Blue Planet II

The toughest animals on Earth are not what you'd expect

By Ella Davies

The ultimate survivors of the animal kingdom.

If you’re looking to the animal kingdom for tough customers, you need look no further than tardigrades. These eight-legged animals can survive extremes of heat, cold – even the vacuum of space.

Tardigrade front on
The remarkable resilience of tardigrades is due to their ability to survive without water. © SEBASTIAN KAULITZKI/SCIENCE PHOTO LIBRARY | Getty
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Blue Planet II

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Wherever they are found – the Himalayas, ocean abysses, volcanic mud or chilling in Antarctica – the remarkable resilience of tardigrades is due to their ability to survive without water. Under extreme stress, the animals enter a state known as ‘cryptobiosis’, dehydrating themselves and protecting their cells with special proteins and sugars. The aquatic species are revived by water, which combined with their podgy appearance has earned them the nickname ‘water bears’.

But before you rename your sports team in honour of these critters, it’s worth mentioning they measure less than 1mm long. If you’re looking for more than a ‘tiny but mighty’ reputation, read on for a round-up of some of the other species showing their mettle in extreme circumstances.

Dancing yeti crabs
Dancing yeti crabs featured in the Blue Planet II Prequel. © BBC

Toughened to temperatures

The coldest water in the world is found below Antarctic ice shelves while elsewhere in the Southern Ocean boiling water spews up from cracks in the seabed. In such a contrary environment you’d expect some oddities but even the wildest imagination is unlikely to come up with the yeti crabs (Kiwa S.).

Named for the abominable appearance of their long hairy arms, these blind crustaceans are adapted to endure crushing pressure and no sunlight at depths of up to 2,300 ft (700m) below sea level. Their survival is dependent on the hydrothermal vents they live near, where water as hot as 400C boils up from beneath the Earth’s surface, bringing mineral deposits with it. Bacteria thrive around the vents, converting the minerals to energy. In turn, the crabs are sustained by the bacteria, but it’s a delicate balance to stay in the sweet spot between the boiling vent water and lethally cold ocean.

Featured in Blue Planet II, scientists observed the species informally known as ‘Hoff' crabs (Kiwa tyleri) (thanks to their hairy chests resembling that of Baywatch star David Hasselhoff) fighting for position around the chimney-like structures created where the fiercely hot water deposits minerals from beneath the Earth’s surface.

    Stacks of 'Hoff' crabs in the hydrothermal vents of the Southern Ocean
    At recently discovered hydrothermal vents in the Southern Ocean, great stacks of 'Hoff' crabs farm bacteria all over their bodies. © Jon Copley

    Living the high life

    From the seabed to mountain tops, you’d imagine there would be a very different set of challenges to overcome. But temperature is just as much a factor at altitude, where mountain peaks can be cold enough for exposed flesh to freeze. That’s why yaks (Bos mutus) sport such thick hides; a double thickness of woolly undercoat beneath their long hair helps to keep essential areas such as the chest insulated. Even with their impressive outerwear, yaks cannot survive beyond around 18,000 ft (5,500m) because the higher you go, the thinner the air becomes.

    A white yak stands in grassland
    Yaks may be tough, but geese are tougher. © Xia Yuan | Getty

    Low air pressure means less oxygen, so species at extreme altitude need physiological advantages to help them make the most of what’s available. Big lungs, specialised muscles and an abundance of blood vessels that transport oxygen to essential tissues are the secrets of success for arguably our highest-flying species: the bar headed goose (Anser indicus).

    Birds are known for some extraordinary feats during their migrations, foremost among them are the geese that cross the Himalayas annually. The birds have been recorded cresting Mount Everest at 30,000ft high, though they usually take a slightly ‘easier’ route through mountain passes that would still leave human mountaineers literally breathless.

    What’s truly extraordinary is that the birds can climb between 4,000 and 6,000 m in just 7-8 hours thanks to their adaptations, and scientists suggest they undertake this athletic feat without any prior training.

    A small flock of bar headed geese
    Bar headed geese are the highest-flying species. © Tariq Sulemani | Getty

    Drying times

    We know that water is an essential element for most of life on Earth, so species that survive in the world’s most arid environments really have their work cut out for them. Of all the desert animals, the camel is easily the most iconic. Whether they have one hump (Camelus dromedarius) or two (C. bactrianus), they can take on temperatures of over 40C and endure water losses greater than 25% of their total body weight.

    An adult and young camel sit in the desert
    Camels iconic and expert survivors. © BGS Photography | Getty

    To set the record straight, camel humps are not used for water storage – they are fatty deposits. While fat can be metabolised into water, animals in dry conditions lose too much moisture through respiration trying to get enough oxygen for the process to make it worthwhile. Instead, the fats can be used for nutrition when food is scarce. They also protect camels from the sun as the fatty tissue is slow to conduct heat and by locating most of their body fat on their back the animals carry less insulation elsewhere, helping to keep them cool. So, think of the humps as heat-shielding picnic parasols rather than flasks.

    While their humps are hard to ignore, some of camels’ best features are internal. Scientists investigating the evolutionary background of camels have identified key physiological mechanisms that represent adaptations to the desert environment.

    Due to highly efficient kidneys and intestines, camels lose very little water through waste. They pee very sporadically; their urine is super-concentrated, and their droppings are exceptionally dry. Their red blood cells are also specially adapted to cope with fluctuating hydration: they can expand to 240% of their initial volume, whereas other species’ will burst beyond 150%. Camels’ red blood cells are also smaller and oval shaped, meaning they can flow more easily even when the blood is thickened through extreme dehydration.

    For our most indefatigable species then, the secrets of survival in hostile conditions are literally in the blood.

    Featured image by Tariq Sulemani | Getty

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