The Hot-Blooded Dinosaurs: Reconstructing Dinosaur Physiology

Among modern vertebrates, some gross generalizations:
Birds and mammals are warm-blooded; that is, they are warmer than the environment around them in typical temperate and colder environments. Crocodilians, lepidosaurs, turtles, amphibians, most fish, and almost all invertebrates are cold-blooded: their bodies are generally only about as warm as the general environment around them, so consequently they feel cool to the touch outside of tropical situations; in contrast, warm-blooded animals have temperatures largely independent of the outside temperature, so they feel warm to the touch. Need to be precise as to definitions of terms. "Warm-blooded" and "Cold-blooded" actually encompass several different (although related) topics:

• Energy Source: whence comes the majority of the energy to "run" the animal?
  • In "Cold-blooded" animals, the main energy source is the sun (and external environments in general): called ectotherms ("outside heat")
  • In "Warm-blooded" animals, the main energy source are extra mitochondria whose main purpose is to convert food energy to heat energy: called endotherms ("inside heat")
• Metabolic Rate: how much food energy ("fuel") is used up over time?
  • In "Cold-blooded" animals, rate of fuel usage is low: called bradymetabolic ("slow metabolism")
  • In "Warm-blooded" animals, rate of fuel usage is HIGH: called tachymetabolic ("fast metabolism")
• Temperature Variation over Time: how stable is the body temperature over time?
  • In "Cold-blooded" animals, body temperature fluctuates with the external environment: called poikilotherms ("fluctuating heat")
  • In "Warm-blooded" animals, body temperature regulated by internal mechanisms and thus more stable: called homeotherms ("same heat")

A typical cold-blooded animal is an ectothermic bradymetabolic poikilotherm: needs to get its energy from the sun and fluctuates with external environment (but can moderate fluctuations by moving from sunlight to shade and vice versa); however, needs very little food (snakes can go weeks without feeding, for example). Cold blooded animals become torpid at night and in colder weather.

A typical warm-blooded animal is an endothermic tachymetabolic homeotherm: its body temperature is stable and activity levels can remain high for long periods of time, at night, and in colder weather; however, needs a LOT of food or will die (imagine the effects of not feeding a cat or dog for weeks!).

Additional issues to consider:

• Resting metabolic rate (RMR) vs. active metabolic rate (AMR): "warm-blooded animals tend to have RMRs 4-10x that of similar sized "cold-bloods", but AMR is similar in both
• Duration of sustained activity: "warm-blooded" animals tend to have longer durations of sustained activity
• Recovery time between periods of activity: often much shorter for "warm-blooded" animals

Why evolve such an expensive trait as endothermy? Some suggestions have included:

• Increased aerobic capacity, allowing for greater total activity levels and greater ability to recover from sustained activity
• Greater environmental tolerance: endotherms can live in wider range of latitudes and altitudes
• Increased metabolic efficiency due to homeothermy: can "fine-tune" physiological systems to a narrow range of temperatures
• Increased ability for parental care: both brooding/gestating at constant temperature, and increased ability to watch over young

Note that it is not just mammals and birds that are "warm-blooded". For example, tunas, billfish (sailfish, swordfish, marlins), lamniform sharks (like great whites and makos), boid snakes (pythons, etc.; but only while brooding), and certain plants (which aren't "blooded" as such, but some can emit internally-generated heat).

When dinosaurs were first discovered, they were interpreted as being no more than gigantic cold-blooded lizards. However, as early as 1842 Owen (in the very paper in which he named "Dinosauria") speculated that dinosaurs may have been warm-blooded like mammals. During most of the 20th Century the model of dinosaurs as cold-blooded returned. Work by John Ostrom (of Yale University) and his colleagues and students (especially Robert Bakker) presented new information that dinosaurs were in fact warm-blooded. This hypothesis generated considerable research (both in support and in attempts to falsify it): this change in thinking about dinosaurs and renewed interest in dinosaurian studies has been termed the "Dinosaur Renaissance".

