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
- When we say that an animal is cold-blooded, we are generally describing an animal that:
- Relies on external heat sources and sinks.
- Allows its body temperature to rise and fall with that of its environment.
- Is, at best, capable of only short episodes of intense activity.
- When we say it is warm-blooded, we mean:
- Relies on internal heat sources
- Maintains a constant body temperature most of the time.
- Is generally capable of prolonged episodes of intense activity.
- Thus, we are really talking about the presence or absence two separate capabilities:
- The ability to maintain constant temperature.
- The ability to exert large amounts of energy over a reasonably long time.
- The ability internally to generate body heat.
- Terms dealing with these three general concepts:
Fundamentally, these abilities are determined by separate and distinct functions of the body, however their consistent association in wide ranges of critters shows that there are some connections between them.
- Constancy of body temperature:
- Homeothermy - the ability to maintain a constant temperature.
- Poikilothermy - the practice of allowing body temperature to vary with the temperature of the environment
- Source of body heat:
- Endothermy - body heat generated internally
- Ectothermy - reliance on external heat sources
- Activity levels:
- Bradymetabolic - low energy
- Tachymetabolic - high energy
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:
- ATP: Inside living cells, the energy for any kind of work is supplied by breakdown of the molecule adenosine triphosphate or ATP. This is a molecule that the cell can manufacture quickly when it needs it. Unfortunately, it is very unstable and "spoils" almost instantly, so it is not a good medium for storing energy or moving it around. For this purpose, glucose is used.
- Glucose: During photosynthesis, the energy of sunlight is captured and stored in the form of molecules of glucose, a simple sugar. Glucose's virtues include its being stable, and easily transformable into more complex sugars. These properties make glucose the ideal medium for storing energy and transporting it. The problem: The metabolic machinery inside the cell, where energy is actually used, can't unlock the energy of glucose directly. It wants ATP.
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:
- Anaerobic respiration: Several exchange pathways produce ATP from glucose within the cell at modest rates, fewer than ten ATPs for each glucose. The problem: These processes create toxic byproducts. Yeast, for instance, produce alchohol. Vertebrate anaerobic respiration produces lactic acid. This causes the sensation of soreness in your muscles you exercise vigorously. Thus, anaerobic respiration is only good for short bouts of heavy activity, after which the organism must rest while its system is cleansed of this toxin.
- Aerobic respiration: Most organisms also have the ability to exchange glucose for ATP at the much better exchange rate of up to 36 ATPs for each glucose, using this pathway. Furthermore, there are no toxic byproducts.
The problem: For aerobic respiration to work, oxygen molecules must be present.
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.
- Now suppose you start doing push-ups. Eventually, most people reach the point at which their heart and lungs simply can't supply all the oxygen that that their muscle cells need.
- When this happens, aerobic respiration shuts down in the muscles and anaerobic respiration takes over. ATP continues to be produced, but lactic acid starts to build up.
- Eventually, the lactic acid reaches the concentration where the cells can no longer function and activity is curtailed. As you lie, panting, on the floor your body switches to aerobic respiration, and begins to clean up the lactic acid mess.
Note: ATPs are now being used up on the lactic acid cleanup. Aerobic respiration supplies this energy. For this reason, the presence of lactic acid is said to reflect an oxygen debt.
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.
An animal makes more glucose available to its cells by:
Oxygen plumbing in archosaurs and synapsids:
- Digesting efficiently: The first amniotes probably ate in the manner of modern squamates, swallowing prey whole and slowly digesting it. Several small peices of food present more surface area to the digestive juices than one big one. In archosaurs and derived synapsids, we start to see teeth that are designed to chop prey into smaller pieces before swallowing. These animals had found a way to release glucose to their cells faster.
- Eating more. Of course, as efficient digestion made more energy available, that energy could be invested in longer hunting, which would yield yet more food.
