Among modern vertebrates, some gross generalizations:
Birds and mammals are warm-blooded; Crocodilians, lepidosaurs, turtles, amphibians, and most fish are cold-blooded.

Old debate in dinosaur studies: were they warm-blooded or cold-blooded?

Owen in 1842 suggested dinosaurs might have been warm-blooded, or at least more warm-blooded than typical modern reptiles.

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 specialized sub-cellular structures 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…).

How can people determine the thermal physiology of extinct animals like non-avian dinosaurs?

Owen suggested dinosaurs might have been warm-blooded because:

• Upright posture: today, all living animals with upright posture are warm-blooded

Many late 19th Century paleontologists considered dinosaurs to be more similar to modern warm-blooded animals in terms of activity levels.

During early 20th Century, shift to lizard-like concept for dinosaurs.

Concept of warm-blooded dinosaurs revived in late 1960s by Ostrom because of a number of lines of evidence:

• Upright posture: as in Owen
  • Problem: No causal relationship ever established: just because all living animals with upright stance are endotherms does not mean that upright stance requires endothermy
• Dental batteries of hadrosaurids and ceratopsids: useful for chopping up food into very fine particles for fast digestion, but bradymetabolic animals don't have fast digestion; suggests tachymetabolism in hadrosaurids and ceratopsids
  • Problem: Most ornithischians and all sauropods lack sophisticated chewing or slicing teeth
    • Does not negate observation that hadrosaurids and ceratopsids have dental batteries
    • Modern herbivorous birds make do without grinding teeth by using gastroliths (gizzard stones), which are found in non-hadrosaurid, non-ceratopsid herbivorous dinosaurs
    • However, gastroliths are also found in some ectotherms, so their presence is NOT evidence of endothermy!
• Sickle claw and stiffened tail of dromaeosaurids: suggested a more dynamic mode of attack for dromaeosaurids than in monitor lizards or crocs
• High blood pressure necessary to pump blood into brains of tall theropods, ornithopods, and (most especially) sauropods: requires powerful, active heart
• Latitudinal distribution: dinosaurs (and therapsids) found in Mesozoic (and Permian-Triassic) polar regions, although not as cold as today would still be cooler than climates preferred by typical modern cold-blooded animals
  • Problem: Earth's climate WAS warmer in Mesozoic
    • However, some polar sites contain dinosaurs & mammals but not crocs, lepidosaurs, turtles, etc., while other sites in Alberta of same age are chock-full of known ectotherms
      • Maybe the dinosaurs migrated out of the polar sites during cold winters?
        • BUT energy requirements for large scale migration might arguably require endothermic levels of metabolism!!
        • Also, baby dinosaurs found in these sites: unlikely to have migrated
• Origin of birds from coelurosaurs: birds are known to be warm-blooded, so their immediate relatives might have been, too
  • Problem: Some argue that early birds themselves could have been ectothermic, with endothermy evolving AFTER Archaeopteryx
• Complex social behaviors for at least some dinosaurs: no causal link, but more typical of modern mammals and birds than crocs, lepidosaurs, and turtles
  • Problem: As with upright stance, no causal link between endothermy and complex social behavior
  • Also, no evidence for such behavior in most dinosaurs

Colleague from France: Armand de Ricqlès added additional line of evidence:

• Bone microstructure: lots of signs of reworking (bone being resorbed as mineral source in metabolism, and redeposited), lots of Haversian canals.
• Typical bradymetabolic animals have little reworking and few Haversian canals; typical tachymetabolic animals have lots. Dinosaurs resembled tachymetabolics.
  • Problem: Suggestion that very old bradymetabolic animals might develop tissue similar to younger tachymetabolic animal
    • However, even baby dinosaurs show endothermic-style bone tissue

Ostrom's undergrad student Robert T. Bakker: main advocate for the "hot-blooded" dinosaurs model. Added his own observations:

• Ecological replacement: many paleontologists argued that therapsids were at least partly warm-blooded, but were replaced by archosaurs.
  • Problem: We do not know for certain that an ectothermic group would necessarily be out competed by endotherms
  • Also, other possible selective factors (i.e., water retention)
• Predator-Prey ratios: we'll discuss these in more detail in a later lecture.

Additional lines of evidence (primarily from 1980s and 1990s):

• Oxygen isotopes: can determine body temperature and (importantly) variation of body temperature over time: dinosaurs show stable temperatures, while contemporary non-dinosaurian reptiles show larger variation.
  • Problem: Large bodied animals expected to have stable temperatures:
    • See gigantothermy later lecture
  • However, baby dinosaurs match adults in stable temperature; don't match poikilotherms from same environment
• Growth rate: fantastic growth rate (see earlier lecture) suggests tachymetabolism.
  • Problem: Maybe due to very favorable conditions of Mesozoic: allow fast growing ectotherms
    • However, known contemporary ectotherms (like giant crocs) show typical slow-growing rate comparable to modern ectotherms
• Presence of feathers on non-avian coelurosaurs: since not flight features, might have been for insulation (which small endotherms need or they lose too much heat).
  • Problem: Not all dinosaur clades show feathers

Not everyone convinced that dinosaurs were fully endothermic tachymetabolic homeotherms.

Two main types of evidence to the contrary:

• Evidence suggested to counter claims of dinosaur endothermy (shown as Problems in the text above)
• Evidence suggest to support dinosaurian ectothermy

Lines of evidence supporting dinosaurian ectothermy:

• Small brain size:
  • Most dinosaurs characterized by brain sizes expected in crocs or lizards of that size; modern endotherms all have much larger brains!
    • Problem: However, no causal link established between brain size and metabolism
    • Also, coelurosaurs at least have larger brains than typical dinosaurs, and proportionately larger brains than contemporary mammals (which are accepted as endotherms)
• Small head size in herbivores:
  • Lack the big maws of large herbivorous mammals: how could they get enough food?
    • Problem: However, many large flightless birds have tiny heads, yet they are endotherms
• Lack of specialized teeth in most herbivorous dinosaurs:
  • Non-hadrosaurid, non-ceratopsid ornithischians and sauropods lack sophisticated chewing or shearing teeth
    • Problem: These other dinosaurs are known to have gizzards, which could process the food
• Overheating:
  • Because Mesozoic was warm, large dinosaurs would overheat if endothermic, so must have been ectotherms
    • Problem: Very large mammals (known endotherms) are found in comparably warm periods of Cenozoic
    • Some dinosaurs may have thermal "radiators" to dump heat (see below)
• Growth lines:
  • Dinosaur bones show "growth rings" (Lines of Arrested Growth, or LAGs), typical of reptiles and (once thought to be) lacking in mammals
    • Problem: Now known in perfectly good endothermic mammals
    • Are a symplesiomorphic feature of vertebrate bone growth; may not signal any aspect of thermal physiology
• Conspicuous potential solar collectors &/or radiators:
  • Stegosaur plates, neoceratopsian frills, sails in mid-K equatorial dinosaurs might be good radiators to dump heat or collectors to get heat
    • Problem: However, might be for display instead
    • Additionally, some endotherms (like elephants and their ears) have large solar radiators

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Last modified: 14 July 2006