Inside: Thermoregulation looks at the way different animals keep a stable temperature.  Animals, including humans, fall into one of two categories when it comes to how they keep their bodies temperature where it needs to be – endotherms or ectotherms.  There are some finer distinctions within these categories and as always, there are exceptions to the rule!
 

It is important for us to keep a stable temperature.  Being human, we’re lucky in a way.  When its cold outside, we can put on extra clothing – a warm sweater, a jacket, mittens, scarf and hat.  In the summer, when it gets too hot, we take off some layers and wear light colors.  But we have other ways to keep warm or cool down.  Our bodies are made to keep a stable body temperature around 36.5–37.5 °C  for those of us in Canada and elsewhere and 97.7–99.5 °F for my American friends.  Notice its a fairly narrow range.  Your body works hard to keep it within that range.  

Here in Ontario, Canada we sometimes experience temperatures that range in any winter week from the low plus side to extremely cold temperatures of -40ºC including wind chill.  Buses for all schools were cancelled this calendar day because of excessively cold temperatures.  Can you believe it, roads were fine to drive but buses cancelled.  This kind of event was a first for our area.  After venturing outside to fill my bird feeder, I could understand why.  Breathing in the cold arctic air was painful.

Behaviors We Use to Keep a Stable Temperature

So, other than putting on clothes or removing them to keep warm, how else do us humans keep a stable temperature?  When I ventured outside today, I began shivering before I finished filling up the feeder.  Live Science provides a great explanation of why shivering makes us warm.  We’ve all experienced a hot summer day that has made us sweat!  The blog, The Conversation, gives a great explanation for why we sweat when we’re hot!

 

Thermoregulation and How Animals Keep a Stable Temperature

Thermoregulation looks at the way different animals keep a stable temperature.  Animals fall into one of two categories when it comes to how they keep their temperature where it needs to be:

  1. Endotherm
  2. Ectotherm

Endotherms Create a lot of Heat Through Metabolism

Endotherms describe us and other mammals and birds.  We have the ability to use our internal metabolism to keep our temperatures within a narrow margin.  Our metabolism creates a lot of excess heat to keep our bodies warm.  Humans keep their body temperature between 97.7–99.5 °F .  Unless we are ill, our metabolism is designed to kick in when that range is exceeded or reduced.  We shiver when too cold.  Our blood vessels constrict to prevent blood flow to our extremities.  We sweat when we’re too hot.  Our blood vessels expand to allow blood to flow to our skin surface.  We can use behaviors to help like wearing warmer clothing in cold weather and less clothing in warm weather.  But it is our metabolism that does most of the work.

Ectotherms Stabilize Temperature Through Behavior

Ectotherms cannot create a lot of heat from their metabolism.  Reptiles, amphibians and insects are typical ectotherms.  They rely mostly on behaviors to keep their body temperatures in a range that keeps them alive.  Ectotherms can survive a much wider range of temperatures.  Turtles when they are too cold find a warm rock to sit on or a sunny place to rest.  Snakes when they get too hot will hide under a rock, in a pile of leaves or in a cave to cool off.

Both Groups are Successful

The world is full of successful organisms in both groups  Endotherms are able to be more active in a wider range of conditions.  Ectotherms will often have periods of being sluggish when their temperatures go too high or low.  But, they can go longer periods without food because they don’t need the extra nutrition to keep a stable body temperature.  Endotherms have higher nutritional needs.  If you’re a parent you might swear your little endotherms need to eat all. of. the. time.

Differences Between Endotherms and Ectotherms

Endotherms

  • use their internal physiology (metabolism) to create heat to keep their body temperature within a narrow range
  • they are very successful at keeping a consistent body temperature
  • their metabolic rate increases steadily as the temperature of their environment falls
  • about 80% of the food energy they eat is used to maintain their body temperature
  • their body cells contain many more mitochondria compared to ectotherms to increase respiration rates providing the body heat

Ectotherms

  • they use mainly behavioral ways that use external sources of heat to regulate their body temperatures
  • they can generate some heat energy from internal respiration as there is always some heat released during respiration
  • their metabolic rate decreases as the external temperature falls
  • because they do not use their metabolism to heat or cool themselves, their energy needs are much lower
  • each of their cells contain far fewer mitochondria than found in endotherms

 

 

Another Way to Classify How Animals Keep a Stable Temperature

Animals are also classified by the overall stability of their body temperature.  Some organisms maintain a very consistent core body temperature.  Other organisms can survive wide changes in their core body temperature daily or seasonally.

