabolic heat production, the hypobaric hypoxia at high altitudes is problematic. To convert fats to energy in the form of ATP, more oxygen is required than to convert the same amount of carbohydrates. The reason they use fats is believed to be because they have it in large stores, but also means that they must eat more or they will begin to lose weight.

A number of rodents live at high altitude, including deer mice, guinea pigs, and rats. A number of mechanisms help them survive these harsh conditions, including altered genetics of the hemoglobin gene in guinea pigs and deer mice.^[14][15] Deer mice use a high percentage of fats as metabolic fuel at high altitude to retain carbohydrates for small burst of energy.^[16]

Other physiological changes that occur in rodents at high altitude include increased breathing rate^[17] and altered morphology of the lungs and heart allowing more efficient gas exchange and delivery. Lungs of high altitude mice are larger, with more capillaries,^[18] and hearts of mice and rats at high altitude have a heavier right ventricle,^[19][20] which pumps blood to the lungs.

At high altitudes, some rodents even shift their thermal neutral zone so they may maintain normal basal metabolic rate at colder temperatures.^[21]

Flying animals

Birds

The Rüppell's vulture can fly up to 11.2 km (7.0 mi) above sea level

Birds have been especially successful at living at high altitudes.^[22] In general, birds have physiological features that are advantageous for high-altitude flight. The respiratory system of birds moves oxygen across the pulmonary surface during both inhalation and exhalation, making it more efficient than that of mammals.^[23] In addition, the air circulates in one direction through the parabronchioles in the lungs. Parabronchioles are oriented perpendicular to the pulmonary arteries, forming a cross-current gas exchanger. This arrangement allows for more oxygen to be extracted compared to mammalian concurrent gas exchange; as oxygen diffuses down its concentration gradient and the air gradually becomes more deoxygenated, the pulmonary arteries are still able to extract oxygen.^[24] Birds also have a high capacity for oxygen delivery to the tissues because they have larger hearts and cardiac stroke volume (mL / min) compared to mammals of similar body size.^[25] Additionally, they have an increased vascularization in flight muscle due to increased branching of capillaries and small muscle fibres (which increases surface-area-to-volume ratio).^[26] These two features facilitate oxygen diffusion from the blood to muscle, allowing flight to be sustained during environmental hypoxia. Bird's hearts and brains, which are very sensitive to arterial hypoxia, are more vascularized compared to mammals.^[27] The bar-headed goose (Anser indicus) is an iconic high flyer that surmounts the Himalayas during migration,^[28] and serves as a model system for derived physiological adaptations for high-altitude flight. Rüppell's vultures, bar-headed geese, whooper swans, alpine chough, and common cranes all have flown more than 8 km above sea level.

Invertebrates

Insects can fly and kite at very high altitude. In 2008, a colony of bumble bees was discovered on Mount Everest at more than 5,600 metres above sea level, the highest known altitude for an insect. In subsequent tests some of the bees were still able to fly in a flight chamber which recreated the thinner air of 9,000 metres.^[29]

Ballooning is a term used for the mechanical kiting^[30][31] that many spiders, especially small species,^[32] 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.^[33] It is the most common way for spiders to invade isolated islands and mountaintops.^[34][35]

Space flight

Before human spaceflight various animals were launched into space, including monkeys, dogs, and insects, so that scientists could investigate the biological effects of space travel. The United States launched flights containing primate cargo primarily between 1948-1961 with one flight in 1969 and one in 1985. France launched two monkey-carrying flights in 1967. The Soviet Union and Russia launched monkeys between 1983 and 1996.

During the 1950s and 1960s the Soviet space program used a number of dogs for sub-orbital and orbital space flights. Most survived and the few that died were lost mostly through technical failures.

Later, animals and other organisms were also flown to investigate various biological processes and the effects microgravity and spaceflight might have on them. Bioastronautics is an area of bioengineering research which spans the study and support of life in space. Certain functions of organisms are mediated by gravity, such as gravitropism in plant roots, while metabolic energy normally expended in overcoming the force of gravity remains available for other functions. This may take the form of accelerated growth.

In May 2011, tardigrades and other extremophiles were sent into orbit on a Space Shuttle.^[36][37][38]

See also

• Aeroplankton
• Altitude sickness
• Effects of high altitude on humans
• Health threat from cosmic rays
• List of microorganisms tested in outer space
• Skylab Medical Experiment Altitude Test

References