Among the lines of evidence supporting dinosaurian warm-bloodedness:

• Bone histology (microscopic analysis of tissue and cellular structure) can test between bradymetabolic and tachymetabolic organisms. Bone becomes reworked (that is, removed then redeposited) as a normal part of vertebrate physiology, as bone is a major store for nutrients like calcium and phosphorus. Bradymetabolic animals show little sign of reworking, as their slower metabolism does not need as much nutrients as quickly. In contrast, tachymetabolic animals show considerable reworking. Examination of the bones of dinosaurs show a high degree of reworking, even as juveniles.
• Using skeletochronology, the maximum rate of growth of dinosaurs can be calculated. When plotted against body size, it is found that dinosaurs had much higher growth rates than ectotherms of the same size; in fact, the dinosaur growth rate is about the same as mammals and ground-dwelling birds.
• Texture of bone also shows signs of the rate of growth. Dinosaurs show fast rates similar to mammals; pseudosuchians and other archosauriforms show an intermediate rate; and ectotherms show a slow growth rate.
• Computer models of the required metabolic rates required for even walking and slow running for large bipedal dinosaurs exceeds the metabolic rate of ecotherms. So if they actually moved, big theropods HAD to have been endotherms.
  • And the data for small dinosaurs and dinosauromorphs are at least consistent with endothermy for walking, and require endothermy if they were even moderate runners (as their skeletons and foot print evidence shows).
• Examining the nutrient foramina (holes for blood vessels into and out of bones) in dinosaurs shows that the amount of blood flowing through dinosaurs greatly exceeded modern ectotherms, and even modern mammals. This suggests that the metabolic rate of dinosaurs was indeed very high.
• Comparison of the oxygen isotope data of dinosaurs with known ectotherms of the same environment (modern-style crocodilians, turtles, and amphibians) finds that the dinosaurs show lower body temperatures in the hottest environments and lower body temperatures in moderate and colder environments. This suggest that dinosaur body temperatures were independent of the local environment.
• Predator-Prey ratios (first considered by R.T. Bakker looked at the trophic relationships in communities to try and determine the thermophysiology of dinosaurs and other extinct forms. His technique:
  
  • In modern endothermic communities very few predators compared to many herbivores (tachymetabolic predators require a lot of food, so only a few can survive in a given region).
  • Bradymetabolic predators require a lot less food, so same amount of potential food can support many bradymetabolic predators.
  • In order to calculate P/P ratios, Bakker had to consider the different sizes of the various populations. Used biomass (# kgs or tons of flesh) rather than number of individuals
  • Found that modern populations had P/P ratios of 0.5-4 %
  • Looking at fossil record, found:
    • Basal synapsid-dominated faunas of the Early Permian: 25-30%, much higher than modern populations. Most paleontologists have accepted this as a cold-blooded community
    • Therapsid-dominated faunas of the Middle and Late Permian and earliest Triassic: 10-20%, seemingly between endo- and ectothermic populations
    • Pseudosuchian-dominated faunas of the Middle and Late Triassic: 10-20%, as in therapsid communities
    • Dinosaur-dominated faunas of the Jurassic and Cretaceous: 0.5-3.5%, as in modern endotherms!
    • Mammal-dominated faunas of the Cenozoic: 0.5-4.5%, known endotherms

Let's consider the equations of life. First, the aerobic respiration equation, the primary means by which animal cells operate:

C₆H₁₂O₆ + 6O₂ yields 6CO₂ + 6H₂O + Energy

(That is, food (glucose) plus oxygen yields waste carbon dioxide and waste water, plus energy).

If an animal's cells can't get enough oxygen, there is a second way of getting energy: the anaerobic respiration equation:

C₆H₁₂O₆ yields 2C₃H₆O₃ + Energy

(That is, food yields lactic acid plus energy (although much less than the aerobic respiration.) Lactic acid itself needs oxygen to break down, so you cannot run on anaerobic respiration for very long.