Homeotherms

  • they are able to maintain a stable body temperature no matter what the temperature of the external environment
  • include most endotherms
Exceptions Include:
  • ectotherms that keep their body temperature stable are considered homeotherms
    • ectotherms that live in a habitat with a stable temperature are considered homeotherms
      • desert pupfish
    • ectotherms that use specialized behavioral mechanisms
  • Upper Image:  Mexicankillis/Mauricio De la Maza-Benignos, CC BY-SA 3.0 via Wiki Commons
  • Lower Image: aes256, CC BY 2.1, via Wiki Commons

 

Poikilotherms

  • their body temperatures vary with the temperature of the outside temperature; their temperatures may actually match the temperature of the outside air
  • includes most ectotherms
Exceptions Include:
  • endotherms that experience torpor can be considered poikilotherms because they change their temperature by slowing their metabolism either seasonally or daily
    • a sleep-like state causing the animal’s metabolism to drop including its body temperature
    • this state is triggered by variations in daily temperatures
      • hibernation is a long-lasting kind of torpor that happens in the winter
      • it is triggered by the change in day length 
      • true hibernators go into a very deep sleep where both their heart rate and temperature drop drastically to conserve energy
        • woodchucks, ground squirrels and bats fall in this group
      • light sleepers go into a light sleep where their heart rate and temperature drop a little and they wake up once in a while
        • bears, skunks, raccoons, opossums are light sleepers
      • estivation is longer lasting torpor that happens in the summer
        • it is triggered by extreme heat and shortage of water and often involves burrowing into the ground to stay cool
        • like hibernators, their heart rate and body temperatures drop to conserve energy but they do not sleep as deeply or as long as true hibernators
        • East African hedgehogs fall into this group
      • some organisms go through a daily torpor to conserve energy
        • the hummingbird has such high energy needs to sustain its method of flying that it goes into a daily torpor each evening slowing its metabolism to conserve energy
        • bats as well reduce their metabolism during the day hanging out in trees or caves and coming awake again to feed in the evening

Keeping Straight the Terminology

What Have We Learned?

So, what have we learned?  Some animals can create body heat through their metabolism to keep a steady temperature.  That makes them endotherms.  Other animals use behaviors to keep their temperature steady which makes them ectotherms.

Most endotherms are homeothermic meaning their temperature stays within a narrow range. We are endotherms and homeothermic.  Our metabolism creates lots of excess heat that we use to keep our temperature right around 37℃ or 98.6°F.  If we get too warm our blood vessels open up and we release heat through our skin often through sweating.  We will alter our behavior as well and put on lighter clothing.  If we get too cold then our blood vessels constrict making sure there is less blood going near the skin.  Less heat is lost and we keep warmer.  Again, we might alter our behavior and put on warmer, darker clothing.

Other animals will also use a combination of metabolism and behavior to keep warm.

If you are an ectotherm, you need to mostly use behaviors to keep your temperature stable.  Most ectotherms are also poikilotherms which means their body temperatures change right along with the external temperature.  When a turtle needs to get warm, it needs to sit in the sun or on a warm rock.  Poikilotherms are lucky in one way.  They don’t need to find as much food to eat to keep warm.  Many can also survive some seemingly impossible low temperatures at least for short periods of time.

Endotherms tend to be very active organisms that can live in a wide variety of habitats.  Ectotherms have a lot of lazy periods when they are low on energy but many ectotherms are among the longest living creatures on earth because of their slow metabolisms.  I’ll leave you with  this video of the Greenland Shark that shows just how long at least one animal can live.

 

How in the World Do Animals Keep a Stable Temperature?-bottom photo by:bottom: Photo by Mircea Iancu from Pexels

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