What are some tips you can think of for animals to keep cool in the summer? For example, peeing on their feet...

"Hot! Hot!" As all parts of China enter oven or steamer mode, everyone becomes sweaty. One gram of water at 35℃ can evaporate and take away 2426 joules of heat, so using sweat evaporation to dissipate heat is a very effective means.

Sweating seems very common to us, but in nature, animals have all sorts of strange ways to dissipate heat, such as spitting, peeing on their feet... Even insects that are very distantly related to us have the same ability to "sweat" as humans.

Ornate box turtles spit to dissipate heat | Peter Paplanus / Wikimedia Commons

Sweat, panting and drooling

Among animals, humans are unique in terms of how much they sweat. Monkeys only sweat 80 to 90 grams per square meter of skin per hour, but humans can sweat more than 800 grams! Therefore, humans are also unique among mammals in their endurance of walking in high temperatures.

Mammals have two types of sweat glands: apocrine glands and eccrine glands. Apocrine glands are usually scattered all over the body where there is hair, and secrete a sticky mixed liquid containing fat, protein, etc.; eccrine glands are rare and concentrated in the palms of the four feet, and more than 99% of their secretions are water. Some animals, such as horses, use apocrine glands to sweat to dissipate heat. Humans use eccrine glands to sweat, and almost all sweat glands on the body surface are eccrine glands. Apocrine glands are concentrated in the armpits, areola and private parts.

Anatomy of the skin glands. No. 7 (red circle) is the eccrine sweat gland and No. 8 (blue circle) is the apocrine sweat gland | www.hegasy.de

Other animals do not sweat as much as humans, but they use other ways to dissipate heat, such as panting. Panting is the most common way for warm-blooded animals to dissipate heat. The most common thing you see is a dog "wheezing" with its mouth open in the summer. Panting evaporates the water in the upper respiratory tract. The surface of the respiratory tract is rich in blood vessels. The heat carried away by evaporation cools the blood, and blood circulation further cools the whole body.

Dogs stick out their tongues not to sweat, but to increase the contact area between the tongue surface and the air | Pixabay

But panting has its limitations. The surface area of ​​the respiratory tract is limited (dogs stick out their tongues to increase this surface area), and the efficiency of heat loss is also limited. Gas exchange during panting only occurs in the upper respiratory tract. Over time, carbon dioxide in the lungs cannot be released and will accumulate, leading to alkalosis. Moreover, when quadrupeds run fast, their trunks flex and extend forward and backward, hindering the contraction of the diaphragm. Therefore, they cannot pant while expanding and contracting like an accordion.

Another way is to use other water as a supplement to sweat. The red kangaroo (Macropus rufus) has sweat glands but they are not well developed. It only sweats when exercising. When it is hot, it will lick its arms to let saliva replace sweat to take away the heat. The kangaroo's front legs have sparse hair, which makes it easy for water to evaporate, and the surface has a well-developed blood vessel network, which can complete the task of heat exchange.

Kangaroos lick their arms to cool down | Pixabay

Interestingly, the ornate box turtle (Terrapene ornata) will also "foam at the mouth" when hot, spitting out large amounts of saliva, using the evaporation of water in the mouth and respiratory tract to carry away the heat.

Sweating cicada

The Apache cicada (Diceroprocta apache) in the Arizona desert, USA, chooses to emerge in the hottest summer of the year, and chirps to mate during the hottest noon, which can be said to be hanging its life on the frying pan of fire. The temperature in the desert can reach 48 degrees Celsius, and when the cicada's body temperature reaches 45.6 degrees Celsius, it will lose muscle control and fall into the hot sand and be scalded to death.

Apache Cicada | Steven Kurniawidjaja/inaturalist

The benefit of embracing the heat is that it can avoid predators. Spring is cooler, but birds are breeding at this time and have a great demand for food. At noon in summer, birds and bees that prey on cicadas dare not come out to move. The Apache cicada will stop moving when it is extremely hot and hide in the shade of leaves. However, what really makes it shine among insects is its ability to "sweat". There are many small holes on the surface of the chest and abdomen of the Apache cicada, which can drain water and evaporate to cool down. In one hour, it can expel 1/3 of the water in its body.

Small holes on the body of the Apache cicada | NEIL F. HADLEY et al. / J. exp. Biol. (1989)

Water is undoubtedly precious in the desert. But Apache cicadas have an inexhaustible source of water: the sap of the wood of plants, which is almost entirely water. Because the nutritional density of tree sap is too low, cicadas will constantly excrete water (in the summer, if you are under a tree, you may be urinated on by cicadas), and Apache cicadas use it to cool themselves down, which can be regarded as a waste utilization.

Bird: A steady stream of water

Birds don't sweat, so some birds, such as storks and vultures, have taken a different approach to evaporative cooling, excreting on their hairless legs and feet, which are rich in blood vessels, allowing the water to evaporate and take away the heat.

The red-headed vulture (Cathartes aura) usually sprays backwards when defecating, but in hot conditions, it will change to dripping downwards, sprinkling its excrement on its feet. This excrement contains 95% water and only a small amount (5 ml) is produced at a time.

Cool bald guy | Juan Rodolfo Lillo Lobos / J. exp. Biol. (1989)

Although the legs and feet of a vulture are not large and the amount of water sprinkled on them is not much, this "heat sink" is very efficient. Experiments show that if the legs of a red-headed vulture are exposed to room temperature, sprinkled with 2.5 ml of water, and then blown with an electric fan to accelerate evaporation, the body temperature can be maintained stable even if the rest of its body is locked in a high-temperature box at 50°C.

In this regard, the American vulture has another advantage: birds are models of water conservation. The nitrogen metabolite of birds is non-toxic uric acid, while that of mammals is toxic urea. The latter requires a lot of water to dilute and excrete (urine), but birds don't. The red-headed vulture can go more than a year without drinking water, and only gets water from the meat it eats. Because it is good at saving water, when it must excrete to dissipate heat, even if there is no water to drink, it has the saved water to use. In order to dissipate heat, the red-headed vulture can defecate 40 times in 4 hours, which can be described as "a long-lasting trickle of water."

Birds' talent for conserving water also helps them adapt to desert life | Andy Reago & Chrissy McClarren / wikimedia

In addition to excreting, the red-headed vulture can also dissipate heat by panting and stretching its bare neck. It is widely distributed, which is naturally inseparable from its ability to withstand heat. You may think that sweating profusely affects your image, but in fact you should be glad that we are not following the path of birds.

References

[1] Hatch, Daniel E. "Energy conserving and heat dissipating mechanisms of the Turkey Vulture." The Auk (1970): 111-124.

[2] Lieberman, Daniel E. "Human locomotion and heat loss: an evolutionary perspective." Comprehensive Physiology 5.1 (2011): 99-117.

[3] Needham, AD, TJ Dawson, and JRS Hales. "Forelimb blood flow and saliva spreading in the thermoregulation of the red kangaroo, Megaleia rufa." Comparative Biochemistry and Physiology Part A: Physiology 49.3 (1974): 555-565.

[4] Heinrich, Bernd. The hot-blooded insects: strategies and mechanisms of thermoregulation. Springer Science & Business Media, 2013.

Author: Little Wombat

Editor: pee pee shrimp

Title image from Pixabay

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