Why can’t humans endure heat like an iguana?

The key is in sweating to lower the temperature and the role of brown fat in the production of heat

In summer, it is frequent that some cases of animals that die from heat stroke reach our ears. However, we are not surprised to see in a documentary that certain species spend their hours in the sun practically undeterred. What are these differences due to? What makes an animal species more or less adapted to heat?

To answer these questions, it is necessary to know the mechanisms that allow thermoregulation, that is, the ability to adapt, within limits, body temperature to achieve a balance compatible with life.

Within these mechanisms, we are going to focus on sweating to lower the temperature, and the role of brown fat in the production of heat (thermogenesis). Without forgetting that almost everything that happens in our body depends on physiological, anatomical and histological factors.

Thermoregulatory mechanisms

There are poikilothermic or ectothermic animals (such as reptiles and fish) that adopt the temperature of the environment without the need to use metabolic pathways, through behavioral mechanisms. For example, they change their habitat: when the temperature rises they go into burrows or in the water, and if it is cold they expose themselves to the sun. These organisms lack sweat glands and brown fat.

On the other hand, mammals and birds are endothermic animals, capable of regulating body temperature and keeping it within operating limits. These limits vary by species, for example, a dog has a normal temperature of 37.5-39 ℃ , range with which humans would have to quarantine at home on suspicion of covid-19.

According to these concepts, a reptile can survive very high temperatures thanks to this physiological condition, which we could consider as an adaptive advantage in a natural environment. However, numerous domestic reptiles come to veterinary clinics with heat burns.

These animals have a much higher threshold of sensitivity to heat, so by the time they realize they are burning, it is too late. Therefore, within the responsibility of the owners is to take care of the environment of their pets in extreme temperatures.

The role of brown or brown fat

There are two different types of adipose tissue: white and brown fat. White fat is found under the skin (in the hypodermis) and maintains heat, while brown fat is specialized in thermogenesis (essential in cold weather). In fact, newborns have a higher percentage of brown fat strategically distributed to avoid heat losses in the early stages of life.

For this reason, it seems logical that those mammals with periods of hibernation in their life cycles have a higher percentage of brown fat. This tissue is rich in mitochondria, the energy engine of all our cells. In this way, these animals can maintain a continuous energy supply for their basal metabolism.

The sweat, uncomfortable but necessary

Perhaps it is unknown to some that certain species do not sweat, or at least not in the totality of their body surface, as we humans do. Stripping sweat of its possibly repulsive aspects, we will emphasize that it is a fundamental mechanism, orchestrated by the parasympathetic nervous system, for temperature control.

Histologically, the glands specialized in the production of sweat for thermoregulation are called eccrine sweat glands. It should be noted that they have only been found in mammals.

Its role in primates (like us) is very important in body thermal control. So much so that a research group in anthropology at the University of Massachusetts has carried out a phylogenetic study to determine the proportion of these glands in 35 different primate species. An important variability in their quantity was observed, according to the different climatic conditions of their habitat.

Something similar has been described with respect to the different bovine breeds, in which the density of sweat glands varies in the same way.

In some non-primate mammals (such as cats and dogs), the eccrine glands are distributed only on the footpads and on the nose (nose). In cases like this, as well as in birds, the main way to control excess temperature is through evaporation from the body surface and what we know as panting.

Dogs, like cats, only have sweat glands on the pads of the feet and on the nose of the nose.

What mechanism is more effective?

The answer is difficult. It all depends on the prism with which we look at it.

It is undeniable that depending on the environmental environment in which the animals are found, both the activity of their sweat glands and the coverage of their fur and fat have greatly contributed to their adaptation to the environment.

These factors have allowed, over the centuries, certain species to better adapt to withstand higher temperatures. Could we then consider that, in general, non-human animals cope better with heat?

To answer, let’s ask ourselves another question: Do you think that an inhabitant of Lapland would endure the high temperatures that exist on the African continent with the same pleasure as a Kenyan? We would surely answer: “They are better adapted.” Well that’s the final answer.

Fortunately, the evolution of species, and even ethnic groups, is an indomitable and prolonged process over time. In the case of thermal tolerance, different paths have been intelligently chosen according to the needs to be covered in space and time.

This article was originally published on The Conversation. Read original here.

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