Since it dies out in the 17th century, the dodo has been depicted as a lumbering, fat bird, ripe for extinction. The reality was very different...
RIVALLING the dinosaurs as a symbol of extinction, the dodo is renowned for being slow, stupid and fat. An evolutional disaster, Raphus cucullatus was doomed to extinction from the day it was discovered by hungry Dutch sailors in the forests of Mauritius in 1598. Wasn't it?
Maybe not. For a start, the dodo's reputation for stupidity and slowness is easy to explain: if you've never met a predator before then why bother to flee club-wielding sailors? The dodo probably evolved from African fruit pigeons of the genus Treron which became stranded on the blissfully predator-free island of Mauritius. And in any case, the dodo's trusting manner was not the cause of its demise. The blame for that lies squarely with the rats, pigs and monkeys which arrived with the sailors and pillaged the dodo's vulnerable ground nests, driving the bird to extinction in the 1660s.
But what about the bird's bulky body? Surely that aspect of dodo folklore must hold up? After all, there are dozens of 17th century paintings and drawings to prove it. Certainly, the dodo was a big bird. With no predators to deal with and an abundance of food, its ancestors gradually lost the ability to fly. So freed from weight constraints, they evolved into flightless giants. Yet tradition has it that the dodo was not just large but overweight, gravitationally challenged by centuries of easy island pickings. Was this really the case?
In 1990, the late Marquis of Bute, then chairman of the board of trustees of the National Museums of Scotland, asked me to come up with a new reconstruction of this famous extinct bird. He saw a golden export opportunity in selling life-size models of the dodo as patio novelties to wealthy Americans. though in the event only a few cuddly toy dodos were ever produced.
Turning to the contemporary evidence, however, I became terribly confused. Like everybody else 1 had assumed that the dodo had a large, round body with a head and two legs poking out of the appropriate ends. However, a close look at the contemporary illustrations revealed an unexpected pattern. The first dodo drawings and paintings, dating from its discovery in 1598 up to 1605, showed much thinner birds than 1 had seen before. The rest, dating from about 1626 onwards, invariably showed the more familiar podgy variety.
In 1917, a Dutch ornithologist called A. C. Oudemans suggested the discrepancy was due to a seasonal fat cycle. The birds were laying down fat in the summer and then burning it off in winter, said Oudemans - just as the pink pigeon (Columba mayeri) and other endemic Mauritian birds do today. Yet none of these birds changes its external appearance as drastically as the dodos depicted. in the drawings and paintings. So I looked more closely at the historical evidence. VVhile the thin dodos were drawn by people who had actually visited Mauritius, the plumper ones, especially the specimens seen in oil paintings, were mostly produced by artists in Europe. What was going on?
I formed a hypothesis. Perhaps the few captive dodos that were brought back alive to Europe were unusually fat for some reason. They might have grown dissolutely obese on the voyage, gorging on an unrestricted diet of ship's biscuits and the weevils that inevitably went with them.
Unfortunately, there are precious few remains of these tubby, captive dodos. In the zoology museum at the University of Oxford there is a dried head, and the Natural History Museum in London boasts a dried foot - both hopeless for reconstructing body shape. Out in die wild, however, the story is different. Since 1865, when George Clark, a schoolteacher on Mauritius, unearthed the first dodo bones, travellers and naturalists have recovered literally hundreds of bones from the Mare aux Songes swamp in south-east Mauritius. These bones are the key to what the dodo really looked like.
Many of the bones are kept in the zoology museum at the University of Cambridge and the Natural History Museum in London, and in the course of a few weeks I was able to measure hundreds of them. Using the mean measurements of each bone, I constructed a wire and cardboard skeleton to one-fifth scale, to which I carefully added Plasticine to represent muscles and other soft body tissues, following the pattern seen in birds today. The result was remarkably similar to the first drawing of the dodo. In the journal of Admiral Jacob Comelius Van Neck, who visited Mauritius in 1598, there is a drawing showing a scene of everyday life in Mauritius. In the background a long-legged and thin-necked dodo strides purposefully into the distance.
