• As size increases, SA/V decreases.
• The mass of an animal, and the heat it produces, is based on its volume.
• The rate at which an animal can gain and lose heat is based on its surface area.
• Therefore, with bigger body size it takes longer and longer for heat to be lost or gained:
  • Become homeothermic without having the energy costs of endothermy!
• Problem: No good living models (elephants once thought to be partial gigantotherms, but does not now seem true; marine leatherback turtles are, but are not in same type of environment)
• Also, gigantothermy might apply to large dinosaurs, but would not apply to small species or to babies.

• 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
      • Ruben suggests 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

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 inosaurs 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 Breathing:

• Mammal-style diaphragm breathing is an advanced therapsid feature; most tetrapods breath by gulping air and by rib breathing
• Diapsids primitively have neck breathing separate from rib breathing
• Crocodilians have specialized breathing:
  • Pubis is mobile, and rocks back and forth pushing & pulling the liver
  • Functions like the mammalian diaphragm, to have additional active 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
• 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, pterosaurs, birds, and ornithischians
• 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

For more on vertebrate breathing, check out the website of experimental work on modern amphibians and non-avian reptiles.

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

Dinosaur Temperature Regulators:

• Some dinosaurs have conspicuous large sails or plates or frills or long necks or long tails that might have been used to dump waste heat
• However, other structures may have also been used to regulate temperature:
  • The antorbital fenestra (also the promaxillary and maxillary fenestrae of various theropods) housed soft tissue air sacs
    • These air sacs may have been useful to transport waste heat
  • Also, many larger dinosaurs have enlarged and/or elaborate nares
    • These may have been useful in dumping waste heat

The enlarged narial regions may support tissues for a different function: recovery of moisture. In living endotherms, rapid rate of respiration would dry out lungs if not for some specialized tissues called nasal turbinates:

• Rare or small in modern ectotherms
  • Click on the "Dynamic Cutaway: Coronal" animation on this website to see the lack of turbinates in a CAT scan of the modern Chinese crocodile lizard
• Extremely large in mammals, where they are scroll work of bone in the snout, supporting thin tissues which trap moisture going out, and rewets on way back in
  • Click on the "Dynamic Cutaway: Coronal" animation on this website to see the turbinates in a CAT scan of a house mouse
• Fairly large in many birds, but are cartilage rather than bony
• Also, some birds seem to rely on air sac system for this purpose
• Most non-avian dinosaurs do not show much evidence for internal nasal turbinates, but the air sac system and/or tissues in the enlarged narial regions of bigger dinosaurs may have served this function

Still much work to be done in interepreting the physiology of extinct dinosaurs

To Next Lecture.
To Previous Lecture.
To Syllabus.

Last modified: 14 July 2006