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Well I’m hot blooded, check it and see…

Posted by: Kirsten Pisto, Communications

“I got a fever of a hundred and three, come on baby…” We’ve all heard the term “hot blooded” in reference to unabashed lust, but this Valentine’s Day we are cooling things off with a little biology recap. Sorry, all you stud-muffins and flirtatious foxes, but using the term “hot blooded” is actually pretty uncool in the zoological community.

In the recent past, hot blooded (or warm blooded) and cold blooded were ways to describe an animal’s thermoregulation. Within the last 30 years, studies in the field of animal thermophysiology have revealed just how wild temperature control is between each species, and it’s pretty sexy stuff!

Flamingos pair up to create a stunning heart-shaped beak rub.  Flamingos are endothermic homeotherms. By constantly ingesting shrimp, these water birds keep their luxurious pink feathers looking bright; the shrimp also keep their metabolic rate nice and even. (Photo by Mat Hayward/ WPZ).

There are three types of thermal stability in animals:

Homeothermy: when an organism maintains a relatively constant body temperature. This temperature is usually higher than the average warmth of the subject’s environment. Most birds and mammals—which includes us humans—are homeotherms, and maintain thermal homeostasis.

Poikilothermy: a poikilotherm is an organism whose internal temperature varies considerably, usually a consequence of variation in the ambient environmental temperature. Most reptiles and amphibians are included in this category and depend on their environment for heat and cooling. 

Grizzlies are heterothermic. A tent to keep warm? Nope, they thermoregulate naturally. By lowering their core body temperature and heart rate during long winter months they don’t need as much fuel (berries and groundhogs) to sustain them. But when spring rolls around, their energy intake will rise substantially… watch out huckleberries.(Photo by Mat Hayward/ WPZ)

Heterothermy: is a physiological term for creatures that share characteristics of both poikilothermy and homeothermy. Heterothermy has been found in a number of mammalian orders, but within the primates so far it seems to be restricted to one family of Malagasy lemurs.

  • Temporal heterothermy refers to animals that are poikilothermic or homoeothermic for a portion of the day, or year. Often, body temperature and metabolic rate are elevated only during activity. When resting, these animals reduce their metabolisms drastically, which results in their body temperature dropping to that of the surrounding environment. This makes them homoeothermic when active, and poikilothermic when at rest. Bats and hummingbirds go into what is known as torpor and bears hibernate. Both are examples of heterothermy; where the internal temperature of the animal drops during specific periods of time, usually when food is scarce.
  • Regional heterothermy describes organisms that are able to maintain different temperature zones in different regions of the body. This usually occurs in the limbs, and is made possible through the use of counter-current heat exchangers. These exchangers equalize the temperature between hot arterial blood going out to the extremities and cold venous blood coming back, thus reducing heat loss. Penguins and many arctic birds use these exchangers to keep their feet at roughly the same temperature as the surrounding ice which prevents the birds from getting stuck to ice sheets!

Got it? Not so fast. There is another factor in determining whether an animal is so-called warm or cold-blooded.

This graph shows us the difference between endotherms and ectotherms. The jaguar keeps her body roughly the same temperature throughout the day. Her fur keeps her warm and she can pant to cool down. She has to find plenty of food to keep her metabolic rate constant. The Burmese vine snake is dependent on its environment to provide shade or sunny places to keep warm. It can slow down its metabolic rate if food is scarce.(Graph by Kirsten Pisto/WPZ)

Organisms can generally be divided into two types of thermoregulators: endotherms and ectotherms.

Endotherms (most mammals and birds) create most of their heat via metabolic processes, burning cell's energy, to produce heat, and are colloquially referred to as warm-blooded (deriving energy from food intake).

For ectotherms (reptiles, fish and most amphibians), temperature is mostly a function of the environment, and they are sometimes referred to as cold-blooded (soaking up sunrays or laying on a hot rock).

Nina, a western lowland gorilla, keeps her energy high with a Valentine’s treat. Gorillas constantly forage for nutritious snacks. (Photo by Ryan Hawk/WPZ)

People are both endotherms and homeotherms; we maintain a constant temperature through our metabolic process (perhaps why we crave more carbs in the winter). Fish are ectotherms: if they live in a small stream that has varied temperatures, then they are poikilothermic; however, if they happen to live in a large body of water which keeps a stable temperature, then they become homeotherms.  Most endotherms are also homeotherms, but not all ectotherms are poikilotherms…agh!

Kalisa and Hubert enjoy an enrichment treat. Lions are predatory, eating very large meals less often. Both species are homeothermic endotherms, relying mainly on food intake to regulate body temperature. (Photo by Lauren LaPlante/WPZ)

So, who holds the advantage? Like many things this time of year, it’s complicated.

Ectotherms get most of their heat from environmental sources such as sunlight energy, therefore they have less dependence on respiration for generation of heat. Because of this, ectotherms such as reptiles can survive on just a single large meal per week. They are less vulnerable to fluctuations in food supply, increasing their chances of survival. On the other hand, tropical ectotherms may be particularly vulnerable to climate warming. 

Endotherms on the other hand, can survive in harsher conditions since they don’t rely on the environment for their warmth. Another advantage is that endotherms can have internal reproduction, which depending on the predators in the environment, can be more successful than egg-based reproduction. Of course, all this internal temperature control takes a lot of energy, placing a strong dependency on food intake.

A bearded dragon manipulates his body temperature by changing locations throughout the day. Bearded dragons are ectothermic, and poikilothermic. During the cool mornings, these lizards take advantage of the sunshine and bask on the warm rocks. If they get too hot, they can angle their body away from the sun or seek shade in a burrow. It is interesting to see them stick part of their body in the sun, and part in the shade to get that perfect temperature. (Photo by Ryan Hawk/ WPZ)

Our advice for Valentine’s Day? Engage in some kleptothermy. Kleptotherms share, or steal, each other’s body heat! Animals that nest together increase their thermal inertia, reducing heat loss and providing each other warmth. There’s no discrimination amongst kleptotherms; an ectothermic animal can take advantage of an endotherm’s cozy burrow!

The zoo can be a romantic spot for hot blooded creatures, especially on chilly February afternoons. Bring your sweetheart and take advantage of some kleptothermy of your own… we hear the Jaguar Cove is particularly cozy. Photo by Rathbone Images.

Whether endo- or ectothermic, all animals regulate and maintain their body temperature with physiological adjustments and behavior. Snakes bask in the sun. Kangaroos lick their arms to cool down. Bears eat more berries and fat-rich salmon in the summer to store energy during the winter. Ostrich stick out their long necks so that they can conduct body heat to the air. Humans sweat when they are too hot, and shiver when they are too cold. (Shivering helps increase heat production as respiration is an exothermic reaction in our muscle cells.) Iguanas soak up the warmth from sun soaked rocks. Crocodiles seek cool mud in the hot afternoon. Orangutans cover themselves with large leaves when they feel too hot. Fox grow thicker fur in the winter. And hummingbirds simply head south.

So, whether you are gearing up for roses and chocolate with your schnookie-pie or celebrating anti-Valentine’s Day with your friends, remember, we’re all just homeotherms trying to keep warm.


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