I could measure the volume of my model by dunking it in water. Scaling this to life size, multiplying by the density of a living bird, and adding 5.6 per cent extra for feathers, should give a body weight for the dodo. The measured densities of living birds vary from 0.73 to 0.94 grams per cubic centimetre. These calculations put the weight of the dodo - or at least the thinner specimens - at between 12.5 and 16.1 kilograms, about the weight of a male great bustard (Otis tarda), the heaviest flying bird alive today.
But what if the dodo were as fat as the later artists would have us believe? How much would it weigh then? I fattened up my Plasticine model until it resembled the dodo models produced early this century by the London taxidermist Rowland Ward. Using the same Archimedean displacement method, I estimated that a fat dodo would have weighed between 21.7 and 27.8 kilograms. This range compares well with the only published record of the dodo's body weight that 1 could find: in 1634, a traveller called Thomas Herbert claimed that the dodo weighed a minimum of 50 pounds (23 kilograms).
The Dodo & Given, by G. Edwards 1750
|To confirm these figures, I tried three other approaches. One involved weighing every bone in a dodo's body. Previous research has shown that a bird's skeleton accounts for a fixed proportion of its body weight. The problem in using this relationship to estimate the weight of a dodo, however, is that most existing dodo bones are broken and damaged, especially the larger ones such as the skull and sternum. Even apparently complete bones may have been repaired with plaster, which is much denser. Taking great care to select the best bones, I calculated a body weight of just under 12 kilograms. This almost certainly underestimates the real body weight, because a few bones were incomplete and many probably dehydrated.||
Gross discrepency: early drawings, like the one done by Carl Clusius in 1605, show a thin bird, whereas the paintings of Roelandt Savery, dating from the 1620s, show a more portly figure
There was another route to estimating body weight. By examining different species of pigeon, I derived a scaling relationship between body weight and the length of the leg bone - a formula I could then use to calculate the body weight of the dodo from measurements of its leg bones. Zoologists have used similar scaling relationships to estimate the body weights of dinosaurs, the extinct flightless moas of New Zealand and the giant teratorn vultures, the largest birds ever to fly.
The scaling relationships for pigeons predicted that the dodo would have weighed between 10.6 and 17.5 kilograms, depending on which leg bone measurement was used. Extrapolating from small, everyday pigeons to the dodo seems like a dangerously large leap, but my confidence in the results grew when a similar relationship (between tibiotarsus length and body weight), formulated for every size of bird from hummingbirds to ostriches, predicted a weight of 13.7 kilograms for the dodo.
Another worry was whether it was justifiable to take data from flying birds and extrapolate it to grounded ones. To find out I collected similar data for parrots, which 1 used to estimate the body weight of the flightless kakapo (Strigops haboptilus), the world's largest parrot. The estimated weight fell within the known range of body weights of this species, suggesting that flightless birds are no denser than their flying relatives.
Eggshells offer a further index of the body weights of birds, as the mass of an egg's shell is known to vary in proportion to the mass of the bird that lays it. Unfortunately, there is only one surviving dodo egg that is even claimed to be from a dodo, a specimen given to the East London Museum in South Africa by Marjorie Courtenay Latimer, the naturalist who discovered the coelacanth in 1938. But this egg is unlikely to have been laid by a dodo: the mass of the eggshell suggests it came from a bird the size of an ostrich (Struthio camelus). In 1638, Frangois Cauche described a dodo's egg as being as large as a halfpenny roll or the egg of a white pelican (Pelecanus onocrotalus). From this, the scaling relationship suggests the dodo would have weighed about 13.7 kilograms.
So according to four different methods, all based on the dodo's bones, the famous flightless pigeon weighed between 10.6 and 17.5 kilograms - which is much less than Herbert's hefty minimum of 23 kilograms. Yet even at this weight the dodo was as heavy as the biggest flying birds and would have provided a formidable meal for hungry sailors.
