How Did Dinosaurs Get So Big? Maybe Because They Were Couch Potatoes

By Eliza Strickland | July 7, 2009 2:45 pm

big dinosDinosaurs are objects of eternal fascination to we puny humans because of their sheer size. Some, like the long-necked sauropods, are thought to have had eight times the mass of an African elephant, the largest modern land animal.

Now a new model from zoologist Brian McNab suggests that the secret of dinosaurs‘ size was really quite simple: They had plenty to eat, and didn’t have to expend much energy in their daily lives. Says McNab: “Like couch potatoes sitting within easy reach of high calorie foods, the gargantuan size of dinosaurs most likely stems from the abundance of resources available, coupled with low energy expenditures” [Telegraph]. McNab’s findings also contribute to the long-running debate between paleontologists about whether dinosaurs were warm-blooded or cold-blooded creatures.

Warm-blooded animals like mammals and birds have to eat a lot more; they expend energy to keep their internal body temperatures constant, and so they have a high metabolic rate. But cold-blooded creatures such as reptiles rely on their environment for body heat, and their internal temperature fluctuates depending on the surrounding conditions [LiveScience]. McNab argues that dinosaurs could only have gotten so big if they didn’t have to expend energy on maintaining body temperature as warm-blooded creatures do, and could devote their energy to facilitating growth.

In the study, published in the Proceedings of the National Academy of Sciences, McNab argues that dinosaurs were “homeothermic,” somewhere in between warm and cold-blooded. They did not have a high metabolic rate, but their internal temperature did not fluctuate like that of  cold-blooded creatures. Instead, their sheer size kept their body temperature constant. “When you’re that big, you can’t cool off rapidly like a small lizard will,” said McNab. “You have a large volume, and you have comparatively small surface area. And so if you’re warm, you’re going to stay warm, unless something unforeseen happens” [LiveScience].

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80beats: How Did the Giant Flying Reptiles of the Jurassic Period Take Off?

Image: Wikimedia Commons

MORE ABOUT: dinosaurs
  • Nick

    We just need to find more dino DNA and sequence it, till we find the genes for warm or cold-blooded, then we can start classifying families of dinosaurs as warm/cold or inbetween blood. No amount of speculation will prove a damn their one way or another.

    Seriously, I’ve seen dinosaurs go from lizards to birds, from cold to warm to inbetween in my lifetime… but the DNA! That is new! Jurassic Park was one of those “wouldn’t it be cool if…” scenarios that actually had a real premise. We have found the DNA. Now we just need more. It’s one of the best legacies life has left us. DNA is the biological computer that understands the world.

  • Cambrico

    “You have a large volume, and you have comparatively small surface area. And so if you’re warm, you’re going to stay warm, unless something unforeseen happens” – Like a steep descent of temperature due to volcanism or an asteriod smashing Yucatan or all at the same time. The ones that didn’t die by the cool weather, died of hunger when their food (other animals or plants) died or just couldn’t eat enough to recover their body temperature or get so sick to recover or reproduce. A lot could be inferred if this study is some day verified. Allways a fascinating matter!!!

  • http://msn roger

    Let me see if I understand this. If I have plenty to eat, don’t work, don’t exercise, etc., I will both broaden and become very tall, is that right? Oh, and if my descendants continue this lifestyle, the trend will continue and perhaps my G-G-grandchilden will top out at forty feet, or so -yes?

  • Gary

    It is established fact the sun was much closer to Earth back then causing more heat. Also, the magnetic poles had changed many times and this affected gravity so that it was only half as much! So, with more heat and less gravity these gas-bag dinosaurs could float along like hot air balloons
    and hardly use any energy at all. Yay for speculative science!

  • hobgot

    It has to do with the ratio of the surface area of an animal to its mass.

    A shrew has a huge surface area compared to its mass and can eat its own body weight every day, an elephant eats only a small fraction of its weight daily.

    The paper above demonstrates that crocodiles increase their body temperature as they increase in mass. If this model is applied to dinosaurs it would indicated that the largest dinosaurs reached a point where their body temperature would kill them.

    While no direct evidence is available it is more then speculation, it is math.

  • Nava

    As Gary says that due to change in gravity the growth and size of dinosaurs were very large..

    I too accept , but due less gravity the mass of an object will only reduce . But in case of dinosaurs , why they have to increase size . I think that due high gravity of earth , the mass and size were big . As the gravity was more to withstand it they had broad feets and their mass was also more up to 5-6tonnes .

  • Tom

    Less gravity? The sun was so much closer that it caused a significant difference in temperature a mere 100 million years ago? Gary pegged my sarcasm meter. Nava needs to invest in a sarcasm meter.

  • Jockaira


    If the sun was closer to earth in dinosaur times, then what is the mechanism that results in the present distance of the sun from the earth today?

    Gravity is completely dependent upon mass. Are you saying that in dinosaur times, the earth was less massive than it is today. If the mass of the earth then was only half, then it means that the earth was similar to the moon in mass. Maybe the moon was larger then and somehow transferred some of its mass to the earth?

    It’s fun to speculate, but come down to earth sometimes. Gravity will help you do that.

  • Masoud Ramson

    I love this post very much. I will certainly be back. Hope that I will be able to read more insightful posts then. Will be sharing your knowledge with all of my friends!

  • Spyder

    Ok,gravity wasn`t less,but the sun was at a brighter,hotter point in its life.Thus plenty of food,warmth,and less requirement for using that energy to heat the body of a cold bloodied animal.But this expansion of size was due to this occurring over a vast scale (in comparison to our time frame to date) of time.So would your 50 generations removed grand kids end up 40 feet tall if this was to occur today?? No,warm bloodied animals get smaller during hot times and bigger during cold times.As this is evolution captured for us,all that is left is to apply common sense……and new data as it comes to hand.Bring on the ice age perhaps?????

    In this case,these evolutions were over millions of years and can be used as a gauge of our suns state of output also.So many things to be learned still though.And will DNA really give us more answers?? Or as most new information does more questions????


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