I assume there must be a mechanical limit on how big a beast can get, just as there is a limit on animal flight. After a point, the muscles needed to pull such strong load-bearing bones need more bone to support them, and so on and so on.

What is the limit on animal size? Could skyscraper-sized animals like Godzilla actually work in real life? Does it make a difference whether the animal is cold-blooded or warm-blooded? (Difficulties of keeping such a large body cool, etc.)

Hi Anna,

this is a very interesting topic! I have been involved in research on it for the last six years, as part of the Research Group on Sauropod Biology (http://www.sauropod-dinosaurs.uni-bonn.de/). We looked at sauropods because they were the largest terrestrial animals of all times, and beat mammals by a factor of 5 to 10, depending at what specific measurement you look.

There obviously are size limits at many levels. You pointed out two examples:
- mechanical limits due to the fact that increasing size causes mass to increase to the power of 3, while muscle forces and bone strength increase to the power of 2. One can adapt, e.g., by having columnar limbs (elephants, sauropods), one can reduce density (sauropods), but there will be a final point where getting even bigger means becoming too slow to collect enough fodder to support the body.
--> Here, the largest sauropods probably apporached this limit, so I guess using Amphicoelias and relatives is a good idea.

- you also mentioned physiology, specifically heat transfer. Again, larger size means a ratio gets worse, in this case that between volume and surface. Overheating is a problem, wherever your heat comes from, so both endotherms and ectotherms (mass-homoiotherms) are affected. Here, sauropods found an approach that helped in two ways: air sacs! They reduce the volume of tissue present, so that less heat is produced, and they massively increase the area available for tranferring it to air, which can be exhaled.

Further limits:
- the size of offspring
  (if you're a mammal the young need care, so you'll be forced to have young so big that you can care for them - imagine an elephant caring for a newborn rat. That wouldn't work. But big young mean few young, and maybe not even one a year (think elephants!), and that again means low populatiuon recovery rates. The tiniest change in the ecosystem can force your population below the threshold..... How did sauropods solve this? By laying eggs, and having precocial young)
- available food --> climate
  (If the climate is unsuitable, there simply may not be any large area on earth that provides sufficient fodder year-round and stably [no recurring draughts, please!] for a species to evolve giant size.
and so on.

A followup question: Why were the dinosaurs so much larger than current land mammals? Was it in part changes in the temperature and thus oxygen content allowing a much greater variety and abundenace of food?

That has been bothering us as well, obviously, not only the maximum size of sauropods.

It seems that egglaying is part of the answer (much better population recovery rates for given body size; see above), better lungs another part, and yes the ecosystems did play a role as well. More O2, more fodder, larger overall available space without seasonal snow/ice cover, etc.

Possibly (but I am jumping ahead of my own research here), the dinosaurian locomotion system was more efficient for slow locomotion at larger body size, too.

Non-mastication is important, too - bulk feeders can be bigger. Now, you probably remember the wonderful tooth batteries of, e.g., hadrosaurs. That's still almost bulk feeding, even if they chewed some to increase surface area versus volume of the bolus. However, compared to animals that really grind stuff into a mushy smear before swallowing, the chop-chop-swallow strategy of hadrosaurs is much faster. In sauropodsomorphs, this was taken to extremes, and the resulting tiny skull allowed the long neck, which allowed enery-efficient bulk feeding (you need not move the body, but only the neck&head).

if you really want to dig deep into the issue, on the example fo sauropods, this open-access publication summarizes 6 years of research by some 40 experts:
http://onlinelibrary.wiley.com/doi/10.1 … 137.x/full


MOST useful and many thanks - always good to learn something new outside one's own field!

you're very welcome! If you want to get the broader and mroe readable version of Sander et al. 2010 I can recommend this book(*):

http://www.iupress.indiana.edu/product_ … _id=317633

*: full diusclosure: there's two chapters by me in it, but I don't get any royalties.

I doubt that differences between Mesozoic and modern environments had much if anything to do with the huge difference in size between sauropods and our weedy modern animals: the Mesozoic spans 150 million years, during which O2 levels, temperature, sea level etc. varied wildly -- sometimes higher and sometimes lower than what we have now.  There were giant sauropods through pretty much all of this timespan.  I would be confident that if you could transplant a 70-tonne titanosaur from the Cretaceous to today, it would do just fine.