If you want to evolve endothermy, you need to:

• Increase glucose intake, plus...
• Increase oxygen intake, plus...
• Increase the speed of distribution of glucose and oxygen throughout the body, plus...
• Deal with excess carbon dioxide, water, and heat.

So, where do we stand on dinosaur metabolism?

• All living dinosaurs (Aves) are endothermic tachymetabolic homeotherms
• The living outgroups (crocodilians, lepidosaurs, turtles) are all ectothermic bradymetabolic heterotherms
• Non-avian dinosaurs show many anatomical features suggesting levels of activity higher and/or more continuous than that seen in modern "cold-blooded" animals
• Non-avian dinosaurs show growth patterns comparable to those of modern endotherms, and unlike those of modern and extinct ectotherms

What would be necessary to justify the above observations?

• Non-avian dinosaurs would need active ventilation (breathing) to power the muscles and to fuel the growing tissue
• Non-avian dinosaurs would need strong, active heart to get the oxygen to the muscles and tissues
• Non-avian dinosaurs would need structures to control heat

Is there evidence for these features in dinosaurs? YES!: Dinosaur Respiration:

• Primitive tetrapods are subject to Carrier's Constraint: the same muscles for breathing are used for locomotion (bending from side-to-side). Thus, most primitive tetrapods tend to hold their breath while breathing, meaning that they rely more on anaerobic respiration and require long recovery intervals.
• Mammal-style diaphragm breathing is an advanced therapsid feature; most tetrapods breath by gulping air and by rib breathing
• Specialized lizards developed neck breathing separate from rib breathing
• Living dinosaurs (birds) have extremely specialized breathing:
  • Pump their lungs by rocking their hips up and down
  • Special air sacs in the torso, vertebrae, and limbs help keep the air flowing in one direction, rather than a simple in-and-out
• Crocodilians have their own specialized breathing:
  • Pubis is mobile, and rocks back and forth pushing & pulling the liver
  • Functions like the mammalian diaphragm, to have additional active breathing
  • Recent analysis shows that they, like birds, had unidirectional flow through their lungs (with passages rather than airsacs)
• Speculation: belly breathing is an archosaurian synapomorphy:
  • In primitive archosaurs, primitive pseudosuchians, and most dinosaurs other than birds, muscles from the pelvis would pull gastralia down, which would inflate the lungs
  • This would give these animals extra oxygen for their metabolism
  • Becomes modified in crocodilians (liver pump with mobile pubis), pterosaurs (a mobile "prepubis"; another liver pump?), birds, and ornithischians (mobile pubes or other parts of the pelvis in some ornithischian groups)
  • Additionally, the air flow in all archosaurs would have been unidirectional
• Furthermore, strong evidence that theropods and sauropods (at least) had air sacs like those of birds:
  • Chambers in vertebrae are very similar to those of birds
  • Air sacs may have been present in other dinosaurs, but apparently did not enter the vertebrae

Dinosaur Hearts:

• Turtles and lepidosaurs have three chambered hearts
• Birds and mammals have four chambered hearts:
  • A "double pump" system, so the heart acts as a control between lungs and body
  • Shunts blood to lungs before going out to body, so all the blood getting to the tissues are fully oxygenated
  • Also, can allow these animals to be taller, since the heart pressure control separates lungs and body, and therefore pressure on lung blood vessels won't get too high
• Crocodilians actually have specialized (NOT primitive) four-chambered hearts:
  • Operate as four-chambered heart on land, shifts to two chambered underwater since doesn't need to get blood to lungs
• Since both birds and crocodilians have four-chambered hearts, assumption is that all extinct archosaurs, including non-avian dinosaurs, did too

Heterometabolism:

