A minute before touchdown Neil Armstrong looks through the window. The rock-strewn surface below is impossible for landing…
The team back in Houston observe as the monitors recording his heart and blood pressure register a sudden massive rise.
Despite this physiological jolt, his core body temperature remains stable. Moments later the lunar module lands safely.
Our bodies, like those of most other mammals, shiver to warm up and sweat to cool down. Most of our body heat, however, is generated by our organs, particularly the kidneys, liver, gut and brain. This is known as ‘non-shivering thermogenesis’.
During normal metabolism, cells break down sugars and transfer the energy released into production of the energy-storage chemical, ATP (adenosine tri-phosphate). This reaction happens in subcellular compartments; ‘organelles’ called mitochondria. These organelles can also interrupt this energy transfer, releasing it instead as ‘waste’ heat.
This process enables animals to generate their own body heat (endothermy). Our bodies, like those of other mammals, control this process internally, and so maintain our temperature within narrow limits (homeothermy). A consistently warm body is needed to allow our neurons to function, operating our large brains and higher cognitive processes.
Internally controlled body temperatures are found in all mammals and birds, and a few other species. A few species of fish, reptiles, insects and even plants are able to generate warmth, at least in some of their body tissues.
Most reptiles, fish and amphibians however are ‘cold-blooded’ (poikilothermic): they allow their body temperature to fall or rise with that of their environment. These animals are highly metabolically efficient, meaning their energy requirements are low. They are however limited in the times during which they can be active. Snakes and lizards must bask in the sun to warm up, and also may need to shelter from the extremes of heat later in the day.
Birds and mammals have independently evolved their body temperature control systems. This means that they can hunt or forage during the cooler times of the day and at night. This comes at a huge metabolic cost: typically, mammal bodies require ten times the energy resources of a similarly sized reptile.
Some smaller mammals and birds release this control at certain times of day, or under certain circumstances; a strategy known as ‘heterothermy’. Small animals have a greater surface area ratio to their body mass than larger animals, so more easily lose body heat. Adopting a heterothermic (variable) body temperature control system allows them to conserve energy.
Larger animals may have the opposite problem: they find it more difficult to lose heat. Operating active cooling systems are also metabolically costly. Elephants and camels release their temperature control and become heterothermic during extreme desert conditions. They conserve energy by resting and allowing their body temperatures to rise, driven by heat from their environment.
The need of many mammals and birds to offset the benefits of homeothermy with heterothermy poses a puzzle; why has this costly strategy evolved repeatedly? Its selective advantage to the animal isn’t speed; when warm, a lizard’s reflexes and muscle performance can match and even exceed that of a mammal. Neither are warm bodies ultimately more active; a poikilotherm’s total hours of activity during its lifetime is similar to that of a similarly sized homeotherm.
Answers to this question currently show no signs of reaching a consensus. This may be in part because each species studied has a unique thermal physiological response which varies with its specific conditions, as well as over the life cycle. In addition, the behaviour of physical matter changes as the temperature shifts. Living organisms are also material. The chemicals they are composed of, for instance the fats that make up the cell membrane, adopt different properties as the temperature varies.
What can we say then, about the evolution of our stable body temperature?
How did regulated body temperatures first evolve?
Whilst cool temperatures slow down the metabolism, excessively high temperatures are lethal. Before any animal can begin to retain or generate heat, the first requirement must be an effective cooling system.
Since soft tissues are seldom fossilised, we cannot easily deduce when the ancestors of modern mammals may have first started to sweat. The earliest evidence for the evolution of an active cooling system in vertebrates comes from the fossilised bones of reptiles known as therapsids. These mammalian ancestors lived in the late Permian-early Triassic (c270-230Ma), when much of earth’s climate was very arid.
Breathe in deeply through your nose.
The cool sensation you feel is caused by water evaporating from bony flanges in your nasal cavity, called turbinates. These surfaces warm and moisten your in-breath, and then condensing much of this moisture and recovering heat as you exhale. This reduces dehydration. Dogs have well-developed ‘maxillo-turbinates’, enabling them to pant to cool down without losing excess water.
Therapsid fossils have maxillo-turbinates in their nasal cavity, indicating that these animals could pant. They also have naso-turbinates and ethmo-turbinates: structures which increase the internal surface area available both for evaporation and also for detecting scent. This tells us that these animals not only had a well-developed cooling system, inferring that their bodies were warm, but also had a keen sense of smell.
Modern mammals such as dogs and cats have highly developed neural systems for processing this level of chemo-sensory information. Similarly sized turbinates in fossil therapsid skulls suggests that these animals too may have had similar rapid neural relays feeding sensory information into a relatively large brain.
Neural tissues operate optimally within a relatively narrow temperature range. For therapsids to have dog-like sensory processing, they would require a body temperature which was maintained within these limits.
Why does our body stay at this temperature?
Our body temperature stays stable at around 36.7⁰C.
Controllable body temperatures extend the time window during which the animal can be active. Foraging during cooler times such as dusk or dawn may have originally relieved competition pressures between smaller, day-active dinosaurs, or helped them evade predators. Animals staying warm for longer due to insulation from fur, feathers, or fat, would have been able to gain more resources.
