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What is torpor and how exactly does it differ from hibernation and aestivation?

When speaking about certain animals and the way they get through the cold months of winter, many of us are quick to speak of hibernation—the condition or period of an animal or plant spending the winter in a “dormant state.” But what we fail to realise is that in reality, not that many animals truly hibernate. Instead, most of them enter a lighter state of sleep known as torpor while a few others utilise a similar strategy called aestivation in the summer months. So what is the difference between hibernation, torpor, and aestivation?

What is hibernation?

“Hibernation is a voluntary state that an animal enters into in order to conserve energy, survive when food is scarce, and minimize their need to face the elements in the cold winter months,” writes Treehugger. Most people usually compare it to a really deep sleep, although hibernating is much more profound than simply sleeping. Depending on the species—from butterflies to bats—it can vary from long, deep unconsciousness to light spells of inactivity.

In cases where the animal is in a state of deep unconsciousness, hibernation can be risky as the dormant animal is vulnerable to predators and the unpredictable climate. Animals may die during hibernation from lack of fat, severe weather or even premature awakening.

How does hibernation work?

A hibernating animal’s metabolism slows and its temperature plunges—in ground squirrels it can fall to -2°C. Breathing slows and, in bats, the heart rate can fall from 400 to 11 beats per minute. Some cold-blooded animals, such as wood frogs, produce natural antifreezes to survive being frozen solid.

Which animals hibernate?

Small mammals, such as chipmunks, dormice, hamsters, hedgehogs and bats. Also, many insects, amphibians and reptiles. “Just one bird is known to be a true hibernator: North America’s common poorwill,” writes Discover Wildlife. “This beautifully camouflaged nocturnal bird is a relative of the nightjar found in Britain, and in winter often hibernates among rocks. It can slash its oxygen intake by 90%, while its body temperature plummets to 5°C, barely registering signs of life.”

To prepare for their hibernation, mammals feed heavily in summer and autumn, storing fat to see them through the winter.

What is torpor?

Torpor is a brief bout of suspended animation, usually lasting less than a day, when an animal’s breathing, heartbeat, body temperature and metabolism are reduced. By doing so, it enables animals to survive periods of reduced food availability. But unlike hibernation, torpor appears to be an involuntary state that an animal enters into as the conditions dictate.

How does torpor work?

During their active period of the day, animals maintain a normal body temperature and physiological rates. But while they are inactive, usually at night, they enter into a deeper sleep that allows them to conserve energy and survive the winter.

‘Waking up’ from torpor takes around one hour and involves violent shaking and muscle contractions. It takes some energy, but this loss is offset by how much energy is saved during the torpid state. One of the main problems with torpor is that the animals are too sluggish to react to predators. Furthermore, if the cold spell is unusually long, the animal may die if its body temperature drops too low.

Which animals enter torpor?

Birds such as hummingbirds and frogmouths, or small mammals such as bats, can go into torpor every day.

What is aestivation?

Aestivation—also called estivation in the US—is another strategy used by animals to survive extreme temperatures and weather conditions. But unlike hibernation and torpor, which are used to survive shortened days and colder temperatures, estivation is used by some animals to survive the hottest and driest months of summer.

How does aestivation work?

Most animals bury themselves in the ground, which protects them from the heat. Here, they wait for the wet season or cooler temperatures. Some land snails climb trees to escape the heat of the ground, sealing themselves into their shells using dried mucus. Similar to hibernation and torpor, aestivation is characterised by a period of inactivity and a lowered metabolic rate.

Which animals aestivate?

Many animals, both invertebrates and vertebrates, use this tactic to stay cool and prevent desiccation when the temperatures are high and water levels are low. Animals that estivate include molluscs, crabs, crocodiles, some salamanders, mosquitos, desert tortoises, the dwarf lemur, and some hedgehogs.

Sadly, large numbers of aestivating animals perish in periods of prolonged drought.

Guess what? One of the world’s largest animals can glow in the dark

According to a study conducted by marine biologist Jérôme Mallefet, three specific species of sharks can glow in the dark. I know what you’re thinking, ‘sharks that glow in the dark?’, get outta here. But no, it is true my friends. And yes, they are just as beautiful as you can imagine. What are these magical creatures, why have they only been discovered now, and how exactly do they glow?

The kitefin shark (Dalatias licha) is approximately the size of your average guitar. In normal light it has a brownish-black skin, with large and slightly googly eyes. But turn the lights off, and it emits an almost translucent and ghostly blue glow. On a 2020 voyage near Chatham Rise, which is just off the coast of New Zealand, a team of international scientists discovered that not only the kitefin shark, but two other species of deep sea sharks have the same gloriously bioluminescent traits. The blackbelly lanternshark (Etmopterus lucifer) and the southern lanternshark (Etmopterus granulosus) have been added to the list, although the kitefin shark takes first place for being the largest known vertebrate to produce such a glow.

The three sharks inhabit the mesopelagic zone of the ocean, which is also known as the twilight zone, and ranges from 200 to 1000 meters in depth—so obviously not a whole lot of sunlight can flood that range, providing an illuminated glow instead. According to Mallefet, most marine organisms that produce bioluminescence contain special chemicals, including a compound called luciferin that interacts with oxygen to produce light. Jellyfish, squid and algae (which are not vertebrates) for example, also enjoy luciferin. But these three shark species do not appear to contain the same chemical properties, so their ability to emit light “remains enigmatic.”

In an interview with Mongabay, Mallefet said that “For the moment our conclusion is that, maybe sharks have a new component that is unknown, But we don’t know.” As anything in nature, everything has a purpose, which is not only beautiful but inspiring when compared to how humans have evolved enough to take advantage of their surroundings.

With humanity set on living in the stars, we have neglected much of the vastness that Earth has to offer. Mallefet continues by explaining that “Many people say the deep sea is less known than the surface of the moon. We hope by highlighting something new in the deep sea of New Zealand—glowing sharks—that maybe people will start thinking we should protect this environment before destroying it. I hope the new generation will carry that message.”

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Bioluminescent sharks emit light from specialised cells on the surface of their skin known as photocytes, but exactly how they do this has long been a mystery, however, the study that Mallefet and his colleagues conducted shows some interesting data: the species control their light emissions using hormones. The luminescence is turned on by melatonin, which in humans, is there to help us effectively mediate dark signals and provide night information. Melatonin has been described by the Psychiatric Times as a “hormone of darkness” rather than the ‘hormone of sleep’ we all hear about. It has also been thought to be an endogenous synchroniser that stabilises as well as reinforces various circadian rhythms in the body—the same goes for all mammals, but sharks however are of the Chondrichthyes class.

So why do sharks, among other creatures, have the ability to light up in the first place? Well, in the deep sea, scientists estimate that around three quarters of all creatures living there are bioluminescent, which in the context of darkness, can be extremely advantageous. Need I say more? I will anyway, because it’s fascinating.

Deep sea animals use this special power to do everything from attracting prey and deterring predators to using it as a means to camouflage by hiding their silhouettes to match the light around them, in turn becoming practically invisible to those big bad scaries lurking below. This trick is called counter illumination.

In the case of sharks, known as the biggest and baddest of many, the fact that there might be something after them too for supper remains a possibility. However, because of the dense concentration of photocytes that were found on the dorsal fins of the kitefin sharks, there is speculation that it might simply happen as a way to help sharks communicate with each other.

For now, it’s important to know that these creatures and their habitats are understudied, and under threat. “A lot of people know that sharks can bite, thanks to Jaws,” says Mallefet, “but few people know that they can glow in the dark.” Now for a second, look down at your hands and skin, and imagine how truly remarkable this really is.