A thick woolen blanket, a cup of hot milk with honey and a fireplace, those were the advises of my grandmother during the cold winter nights (yes, it does get cold in Spain during winter, it´s not only about sun and sangria!).
However, not every animal has the ability nor dexterity to shear sheep, milk cows, harvest honey or produce fire, so say thanks to your opposable thumbs and many hundred-thousand years of evolution which have allowed you to enjoy the “cozy-comfiness” of your living-room.
Let´s get to it. First of all, dealing with cold climatic conditions is as hard as dealing with warm environments. It´s all about how a certain organism is able to cope with the heat exchange between its body and the surrounding environment. This is extremely important at physiological level, as the utmost goal of your body -or any- is to allow your tiny little cells to function efficiently, so they can transform sources of fuel into energy that can be used to get more fuel (the eternal cycle of life- the old born, grow, reproduce, die).
Sometimes it might be hard when you have to deal with temperature gradients of more than 80ºC. The reindeer (Rangifer tarandus) is a good example. This arctic ungulate keeps a nice 36-38ºC body temperature despite the harsh conditions with ambient temperatures that can drop down to -50ºC [1],[2]. But, how do they do it?
1) Nasal cavity adaptation to reduce cold exposure; 2) Reindeers have an structure within their noses called concae -a concatenation of spiral structures- which restricts the heat and water loss from the respiratory tract; 3) Polar bear pregnant females undergo an hibernation period during winter season; 4) Penguins huddle to preserve the heat.
The animal kingdom has developed several strategies to deal with thermoregulation (heat exchange), and some of them are based on the question whether you rely on the external environment for your own thermoregulation or you do it by yourself. Organisms that depend on the external environment to regulate and generate their own temperature are called ectotherms. These animals can regulate their temperature behaviourally, by moving into warmer/colder areas. In the opposite, endotherms create the heat they need from internal chemical reactions[3].
As you can imagine, there are pros and cons whether you are from the ecto- or endo-team (as well as the many mid-term organisms such as poikilotherms, mesotherms, homeotherms, heterotherms, stenotherms, eurytherms – I hope you are good at greek to decipher each of these terms!). Ectotherms are very good using energy, as their metabolic rate is usually lower than endotherms, meaning that they need to burn less fuel to live their life, and this difference can be up to 12-20 times for animals with similar body mass[4]. For example, a 1 kg bird will need to produce ~5,000 kj/day while a reptile of the same size just ~500 kj/day. This is a clear advantage as you require less food to survive than your endotherm fellows, and you can use that energy to grow and reproduce. The same applies to oxygen consumption, as ectotherms have 24-30 times less aerobic activity levels[5]. So far, it seems that we should be training snakes and frogs for the space, it will be more economic at least.
Imagine the David Bowie song “Major rattlesnake to ground control”, sounds good, doesn´t it?
Nonetheless, there are some disadvantages if you are an ectotherm. First of all, you entirely rely on an external source of more or less constant heat, meaning that you can inhabit only certain places in the planet. This is what scientists call “geographically disparate macrophysiological constraints”[6] (what a sentence, uff). Put it in other words, there is an opposite latitudinal gradient where ectotherms and endotherms can exist. In fact, ectotherms richness is higher in lower latitudes (equatorial and tropical zones) but endotherms are more abundant where primary productivity is higher, towards the poles and temperate areas.
This concept applies not only to land-based animals, but also to aquatic. In colder environments, we can find a higher richness of endotherms because they are able to maintain a constant body temperature (~36ºC), allowing them to deal with extreme temperature ranges. Some of the advantages are the ability to occupy thermal niches that exclude other ectothermic organisms, a high degree of thermal independence from environmental temperature, high muscular power output and sustained levels of activity, which increases the chances to capture preys more efficiently. But the price to pay is an elevated metabolism to maintain the body, and it is energetically expensive[7], [8].
1) Distribution map of endotherm (red) / ectotherm (blue) richness; 2) Metabolic rate of endotherm/ectotherm related to ambient temperature and burst speed of different endothermic species compare to fish (ectotherm) and related to water temperature[7].
Being pushed to limiting environments where temperatures are very low, made few behavioral, physical and physiological changes in the bodies of the endotherms. Some responses have been[9]:
Reduce the surface exposed to the hostile environment by “balling up”, huddling or shelter building.
Adapt the thickness of the external layers like fur, plumage or blubber, depending on the season.
Develop circulatory systems that reduce the heat loss by conduction by cooling the peripheral tissues by vasoconstriction/dilatation.
Restrict the evaporative heat and water loss from the respiratory tract.
Response to cold with shivering, which aims to increase the metabolic rate and warm up the body to avoid hypothermia.
Become intermittently active depending on the light availability (torpor, hibernation).
These are some of the answers given by endotherms to adapt to high latitude environments. Nevertheless, we are facing a scenario of constant increase of temperatures (climate change, in case you haven´t heard of it) and these extremely adapted organisms will confront a crude reality: adapt or die. Some scientists are trying to asses which species will be the “winners” and the “losers” of this gruesome lotto. One of the parameters used is the thermal limit of each species, which is the maximum and minimum range of temperatures they can deal with[10],[11]. If some species, such as polar bears or reindeers cannot adapt, they risk for example, overheating and the consequent melting of the brain. How quickly or resilient the species can be to adapt to a new changing environment will be the key for the successful of each of them.
Lesson learnt today: be a good endotherm, otherwise we will have to massively travel to Greenland and start shearing Rudolfs to “adapt” them to their new reality.
Peace
References [1] Nilssen et al. (1984). Regulation of metabolic rate in Svalbard and Norwegian Reindeer. [2] Blix et al. (2011). Regulation of brain temperature in winter-acclimatized reindeer under heat stress. [3] Biology dictionary. Link. [4] Nagy (2005). Review. Field metabolic rate and body size. [5] Gilloly et al. (2017). A broad-scale comparison of aerobic activity levels in vertebrates: endotherms versus ectotherms. [6] Buckley et al. (2012). Broad-scale ecological implications of ectothermy and endothermy in changing environments. [7] Grady et al. (2019). Metabolic asymmetry and the global diversity of marine predators. [8] Hedrick and Hilman (2016). What drove the evolution to endothermy? Link. [9] Blix (2016). Adaptations to polar life in mammals and birds. [10] Huey et al. (2012). Predicting organismal vulnerability to climate warming: roles of behaviour, physiology and adaptation. [11] Somero (2010). The physiology of climate change: how potentials for acclimatization and genetic adaptation will determine ‘winners’ and ‘losers.