Wednesday, September 23

Thermoregulation in humans and animals


Red Tarn in the English Lake District National Park.

TODAY the magic number of the normal human temperature at 37 degrees Celsius is indelibly printed on our foreheads or under our tongues as thermometers have now become household objects. Infrared temperature scans and hand sanitiser, as we go about our new normal lives, will be with us for some time. The average human temperature can range from 36.1 degrees Celsius to 37.8 degrees Celsius according to circadian cycles and one’s age. Obviously a diurnal variation exists, which is dependent on periods of rest and activity with the lowest temperatures between 11pm and 3am, and peaking between 10am to 6pm.

As a 20-year-old hockey player, in a late summer afternoon match, an opponent’s stick smashed into my cheek badly snapping off a molar tooth. With blood gushing in my mouth, I quickly ran to my dentist whose practise was about to close for the day. Pouring in perspiration, I sat on his chair and with a dose of nitrous oxide I was asleep. I awoke from his removal of the rest of the tooth to feel him slapping my face and calling my name. With blurred vision this white apparition appeared before me and I exclaimed, “It’s a miracle!” He replied, “It was a miracle how long I took, with the help of my staff, to anaesthetise you and the long time you took to come around.” Upon reflection, in my stupor, I had fallen down a long dark tunnel whilst asleep and awoke to what I thought was St Peter at ‘The Pearly Gates of Heaven’ – hence my exclamation! It was, I now believe, hyperthermia that took me near to death’s door.

What is hyperthermia?

Usually this occurs when bodily temperatures exceed 37.8 degrees Celsius in ambient temperatures of over 40 degrees Celsius for a prolonged period of time. As a geographer, I often took my senior students on fieldwork courses to collect and analyse data. It was on such a course in 2000 at Danum Valley in Eastern Sabah that a female student overheated through a combination of sunburn and because, in rainforest air of such high relative humidity, her sweat could not evaporate to cool her skin (we have three million sweat pores on our bodies) together with dehydration she needed immediate hospital treatment. After a two-hour drive to Lahad Datu, three hours of excellent treatment and observation there, she returned to rejoin the group. Fortunately, I had taken with me a young Sabahan family friend, who was studying medicine in the UK. Her swift diagnosis of hyperthermia and action certainly saved this young student.

Hippos cool down in a river in Namibia.

What is hypothermia?

This condition is caused by the loss of bodily heat faster than it is able to produce. This usually occurs when a human’s core bodily temperature is lowered below 35 degrees Celsius. On another fieldtrip in the mid-1970s, I took 20 of my A-level students to the Lake District in North West England to measure, for the first time ever, using a transponder, the depth of Red Tarn, the highest tarn (corrie glacier made lake) in the UK and just beneath the summit of Helvellyn (950 metres high).

All the students were physically very fit individuals and were determined to accomplish this exercise in early May. Fully equipped with emergency rations, changes of clothes, first aid equipment including a stretcher and tent together with an inflatable rubber dinghy, we climbed for two hours in temperatures of 17 degrees Celsius perspiring heavily. Within an hour of reaching the tarn, the temperatures fell to about 5 degrees Celsius and a heavy snowstorm lasted for an hour. We sheltered behind a low wall of a disused sheepfold. The sun came out and the dinghy crew embarked on their traverses of the small lake.

Within two hours, because of the lake temperatures, two of the crew developed hypothermia with signs of slurred speech, rolling eyeballs, and shivering. We immediately put them in sleeping bags inside the emergency tent and, against all safe camping rules, lit a primus stove inside the tent with a couple of the students taking turns to keep both students awake whilst the rest completed their assigned tasks. Within an hour they recovered and we finally descended the mountain range as dusk descended having determined that deepest part of Red Tarn was at 25 metres!

Today, stringent health and safety risk assessments are in place for all students on any type of fieldtrip and however well they are recorded, the unexpected accident can happen.

How do animals respond and adapt?

I admire a veterinary surgeon’s not only ability to take the temperatures of animals of all shapes and sizes but also to know their ranges in normal bodily temperatures. For zoo vets, it is even more complicated when examining exotic species of animal. Most of us do not know the temperatures range of our own domestic pets let alone relate them to the shape and body mass.

Animal species can be generally divided into two types: endotherms and ectotherms, warm-blooded and cold-blooded respectively. Endotherms include mammals with temperature ranges from 36.1 degrees Celsius to 39.4 degrees Celsius, and most species of bird with a temperature average of 41 degrees Celsius. These all need lots of energy to maintain a constant body temperature for their loss of bodily heat is proportional to their surface area and the heat they produce is proportional to their mass and thus they tend to be larger animal species whose potential energy is derived from food. Hippos often take to rivers and ponds by day to cool down and graze in the early morning and night lower temperatures.

Ectotherms don’t waste energy in generating their own heat and thus have lower metabolic rates resulting in less pressure on them to regularly eat. Their heat is derived the outside ambient air temperatures and so they regulate their bodily temperatures to meet outside conditions. Pythons have been known to forgo food for a year and adders and turtles are often seen basking on rocks in the heat of the day.

In all cold-blooded animals, there is a critical temperature below which their metabolism ceases to work as all their chemical reactions slow down. These animals tend to be active in winter and the further north one moves towards the Arctic Circle the rarer they become rarer.

A lizard builds up energy whilst basking on a rock.

Hibernation and estivation

These words are derivatives of the Latin ‘hibernare’ – to spend the winter, and ‘aestus’ – heat. The former is a method for animals to cope with reduced food resources in wintertime. Such animals as brown and black bears spend copious hours in the summer months building up their brown fat resources before entering their dens for a long winter’s sleep. Black bears often sleep for 9-10 months but brown bears with variable core temperatures often awake and wander out of their dens in search of prey during the winter months and thus often creep out of the forests to raid villagers’ livestock and rubbish bins.

Estivation, however, occurs during prolonged heat-waves allowing animals to survive high temperatures and desiccation. Various types of beetle, tortoise, crocodile, salamander, toads, and frogs often bury themselves in mud baths or deep underground dens awaiting the return of rain.

Effects of climate change

Autumn 2019 saw the publication of a paper by researchers at the University of California, Berkley, on the effects of rising temperatures on birds and animals in the Mohave Desert. From 2014 onwards, the warmest five years on record were recorded by the World Meteorological Association. It is anticipated that the frequency, intensity, and duration of heatwaves will increase in the future thus having a devastating effect on both marine and terrestrial ecosystems.

Heat stress has been recorded in insects, coral reefs, and with tree dieback. Population crashes are occurring in bird and bat species in Australia with extreme temperatures being the catalyst for an ever growing number of local extinctions.

What is happening is best expressed in the words of an eminent professor of ecology at the University of Arizona, John Wiens, “It is estimated that if there is moderate global warming, 16 per cent of all species would be lost; if there is more severe warming then there will be a 30 per cent loss. The big picture is that one in three species could become extinct over the next 50 years.”

Birds suffer more from the highest exposures to climate change because of their daily movements often during the hottest times of the day. Certainly we need to work at the thermoregulation of our world’s climates, for human kingdoms and animal kingdoms risk further loss of life at the present rate of global temperature increases.