The alternative explanation for the dodo's reputation as a fatty, the one favoured by Oudemans, is that the dodo was subject to enormous seasonal changes in the quantity of its body fat. This, though, would have required the storage of improbable amounts of fat. The podgy bird would have had to starve for five months to slim down to its thinner self, before embarking on a gluttonous orgy to regain the fat. This would have been an incredible feat for a large bird with a relatively low metabolic rate on a subtropical Indian Ocean island with limited seasonality, even taking into account the odd hurricane-induced food shortage.
|All in all, then, it seems that the dodo was much slimmer than we thought. This raises a further question: could it have won the Caucus-race in Alice in Wonderland, the method chosen by Carroll's fictional dodo and animal friends to dry themselves after swimming through Alice's tears? To find out, I adapted a method which has been used to predict the running abilities of dinosaurs and moas - comparing the cantilever strengths of their leg bones with those of similar animals.|
Cantilever strengths are the theoretical maximum forces that would be expected to break bones, expressed in relation to body weight. They can be used to compare the running abilities of animals of different sizes. Some years ago McNeil Alexander of the University of Leeds measured the weights and cantilever strengths of the leg bones of some terrestrial African birds known to be swift of foot. They ranged in size for the guinea fowl, (1.25 kilograms) to the ostrich (41.5 kilograms).
|Ken Joysey of Cambridge University's zoology museum agreed to let me slice in half some average-sized dodo leg bones so that I could measure the thickness of the bone wall. Taken with the bones' original lengths, this allowed me to calculate the cantilever strengths of the three main leg bones. I found that the cantilever strengths of the tibiotarsus and the tarsometatarsus bones of the dodo fall within the ranges for these other birds studied by Alexander - but only if the dodo is in its thin guise. Fat dodos would have barely managed a gentle trot.|
So was the living bird an athlete? Sadly, most accounts of the time are concerned with the culinary potential of the dodo rather than its behaviour. But one eyewitness account is relevant. In 1662, Volquard Iversen become shipwrecked on a small island off the southeast coast of Mauritius. Rather than soaking up the sun until the next ship arrived, he described the birds he saw. "Amongst other birds were those which men in the Indies called doddaersen ["round bottoms"]; they were larger than geese but not able to fly. Instead of wings they had small flaps; but they could run very fast." The simple mechanical analysis seems to confirm that the thin dodo would have been a hot favourite for the Caucus-race.
One problem remained. The dodo's femur had a lower than expected cantilever strength. The only reasonable explanations were that either dodos held their femurs in a more vertical position than other birds, so that bending stresses would be much lower, or the bone I had measured was anomalously weak. Sadly, I could not justify slicing up more bones to find if all the femurs were the same. So that is where 1 had to leave my research. On it is based a new life-size, slimline, athletic dodo model which was produced for our global nature conservation exhibition, "World in Our Hands", where it is on permanent display in the Royal Museum of Scotland in Edinburgh.
Then, in 1991, something rather disturbing happened. I was informed by Boudewyn Buch of Dutch TV that some original unpublished dodo drawings made in 1601 and 1602 had been rediscovered in the Hague after having been lost for 150 years. If my theory about the fat captive birds was correct they would show thin birds. To my eternal relief, all the drawings show much thinner birds than tradition dictated, and they all seemed to have much longer legs than 1 expected. By tilting the femur downwards, the dodo could extend its leg length - a change which, by placing the femur in a more upright position, would reduce the bending forces it would have to withstand (hence its unusually low cantilever strength). Perhaps the highly conservative pigeon body plan precluded the evolution of longer leg bones, or loss of the flight muscles led to a shift in its centre of gravity, which was compensated for by swinging its femur downwards.
For more than 350 years the dodo has been thoroughly misrepresented as plump and immobile. The reality is, however, that in the forests of Mauritius it was lithe and active. Like other Mauritian birds it would have undergone a seasonal fat cycle to overcome shortages of food, but never to the extent that those wonderful oil paintings suggest. Sadly, it is from these portraits of the last captive individuals that most people have gained their impressions of the world's largest and most famous pigeon.
Andrew Kitchener is a curator at the Royal Museum of Scotland.
New Scientist, 28 August 1993
© 1996 IPC Magazine, 1993
Used by permission
For more science news and views, check out New Scientist Planet Science
Back to information links