An animal makes more oxygen available to its cells by:
- Moving more air through its lungs: Modern lepidosaurs breathe by expanding their rib cages. The side-to-side flexion of the thorax during locomotion interferes with breathing, often requiring them to hold their breath while running. In archosaurs, the body is held straight during locomotion, reducing this interference. In synapsids, it is held straight or flexed vertically. In both groups, breathing is aided by a muscular diaphragm.
- Separating oxygen rich blood from oxygen depleted blood. The ancestral amniote 3-chambered heart did not effectively separate oxygenated and deoxygenated blood. Both living synapsids (mammals) and archosaurs have 4-chambered hearts that accomplish this separation, allowing a much higher concentration of oxygen to reach their cells.
- Of course, an organism that can move oxygen back and forth effectively, is also good at moving CO2 and other wastes.
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:
- In the oceans: The problem is not so severe for marine animals, since they live in environments whose temperature is very constant.
- On land: Here, the problem is severe, since every day the temperature may vary greatly. Land organisms must somehow warm up quickly to become active, but not overheat and die.
Organisms use two basic heat sources:
Eliminating excess heat:
- The environment: Heat from the environment will diffuse into organisms when given the chance, so that an animal can warm up by just sitting in a warm place. On a cool day and you will see turtles stretching out their legs, making ever effort to pick up heat from the sun. Many animals begin their days by basking, warming up until they enter the range of maximum enzyme efficiency.
- Metabolic heat - the first big connection: Aerobic respiration generates much heat as a by-product. (Think about how your body heats up when you are exercising, metabolizing energy quickly.) Animals with a high aerobic scope have the option of deliberately stepping up their metabolic rates to elevate their temperature. Some have specialized brown fat cells whose job is simply to generate heat. Many also use their muscles for this purpose. When you shiver from cold, your body is kicking into "metabolic overdrive," using your muscles to generate emergency heat. Note: animals with low aerobic scopes cannot do this.
This requires that heat be shed to the environment:
- Heat sinks: An animal can accelerate its heat loss by sitting in a cool spot. Small animals with high surface area to volume ratios have an easy time, since they lose heat more quickly, and because any little spot of shade will suffice as a heat sink.
- Large animals cannot shed heat quickly, and so must avoid overheating to begin with by curtailing their activity (and hence not warming themselves up with their own metabolic heat,) and spending hot periods immersed in water, the ultimate heat sink.
- Altering surface to volume ratios: One way for a large animal to regulate its temperature is for it to assume a shape that allows a greater surface area to volume ratio.
- Sweating: Some mammals produce sweat, which accelerates the dumping of excess heat through evaporative cooling. No archosaur is known to do this. The problem: sweating requires one to lose water.
- Panting: Accelerating the flow of air over respiratory surfaces accelerates the rate of heat loss. These surfaces are usually moist allowing evaporative cooling to to play a role. This strategy is seen in mammals and archosaurs. The problem with evaporative cooling driven by the lungs: Only animals that can move large volumes of air through their lungs are really good at this. Thus, squamates don't usually bother.
Thus, the second big connection: It stands to reason that an animal that can move enough air through its lungs to pant effectively can also move lots of oxygen into its blood stream. Therefore, ironically, the animals that are best at shedding excess heat by panting tend to be those that are able to generate lots of metabolic heat.
Thus we see that the mechanisms of energy metabolism and temperature regulation are connected.
- Rapid metabolism ====> heat that can be used to warm the body up.
- The use of panting to reduce temperature also makes more oxygen available to tissues.
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.
- Daily heterothermy: torpor occurs at night. Hummingbirds have such energy requirements that they would starve overnight but for the ability to become torpid.
- Seasonal heterothermy: torpor occurs during winter. E. G.: Small mammals.
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.:
- Many marine pursuit predators (like mako sharks and tuna) use the heat of venous blood returning from trunk muscles to warm arterial blood leaving the gills.
- Some large bodied insects do the same. Bumblebees can actually decouple their flight muscles from their wings. By vibrating them for several minutes before takeoff, they can begin flight at optimum temperature.
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.