Physiology and metabolism in the fossil record I: The living base-line

John Merck

Robert Bakker, warm-blooded dinosaur apostle from Wikiquote

We long to know how ancient vertebrates lived.

To that end, we:

But one thing we have real trouble assessing are the characteristics of their metabolism. In the public eye, this often leads to a simplistic debate over whether they were warm-blooded or cold-blooded, with outspoken protagonists on both sides. (E.G.: Robert Bakker (right) and John Ruben.)

But what does it mean?

Fowler's toad - a bradymetabolic poikilothermic ectotherm
Being warm or cold blooded actually encompasses several different biological issues:

When we say that an animal is cold-blooded, we mean:

Red-browed finch - a tachymetabolic homeothermic endotherm
When we say a creature is warm-blooded, we mean:

Thus, we are really talking about three separate capabilities:

Each of these concepts has its own set of terms:

Why does temperature matter?

Metabolic processes are mediated by protein catalysts that are sensitive to temperature: Thus, to use metabolic energy effectively, a creature needs to keep its body within a narrow temperature range. This is easy for sea creatures, but a real challenge for land animals that inhabit environments with variable temperatures. Land vertebrates approach this problem with two (generally) distinct strategies:

A gray tree frog copes with hypothermia from Winnipeg Free Press


Body temperature is allowed to fluctuate with the environment. Behavioral responses including: are used to cope with or avoid the ill-effects of overheating or hypothermia.

From Bob the Ranger Wildlife Videos


Body temperature is maintained within a narrow range.

Eastern painted turtle soaking up the sun

How is homeothermy achieved?

Ectothermy: By sitting in microenvironments that serve as heat sources or heat sinks, animals can maintain elevated constant temperatures much of the time. Depending on the animal's size (small = low surface/volume ratio) and environmental conditions, these behaviors may take up much or little of its day. A special case:

Inertial homeothermy: For large animals with low surface/volume ratios (Galapagos tortoise), body temperature changes slowly. Such poikilothermic animals inhabiting mild climates can maintain a constant temperature with minimal behavioral modifications.

The take-home: achieving homeothermy requires both:

Achieving these goals requires separate and distinct behaviors and adaptations.

Brown and white fat cells from Sci.en.gist
Endothermy: Internal heat as a by-product of glucose metabolism: The take-homes:

Anaerobically metabolizing squamate from Ancestral Movement

Glucose and ATP:

Where does the ability quickly to metabolize glucose come from? In fact:

Energy pathways: There are two general ways in which a body cell can convert glucose into useful energy:

Exhaustion from Ask Naij
Vertebrates "prefer" to respire aerobically, but can add on anaerobic respiration during intense activity. Intense exercise:

The only creatures that can live entirely off of anaerobic respiration are single celled organisms that can eliminate toxic by-products by simple diffusion.

Side note: Metabolizing ATP requires access to phosphate ion (PO3). Where might vertebrates find a supply of that? (Nudge, nudge..)

Chelonoidis nigra vandenburgi - a bradymetabolic homeothermic ectotherm
Homeothermy, endothermy, and tachymetabolism usually go together but not always: From this we infer that the matter is not straightforward.

Hoverfly from Wikipedia

Resting metabolic rate: The base rate at which an organism metabolizes glucose while at rest. In endothermic homeotherms like mammals, this can be roughly ten times what it is in ectothermic creatures.

Aerobic scope: The factor to which an organism can elevate its activity levels over the resting state without slipping into anaerobic respiration. For most mammals, that is 10 - 20. The aerobic champs are insects like hoverflies, some of which have aerobic scopes approaching 300.

And yet, when they are at rest, hoverflies are poikilothermic!

Nevertheless, among vertebrates, there is a general relationship between one's competence as a homeotherm and one's overall metabolic rate.



Whether you need to burn it for heat or for chemical energy, you need glucose and oxygen. The body cells of animals are not that different in their ability to metabolize glucose. Where animals differ is in the arrangement of their plumbing. For an animal to maintain a high metabolic rate, its cells must be well-supplied with:

Chicken gizzard from Summa Gallicana

Glucose plumbing:

To make more glucose consistently available to its cells, an animal must:

Avian respiratory system from Zoology Goa University

Oxygen plumbing:

To make more oxygen available to its cells, an animal must:

Duck-billed platypus cooling off

Shedding Heat:

Obtaining heat is only half of the homeothermy problem. How do you eliminate excess heat?

So, cooling is distinct from warming in homeothermy. Is there, nevertheless, a connection between the two?