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On hot summer days, we might hide inside the cool confines of our house. For many flora and fauna, such ...
2020-07-21 09:00:00

Radiate and Drink
Nature’s Heat Survival Strategies

Daniel Mróz – drawing from the archives (no. 503/1954)
Radiate and Drink
Radiate and Drink

To survive heat, lizards dance on hot sand and beetles hunt tiny pieces of fog at dawn. People shelter under trees, and trees under their bark.

Read in 9 minutes

Heaven has broken into two uneven pieces. The higher one is an exhausted, worn-out, faded blue blurred into a beautiful gradient ending in a deep azure over my head. Heat pours down from it. The smaller part, squeezed in right over the grey space spreading out in all directions, is as thick as the sweet local wine. It undulates like crazy, breaking into vibrating frills, the light rays rushing over the horizon. I sit under a spreading cork oak, Quercus suber. Both of us pretend we’re doing well. Me, by telling myself this Portuguese hell is a well-deserved rest from the nearby scientific conference. The oak, in the hopelessness of its motionlessness, becomes even more motionless, freezing the frames of photosynthesis and biochemical cycles and holding its breath, squeezing shut its billions of microscopic mouths. On the back of my head (yes, you talk with a tree by feeling its fixity on your back and the back of your head) I sense the nodes of cork bark, puffed out by years of growth, surprisingly not warm on this inferno of a day. It doesn’t surprise me at all; after all, how could it? Instead, I admire the engineering genius and foresight that, without reading even a single chapter about the secrets of thermodynamics, allowed evolution to get carried away, clothing the oak in a climate-controlled caftan.


The cork oak’s bark is a pure utilitarian design. Silvery bright – because it is optical armour, to reflect the wild, all-boiling Portuguese sun. It’s saturated with a tangle of strange molecules whose names are insignificant (significant only insofar as one of them, suberin, is contained in the Latin name of my oak). They’re so strange and so boring that nobody has even completely drawn them yet; there’s no book that comprehensively describes how their molecular branches run and which precise atoms they blossom into. But we don’t need that; it’s enough that we know these molecules hate water. So we have a wall against light, water, and also heat. Each layer of cork bark is made up of the decades of air enclosed in it. It’s difficult to refrain from wandering off into this air, all of it diligently packed away in cells – each with a different taste and from a different era, breathed out by someone else, snorted in by someone else. Now I’m a biologist, and I analyse the structural genius of oak, which harnessed physics to escape from heat. Thanks to its shapely cork structure, on the most blazing day the oak trunk sits calmly in its pleasant coolness under the bark, calmly growing outward in the cool, and in the cool transporting water and nutrients between the cold roots and the leafy head, heated by the sun. Biological polystyrene, au naturel.

It’s a sweet irony of fate that while taking a break from the noise of a conference on how life adapts to the environment, it was a cork golem that I ended up leaning against. Because heat is one of the oldest oppressors of life that has been attuned through billions of years of evolution to a quite narrow range of temperatures. My oak – and life in general – observes a simple biological mantra when an excess of heat comes. Fleeing from thermal highs, life has just a few options to stay in cosy optimum.

Hide. Don’t touch. Steam and radiate. Drink.

Hide away

The cork oak has clearly bet on the option of introversion. Deep under its thermal armour, it has hidden away what’s most important. It doesn’t have much of a choice – like most land plants. The motionless can’t flee from the sun; they can’t crawl into a cave to wait out the heat, they can’t bury themselves under the earth, abashed by their sensitivity to heat (though not entirely – more about that in a moment). But they can hide inside themselves, under layers of insulating tissue, or get rid of everything that in the heat would only expose them to a loss of precious water. Here, biological correctness would suggest recalling the cactus. Evaporating water, the cactus’s costly leaves changed into needles, and its body puffed out into spherical-cylindrical shapes that minimize (as much as possible) the plant’s surface (each square centimetre is space for potential loss of water vapour) and simultaneously maximize its volume (in the end, additional cubic centimetres of pulp are more storage for precious water). But cactuses seem to be very conservative, if you look at the silvery, scaly fingers of Avonia papyracea. This marvel, looking like dragon’s claws grown in a pot under a covering of silver scales, went a step further than the cactus. Its leaves are tiny balls, hidden under scaly bracts that are clear as a shop window. Not only are they just as economical as a cactus’s needles, they’re also cleverly tucked under filigree shields protecting them against the heat and glare of the African skies.

There are strategies that are captivating in their totality. The ‘Baby Toes’ (Fenestraria rhopalophylla) succulent, for example, digs itself into desert sand (told you!) and pokes just the tips of its leaves out over the surface. Each leaf is a cylinder of green pulp, surrounding a clear, water-storing pith. Under the surface of the sand, Baby Toes avoid overheating, simultaneously letting light into the buried leaves through a ‘lens’ that sticks up out of the sand. In turn, the queen of the night (Epiphyllum oxypetalum), a Mexican cactus, hides in time. Everything that’s important – blossoming and sprinkling its pollen – the queen carefully hides in the darkness of the cool night. And it blooms only one night; in the morning the cups of its flowers are just twisted, fertilized coils of seized opportunities, calmly growing into fruit.

It’s infinitely easier for animals. They have mobility – this marvellous ability to change their spatial coordinates – and the derivatives of mobility: seeking, hiding, fleeing. Avoiding overheating, animals roam where the sun can’t reach. Like the legless, eyeless skinks of the Namib desert, reptiles that dig into the surface layer of the gravel, or the golden moles, South African mammals that flit about just under the surface of the heated desert sand.