• Changeable metabolic rate: tachy- to bradymetabolic.
• Two main types: behavioral and ontogenetic:
  • Behavioral heterometabolism:
    • Normally operate as bradymetabolic, but shift into "high gear" in certain circumstances
      • Living examples: sharks in feeding frenzy; pythons while brooding
      • Some have suggested that specialized breathing structures may have let dinosaurs be "turbo-charged", but have fully ectothermic physiologies
  • Ontogenetic heterometabolism:
    • In most animals, metabolic rates slow down as age (and thus size) increases
    • Perhaps in dinosaurs was more extreme
      • Problem: No good living examples

Ancient Atmospheres
Even as dinosaurs were evolving, the atmosphere they were breathing was evolving, too. Geochemists have seen that the ratios of various gases, including oxygen, have varied over geologic time. At least some models suggest that the Middle Jurassic though the end of the Cretaceous had oxygen levels exceeding the present 20%. This would mean that every breath a dinosaur took would have more oxygen, making it easier to power a high metabolism.

Furthermore, experiments of growing plants of Mesozoic varieties under Mesozoic-style atmospheres suggests that their productivity (essentially, the amount of nutrients they produce per area per unit time) could go up 2 to 3 times present day conditions. If so, then there would have been more food available per unit area for the herbivores (and from this up the energy pyramid), again making it easier to be an endotherm in these conditions.

Warm-blooded Protocrocs?: Most studies assume that endothermy evolved sometime after the bird lineage (Ornithodira) and the crocodilian lineage (Pseudosuchia) diverged from each other. This is because crocodilians are ectotherms, as are all the next several outgroups (lepidosaurs, turtles). However, what if crocodilians were not ancestrally ectotherms, but instead reverted to a cold-blooded physiology from warm-blooded ancestors?

There is some evidence that this is the case:

• Most extinct outgroups to the living crocodilians had a more upright stance (in fact, many of them parasagittal) allowing for more aerobic breathing (and at least one genus shows growth rates comparable to dinosaurs, pterosaurs, and mammals)
• Crocs retain many ancestral features (unidirectional flow in the lungs; four chambered heart; etc.) that are useful for endotherms but not at all required for ectotherms (since lepidosaurs, turtles, and amphibians do fine without them)
• Crocs have mitochondria of the "leaky" type used in endotherms to generate extra heat: however, they have far fewer of them per cell than living birds and mammals.
• Living crocodilians are aquatic ambush predators living in thermally stable environments: these are conditions which would favor selection away from high metabolic rates to slower ones

This has led to speculation that the ancestral archosaurs were in fact more warm-blooded than crocodilians, and that the latter evolved "cooler blood" after the divergence of their lineage from other types of pseudosuchians. Thus, the origin of avian warm-bloodedness would not have occurred within Dinosauria, but at least in part before the bird line-croc line split.

A New(-ish) Idea: Mesothermy

In 2014 a study came out proposing that dinosaurs were intermediate between endotherms and ecotherms, and the authors termed them "mesotherms". (In fact, Dr. Scott Sampson had proposed the concept and the name "mesothermy" years earlier...). The particular study estimated both the maximum growth rate of fossil dinosaurs and their inferred metabolic rate (based in part on growth rate, so the whole study may wind up being a circular argument...). They found that most Mesozoic dinosaurs (including Archaeopteryx) fell in a range intermediate between where modern endotherms and modern ectotherms plotted (but in the same region as such animals as tunas, sharks, echidnas, etc.)

The authors interpreted this to mean that dinosaurs had the ability to generate internal heat, but did not greatly regulate their body temperature. So in fact, what they call "mesothermy" is technically not intermediate between endothermy and ectothermy, but between homeothermy and poikiliothermy. And thus dinosaurs in their interpretation would be in terms of this course endothermic mesotherms. In their interpretation, the rise of actual warm-bloodedness in the bird lineage occurred somewhere well within Pygostylia. Future analyses will have to be done to see if this model is upheld.

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A relevant video:

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Last modified: 12 November 2019