 How to Weigh a Dinosaur
by Anne Schulp*
Dinosaurs. Some of them were pet-sized, most of them much bigger, and some dinosaurs were just gigantic. But how on earth can we know how heavy they were? After all, they are all extinct: no-one has ever seen a live dinosaur, let alone put one on a scale. With some simple experiments, you should be able to find out more about the size and the weight of dinosaurs.
You won’t need more than a few toy dinosaurs or plasticine clay, some measuring gear, and a few dinosaur books to weigh a 30-metre dinosaur. You will never ever again take the signs in museums at face value, once you discover the story behind the art of weighing dinosaurs.
Playing with dinosaurs
When strolling through a museum, or reading a book on dinosaurs, you may be amazed to see that the weight of those extinct animals is often listed with tremendous precision. ‘An adult Tyrannosaurus weighed 6.4 tonnes,’ one book states. Let’s do some simple experiments to put such claims to the test. Were the scientists right? How much did T. rex actually weigh? And just how accurate are those toy dinosaurs you’ll find by the dozens in museum shops?
Usually, all that's left of a dinosaur are its bones. Weighing just these does not make much sense; the bones themselves have turned into rock, making them heavier, while the remaining part of the dinosaur, the skin, the muscles, and everything else is gone. But we can get around this.
Although we can’t actually weigh the dinosaur, as a first step we can determine its volume (V). And once we have a good assumption of the density (ρ) of the animal, we can easily calculate its mass (m), using the formula m = ρ · V
Of course you could determine the volume of any given dinosaur by putting a life-size model in a swimming pool, and filling the remaining space with water, painstakingly counting the litres as you pour them in. Subtracting the amount of water you just poured from the capacity of the pool should give you the exact volume of the dinosaur. It should work, but be prepared for a tedious job!
So why not take a small dinosaur model? Find some plastic toy dinosaurs, or try to make some dinosaur models yourself, perhaps using plasticine modeling clay. Measure their lengths from head to tail. And try to determine the volume of the models by submerging them in a suitable water-filled container. Mark the water level with and without the dinosaur submerged. If you have a container with a detailed volume scale, even better, but if the container is round, you can also calculate the volume, in cm3, by multiplying the surface area of the container with the distance between the two water level measurements:
Surface area A of a round container = π · r2 (r in cm)
Height difference between the two measurements: ∆h (in cm)
Total volume of water displaced by the submerged dinosaur: A · ∆h (in cm3)
You can make a table listing all your measurements, perhaps something like this:
| Toy dinosaur name |
Head-to-tail length (cm) |
∆h (cm) |
Volume (cm3) |
| Tyrannosaurus
|
14 |
... |
... |
... ...
|
... |
... |
... |
... ...
|
... |
... |
... |
 Now you have the data on the toy dinosaurs, it is time to find out about the size of the corresponding ‘real’ dinosaurs. If you are lucky, you may have a museum nearby with a real (or replica) dinosaur skeleton, perhaps even a Tyrannosaurus. Ask the museum staff about the length, or perhaps you could even get permission to cross the barrier and make some measurements yourself! But of course the library and the internet provide plenty of information about dinosaurs and their sizes, too. Don’t forget to write down the claims on the dinosaur weights, too, should you come across some!
Perhaps you could make a second table, listing the data you collected about the ‘real’ dinosaurs. Maybe something like this:
| Dinosaur name |
Length (m) |
Which source? |
Any claims on weight? |
Titanosaurus
|
11 |
Museum |
none |
| Tyrannosaurus
|
12 |
Dinosaur data book |
6,400 kg
|
... ...
|
... |
... |
... |
Next, you’ll have to calculate the ‘real’ volume from your scaled dinosaur. Remember that a 12-metre dinosaur is 1200 / 14 = 86 times as long as a 14-cm scale model. If you scale up a dinosaur only two times, the length will increase two-fold, but its volume will increase eight-fold: the beast becomes twice as long, twice as wide and twice as tall, leaving you with:
2 · 2 · 2 = 23 = 8 times as much dinosaur meat. Similarly, a dinosaur 86 times as long as your toy dinosaur, would have a 863 = 630.000 times higher volume.
Now calculate the volume of the ‘real’ dinosaur by combining the data you collected from the toy and the length measurements of the ‘real’ dinosaurs. The only thing we still need to know is the density of dinosaurs. Most reptiles (and most other animals, for that matter) have approximately the same density as water (1 kg / dm3). This shouldn’t come as a surprise, because in water, most land animals will just float, but with very little sticking out above the surface. In short, this means that you can safely convert the volume of the dinosaur (in litres) to its weight (in kilogrammes) using a 1:1 ratio.
 Now it’s time to get back to the claims you found in books, on the web and in museums. How do your results compare to that? Did you find any differences? How much difference does it make taking a cute, fat toy dinosaur compared with a slender one? How reliable do you think your data will be? And how much variation did you expect? Can you think of ways to improve this experiment?
By now, you understand that you will always have to be critical of scientific-sounding claims. Decide yourself if such claims are true by experimenting.
 Who is Anne Schulp?
Anne writes articles and books on fun experiments that you can do at home. He is a palaeontologist who specialises in dinosaurs. He is a curator at the Natural History Museum in Maastricht, the Netherlands. The museum is worth a visit when you are in the neighbourhood. They have interesting biology, geology and fossil collections.
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