Don’t touch

This recommendation is harder to achieve, because it requires self-discipline, and sometimes also acrobatic ability, and acrobatic fitness, and an exceptional sense of timing. The Namibian dune lizard Meroles anchietae, to avoid overheating its body, dances rhythmically on the hot sand. Raising each leg in turn, it reduces the average time any given limb touches the desert. Namibian snakes and horned rattlesnakes from the Mojave Desert use a similar trick: when moving, they don’t slide smoothly and soundlessly through the yielding sand. Instead, they transform their bodies into sinusoid curves, throwing them forward. They leave a trail that’s a series of horizontal lines, barely marked by the brushes of sand. Because that’s another idea: throwing its body forward, the snake curves it ideally symmetrically and repetitively in a wavy figure, touching the heated sand in two places, reducing contact with the high temperature to the absolute minimum necessary. Efficient, geometrical, two-point acrobatics.

‘Don’t touch’ can apply not only to hot sand, heated stones, sizzling rocks. You can also not touch more quietly, discreetly, for example a stream of heated solar photons. Plants are monuments to motionlessness – or so we think. All because of an inhuman time scale, because of the immersion of their motor skills into completely different speeds and visibilities, far away from our impatience and demand for an immediate effect. I became convinced of this recently at a certain Australian university campus. It took me a while to realize what was so unusual about the eucalyptuses that surround the buildings. Finally, it hit me: there’s almost no shade! Though they’re covered with thick, waxy leaves, even at high noon (especially at high noon!) the eucalypts allowed quite a lot of sunlight to pass through their crowns. If we did a time-lapse film of one of these branches, we’d see the grotesque ballet of the eucalyptus leaves. Suddenly invigorated, trembling in incomprehensible convulsions (and after all, these are only lazy minutes and hours in the real world), turning their faces in sync to cut along the ray of the sun and expose as little of their surface as possible. We know this kind of sun avoidance, leaves turning their edges toward the heat-bearing light, from other trees too. The black locust, popular in parks, also does this; on a hot day it can suddenly wilt, dropping its complex leaves and orienting their surfaces vertically. This is also the case with the Mediterranean compass plant, which grows so as to avoid the north-south compass axis as much as possible, and thus to absorb as little light as possible at noon.

Steam and radiate

Heat is a very interesting currency. It’s easy to lose if we only know how to waste it efficiently, or we know what costs most in this currency. Pour a little alcohol onto your hand, and right away you’ll feel a pleasant coolness. That’s the alcohol molecules stealing your heat, breaking away from the liquid and flying away as a gas. Evaporation is an effective thief of heat, and some molecules steal particularly large amounts of it; water, for example, is particularly greedy. Kangaroos know this. When hiding in the shade of a tree isn’t enough, they often resort to a thermodynamic ruse: they lick their forearms and shins profusely, so the evaporating saliva will cool their bodies. In fact, almost all land vertebrates use water. Humans sweat. Birds, predatory mammals and rodents pant, throwing off incredible amounts of heated water vapour. In the desert, fennec foxes circulate enormous amounts of blood through their huge, flat ears; it gives off heat, cooling down, and returns to the rest of the body to cool it.

Radiating heat out into your surroundings is always a good method when you have enough bare skin. It doesn’t always turn out to be pleasant and aesthetic. Turkey vultures – like all other birds, in fact – can’t sweat. To achieve the desired effect of evaporating water from their bodies, they use their thick, whitish urine to moisten the bare skin of their legs. When it’s very hot, they can lower their body temperature this way, even if the aesthetic effect is far from ideal.


At the end of the mantra comes something obvious – at least for us humans. Water: the basis of everything living. In its coolness, all reactions, genes, impulses and thoughts are dissolved, and thanks to its aforementioned voracity for heat (known as a high specific heat), water is the perfect coolant. But the problem with water is it’s often not there. It would be silly to dilute our inquiry here with simple drinking of water. It’s more interesting when survival in the heat requires water that isn’t available in its liquid form. I read once that the uniqueness of our planet results from its thermodynamic extravagance. Because it’s only in this corner of the universe, on a cosmically microscopic crumb of rock dust, that water is available in all its forms. Flying through the frozen cosmos, the Earth looks on the sun from distances and at angles that allow it to adjust its temperature to all flavours and forms of water. We use the liquid with the greatest of ease; the kind solidified into a crystal is unavailable in hotter places. Invisible water is created, lonely molecules of water chaotically bouncing through the air, water ever-present but cruel, because it’s hard to drink it that way. Evolution would be exceptionally ineffective if it couldn’t use that kind of water.

Thus the Moloch horridus, the horny devil, a lizard: crossing the Australian desert, it hunts dew spots, where water vapour is on the point of becoming liquid. From the fog it sucks out water particles, which stick to its prickly scales, run into small grooves in its skin and are transported to the corners of its mouth. There are also the Namibian ‘fog beetles’, Stenocara gracilipes. Every day, they travel to the tops of desert dunes, where water vapour compressed over their edges condenses, covering the beetles with a coat of mini-lenses, droplets budding straight out of the air. Marvellously effective – and surreal, because, so unaware of the genius of their most important daily ritual, every day at dawn the beetles chill out on the ridges of the red dunes, believing or disbelieving in thermodynamics, the pressure of vapour over solutions, and all the laws of gasses. How did wandering evolution stumble on this idea, and all its other ideas?

I’d swear that the oak snorted, leaning on my uncomfortably movable back. The cool inside calculates, and now knows for certain that in the evening it will exhale broadly, opening its billions of microscopic mouths.


Translated from the Polish by Nathaniel Espino

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Szymon Drobniak

is an evolutionary biologist, researcher on colour, ornithologist, illustrator and author. A passionate fan of diagrams, cross-sections and pea soup. In his spare time, he collects roots (not the mathematical kind).