Thermal Strategies

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Basic concepts in thermal matabolism: In the last 20 years, a big issue hs been made of whether or not pterosaurs and dinsaurs were warm blooded or cold blooded. Indeed, this debate was one of the things that first peaked my nterest in dinosaurs.

Basic function 1: Metabolism
The set of chemical processes by which organisms capture energy from the environment and use it to perform work or build tissues
. Green plants capture energy from sunlight by means of photosynthesis. Non-photosynthesizing organisms steal energy from other creatures by eating them. In the body, energy is stored in the form of chemical bonds. Releasing that energy to perform work requires special chemical pathways.

Two energy currencies:

Respiration: The cell's currency exchange:
So we have two energy currencies: glucose, the extracellular energy currency through which energy is stored and moved around; and ATP, the intracellular energy currency that the cell needs to actually perform work. Fortunately, several means of currency exchange exist, but their efficiencies vary greatly. The processes by which glucose is broken down and its energy transfered to ATP are collectively called respiration. There are two basic types.

Exercise and oxygen:

Organisms try to use both pathways to their best advantage. For normal activity levels, aerobic respiration produces all the ATP your cells need, as long as you have eaten enough glucose and breathed enough oxygen.

Aerobic scope:
The degree to which an animal can raise its rate of glucose metabolism from a resting state using only aerobic respiraton
is called its aerobic scope. Intuitively, we can see that a bird or mammal has a greater aerobic scope than a snail or sea urchin. In fact, the aerobic scope of most mammals is between 10 and 20, meaning their peak energy output is 10 to 20 times their resting energy output. The modern metabolic champions are insects, some of whom have aerobic scopes approaching 300.

What accounts for these differences? Within the cells, organisms, be they humans, hummingbirds, or turtles, exchange glucose for ATP in the same way. The amount of energy we can actually produce is mostly a function of things going on outside the cell.

To metabolize quickly, the cells must be well supplied with glucose and oxygen. It doesn't matter how capable the cells, themelves, are if an organism's plumbing is not designed to deliver these things to them.

When we consider differences in metabolic scope, we must concentrate of differences in "plumbing" anatomy.

Glucose plumbing:
An animal makes more glucose available to its cells by:

Oxygen plumbing in archosaurs and synapsids:
An animal makes more oxygen available to its cells by:

Metabolism II: Temperature regulation

Enzyme problem: the enzymes on which biochemical processes depend only operate within a narrow temperature range. When they become too cold, they slow down, eventually to a point at which activity ceases. When they get too hot, they get cooked and cease to function (i.e. their possessors die).

The result: Organisms (especially active ones that use a lot of energy) must keep their temperature level within a narrow range.

The temperature challenge

Obtaining necessary heat:
Organisms use two basic heat sources:

Eliminating excess heat:
This requires that heat be shed to the environment:

Composite Thermal strategies

Cold-blooded animals: Ectothermy + poikilothermy. Animals that lack the aerobic scope needed to generate much metabolic heat. On land, they usually depend on on locating environmental heat sources and heat sinks to maintain optimal temperatures. They tend to be small so that they can exchange heat with the environment rapidly. Being cold-blooded has the disadvantage of requiring an animal to spend a lot of time hanging out in heat sources or sinks. On the other hand, they can function using a low aerobic scope that can be maintained with small amounts of food.
Modern example: squamates, turtles, amphibians.

  • Warm-blooded animals: Endothermy + homeothermy. Animals that possess the metabolic scope to generate enough metabolic heat to stay warm, and have evolved mechanisms such as panting or sweating to shed excess heat. Thus they are less dependent on environmental heat sources and heat sinks (although these may be used, too. Go to a swimming pool in summer to see.) Warm-blooded animals must generally eat at least ten times as much food as cold-blooded ones the same size per unit of time in order to achieve this aerobic scope. Thus they needn't waste time looking for heat sources and sinks, but must spend much time satisfying their ravenous apetites. Unlike small ectotherms, small warm-blooded animals must prevent heat exchange with the environment by some sort of thermal insulation like hair or feathers.
    Modern examples: Mammals, birds.

  • Heterothermy: Endothermy + partial poikilothermy. When environmental conditions require an energy-intensive animal to remain inactive so long that it would starve, one response is to enter a state of torpor during which one is poikilothermic until activity can be restored.

  • Inertial homeothermy: A large ectotherm living in a thermally benign environment can maintain a nearly constant temperature by virtue of its low SA/VOL ratio.

  • Partial endothermy: Some animals elevate their body temperatures toward their optimum during intervals of activity by exploiting the heat of their muscles. E.G.:

    Ancient case studies

    Dimetrodon and other Pennsylvanian sail-backs.

    Late Permian and Triassic synapsids

    Many of these were quite large animals living in seasonally arid environments. How did they shed excess heat without sailbacks? Is it possible that they used cartilagenous versions of the nasal turbinates used by modern mammals?

    Typically, ectotherms don't huddle for warmth. So what is the meaning of this fossil, in which two individuals of Diictodon (small dicynodonts) died snuggled up in their burrow?


    What does postural evidence suggest about the likely aerobic scope of the Triassic theropod Coelophysis?

    Mid-Cretaceous thermal max - Return of the sailbacks

    An odd thing about the mid-Cretaceous - the thermal maximum of an ice-free world: During this time, several lineages of dinosaurs produced sail-backed morphs, but only those inhabiting the tropics.