The unconsidered fatness of dinosaurs

I am always interested in ways of measuring or estimating the mass of organisms. While estimates are generally given as a range rather than an exact figure (which makes sense of course) the range is generally there to provide for possible errors in the methodology – were the animals good runners so the legs should have a bit more musculature, was the tail a bit longer, were the neural spines a bit longer? But there is also a further component that is rarely considered (or at least not discussed) – how robust (or lets face it, fat) is the animal?

Now I am not going to criticise estimates for not considering fat tyrannosaurs (though John Hutchinson for example does work with ‘gracile’ and ‘robust’ morphs which accounts for the possible variation present), but those working from them (for e.g. mechanical analyses) should give them a little more thought in my opinion. While it is easy to look at a human population and spot, shall we say, some highly robust morphs, that is a result of excess food being available, not just changes in form or build. While at London Zoo, I worked with a pair of female giraffe where one was probably half a meter taller than the other, but a fraction of the girth – the latter (while a guesstimate) was probably 10% heavier despite a difference in height (measured) of around 7%, with the shorter animal being the heavier. At a more familiar level, while discussing mass estimates in Munich, my fellow pterosaur researcher Mark Witton revealed that he weighs just 65 kg, compared to my then (to my shame, and I desperately point out, after a major sports injury that had prevented any real exercise for months) almost 100 kg, despite the fact that we are near identical heights and similar in build (well, broad shouldered). Even at my more normal ‘fit’ weight of about 85 kg there is clearly a big difference between us despite our similar height (and build actually, at least in terms of shoulder width). Mark is simply far more gracile than me for want of a better term, despite an apparently similar osteology. Either of us would probably be considered well within the normal range of humans, (and perhaps for other animals – i.e. about 20% for a given size) but it is really quite dramatic.

Assuming (i.e. not checking) this is normal, even a near perfect estimate that gave T. rex a mass of say 8 tons, means that for a given skeleton, anything from 6.4 – 9 t might be in the normal, let alone extreme range for the species. That is a big range and can have a considerable effect on wider analyses (such as running speed, ecological biomass etc.). Again, it might be difficult if not impossible to include this realistically into mass estimates (you are looking to get a narrow range after all for practical work, not be left with a hugely unworkable range that encompasses every concievable extreme) but it is worth keeping in mind what kind of variation is possible. Yes our hypothetical 8 ton rex might be a perfect estimate for that specimen when it was alive, but it could have been much bigger or smaller. Perhaps a very gracile form was normal, and rexes of that size rarely got above 6.5 t in which case an analysis of their food requirements for example could be severely affected. If the results come out for some ecological analysis for example suggesting that at an average of 8 t, Tyrannosaurs was badly overpopulated compared to the amount of prey, remember that he might not be quite the giant you think…

This is a revised Mk.1. post, to see the original with comments etc., go here.