The effectiveness of insulation in retaining body heat is shown by the cold tolerances of modern arctic animals. Polar bears, like walruses and seals, have a layer of blubber under their skin. Musk ox fur is 8-10cm thick, and snowy owls have dense feathers that cover even their legs and feet. Thanks to their respective forms of insulation, these animals can maintain a stable core temperature of around 38-40˚C, even when the external temperatures drop to -50˚C.
Warmth may also have brought our mammalian ancestors better disease resistance. Here is why.
Most fungi cannot grow above 20-25⁰C. Arturo Casadevall suggests that the benefit of offsetting fungal infections, opposed against the metabolic costs of warm-bloodedness, predicts a thermal optimum body temperature of 36.7⁰C. This is our body temperature, and also that of most mammals.
Exceptions to this include the duck billed platypus. We share a distant early mammal ancestor with these odd, egg-laying animals. Their lower body temperatures mean they are susceptible to a number of fungal infections that do not affect us and other modern mammal species.
Warm bodies, then, bring various potential survival advantages for survival, particularly in cooler climates. The ability to better maintain their body temperature would have enabled early mammals to expand their ecological ranges into cooler habitats, and also better survive and adapt as the global climate began to change.
What can birds tell us about why homeothermy may have evolved?
Ever held a nestling? It’s warm. Birds evolved from dinosaurs: their thermal regulation mechanism has arisen independently. Birds typically have a more variable (heterothermic) daily temperature regime than mammals. Lowering their resting temperature conserves energy.
Although much of their heat is generated by flight, birds also have ‘non-shivering thermogenesis’. In mammals, the mitochondria from specialised brown adipose tissues produce ‘uncoupling proteins’ that interrupt the transfer of energy to ATP, releasing it instead as heat. Birds have similarly specialised heat-producing cells, called adipocytes, with heat-releasing mitochondria. However their mitochondrial uncoupling proteins are different from those found in mammals.
Why did birds and mammals both evolve to generate heat on demand? Both types of animals provide high levels of parental care. The majority of early deaths in all vertebrates occur at the juvenile stage. Feeding and protecting their young in a nest greatly improves the survival of offspring, although delivering this care involves intense and sustained activity.
Colleen Farmer suggests that the better survival of offspring from parents delivering a more sustained parental care may have actively selected for both warm bodied birds and mammals. If so, evolution has driven these animal groups towards being better able to deliver this care, and consequently having more stable body temperatures.
If evolution has selected for warm bodies (homeothermy) under certain conditions, then other conditions should prompt homeotherms to re-evolve poikilothermy, or at least some forms of heterothermy. Alligators have a heart and circulatory system that suggests they have re-evolved poikilothermy from a warm bodied ancestor, and as an adaptation to hunting in water.
Alligators are also unusual amongst reptiles in that they can be highly active on land, and even protecting their offspring as a brood for the first year of their lives. This supports Farmer’s suggestion that selection for this behaviour has prompted major shifts in the physiology and metabolism of these animals, as it did in the early ancestors of modern mammals and birds.
However evolutionary theories require testing. As the study animals needed are now extinct, debates on the evolution of temperature control will retain their heat for some time.
- Our bodies maintain a stable body temperature by generating heat and operating cooling mechanisms. Movement and exercise generates heat, although most of our body warmth is generated by metabolic processes in our internal organs. We also have ‘brown adipose’ tissues containing mitochondria which are specialised to release heat through a process known as ‘non-shivering thermogenesis’.
- Mammal body temperatures are kept within narrow limits, typically around 36-37⁰C. Our cells, and particularly the cells of our nervous system, the neurons, function only within these narrow thermal limits.
- Most reptiles, fish and amphibians are ‘poikilothermic’, meaning they allow their body temperatures to more closely match that of their environment. This is metabolically efficient, although limits the times at which some of these animals are able to be active.
- Self-regulation of body temperature, known as homeothermy, has evolved independently in mammals and birds, and operates in slightly different ways in these animal groups. This is metabolically costly.
- Some mammal species and many birds operate a modified version of thermal self-regulation, known as heterothermy, in which their body temperatures vary more in line with the environment. For small animals in particular, this helps conserve energy when at rest or asleep.
- The earliest signs of animals having evolved a stably-regulated body temperature come from the fossils of mammal-like reptiles with nasal turbinates – bony flanges that expand into their nasal cavities, providing a simple cooling system through evaporation of moisture from these damp surfaces.
- Many species retain body warmth by using insulating layers of fur, feathers or fat. Warm bodies stay active for longer, allowing them to gain more resources. This may have driven the evolution of better thermal control.
- Warm bodies may have evolved through selection for mammals and birds with better resistance to fungal diseases. Most fungal infections cannot survive at our resting body temperatures.
- The increased levels of prolonged activity made possible by a consistently warm body temperature may have been driven for selection for better levels of parental care in birds and mammals.
Text copyright © 2015 Mags Leighton. All rights reserved.
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