In Barcelona, summer has just arrived. Temperatures are rising and behaviours begin to change. We start to drink more, wear less and switch on the air-conditioning in our homes and offices. Nevertheless, as people travel around the city, there is no shortage of red faces and sweaty bodies!
These are some of the ways we adapt to seasonal changes, but how about animals? What are the limits of human survival? What behavioural and physiological adaptations do animals employ to survive in extreme temperatures?
In this article we’ll explore these questions and take a look at some survival specialists from the animal world!
What is thermoregulation and why is it important?
Thermoregulation is the ability of an organism to maintain its body temperature within a specific range. The process is one aspect of homeostasis, the state of dynamic stability of internal conditions.
In humans, the normal range is 36.5–37.5°C, however temperatures can vary depending on an individual’s age. Older people generally have a lower body temperature. The circadian cycle is also a factor, with our body temperature dropping slightly early in the morning and increasing in the late afternoon.
It’s important that we keep our internal temperature within this narrow range, as 37°C is the temperature at which the body’s enzymes work best.
What are the limits of human survival?
Fluctuations beyond this range, for example caused by exposure to extreme external temperatures, can be extremely dangerous.
If your internal body temperature drops to 35°C or less for a sustained period, a state of hypothermia will ensue. You will start experiencing intense shivering, numbness in your extremities and your skin will start to turn blue.
At 33°C there will be a loss of movement in your fingers, you will feel confused and drowsy. Soon, the shivering will stop, your heart rate will slow and you’ll likely lose consciousness. A person usually expires if their body temperature reaches 21°C.
Exposure to extreme heat, leading to hyperthermia, is equally dangerous. When your body temperature hits 38°C, you’ll experience heat stress and heat exhaustion. Besides sweating profusely, symptoms include headache, nausea, dizziness, weakness, thirst.
At what point does hyperthermia become fatal? This is highly variable, as it also depends on humidity, which alters the apparent temperature. To give an example, if the air temperature is 29˚C, but there’s zero humidity, it will actually feel like 26˚C. But, with 80% humidity, the same temperature will feel like 36˚C!
Thermoregulation in animals
You’re probably familiar with the distinction between cold-blooded organisms (ectotherms) and warm-blooded organisms (endotherms). Most fishes, reptiles, amphibians and invertebrates are ectothermic, whereas most mammals and birds are endothermic.
Both forms of thermoregulation have their advantages. Ectotherms don’t waste energy in generating their own heat, so have lower metabolic rates. This means that there’s less pressure on them to eat so frequently. In fact, pythons can go a whole year without feeding!
That said, for endotherms, having control over their body temperature can be a real advantage, as they are far less vulnerable to fluctuations in environmental conditions. Whatever the weather, they can carry on grazing, hunting or mating.
However, in nature there are always some exceptions. Not all mammals need to keep their body temperatures within the same narrow range as humans do. The arctic ground squirrel has a critical range of 18-36°C and, during hibernation, they can lower their body temperature to -3°C. So is it correct to refer to them as ‘warm-blooded’.
Likewise, not all fish are ‘cold-blooded’. The opah fish (or moonfish) is one of the only species of fish that can stay warm, independent of the temperature of the surrounding water. It achieves this thanks to a system of heat exchanging blood vessels in its gills.
Now, let’s take a look at some more extraordinary animal adaptations.
Survival in extreme cold
This species is the ultimate cold-weather specialist. Musk oxen live on the frozen arctic tundra and have to endure temperatures as low as -40°C. They accomplish this thanks to their incredibly thick fur, which is made up of two layers: outer hairs, called guard hairs, and an undercoat for additional insulation. As musk oxen are herd animals, living in groups of 20-30 individuals, they also huddle together to reduce heat loss. This is an example of a behavioural adaptation.
Wood frogs live across North American, as far north as Alaska’s arctic forests. To survive the winter months, which see temperatures drop as low as ‑28°C, these frogs employ a special strategy – they freeze. As ice starts to form around them, sugars are released throughout the body, which protects the frog’s cells from shrinking. Even though their heart stops beating and as much as 70% of the water in their body may freeze, they can still survive the winter. When temperatures rise, their heart starts beating again and their body replaces any damaged cells.
Weighing up to 1.5 tons, these supersized aquatic mammals live comfortably in the harsh Arctic seas. Physiological adaptations include a small surface area to volume ratio, to minimise heat loss, and an extremely thick layer of blubber for insulation. As an endotherm, the walrus relies on metabolic heat to survive. An adult must consume around 25 kg of food per day, usually in the form of clams, snails, crabs, shrimps, as well as the occasional seal carcass!
Survival in extreme heat
While insulation is key to survival in cold conditions, the opposite is true for desert dwellers. As with walruses, camels also have large reserves of fat, which is broken down to produce water and energy. Instead of storing the fat all around their body like walruses do, camels keep it all in one place: their hump. This way, they limit any unwanted insulation.
This species of invertebrate, endemic to Australia, builds elaborate mounds that reach 4m in height and can house a million individual termites. Their mounds are always oriented in a north-south direction, which is how they got their name. This detail ensures that their nest receives heat from the sun on their eastern and western sides at dawn and at dusk, while exposing less surface to the sun in the middle of the day, when there is a much greater risk of overheating.
The fennec fox (main image) is very well-adapted to the inhospitable heat of the Sahara desert. It has evolved massive ears that can grow to be half the size of its body. As well as helping them to detect prey, they contain an extensive network of blood vessels that allows rapid heat loss through a process called vasodilation. As for behavioural adaptations, they display nocturnal habits. They hunt at night and retreat to their underground dens during the heat of the day.