ife cycle of the mysterious and long-dead dodo revealed by bone study

From egg-laying to moulting, a new bone analysis has unpicked the biology of the long-extinct bird

The long-extinct dodo has inspired a panoply of research, but until now much of its life cycle has been a mystery.
 The long-extinct dodo has inspired a panoply of research, but until now much of its life cycle has been a mystery. Photograph: Kevin Webb/The Natural History Museum, Lond

ump and puny-winged, the dodo has been immortalised by humans in art, literature and song.

But while the peculiar animals have inspired a panoply of research, not least as to whether they were really bird-brained or as corpulent as portraits implied, much about the dodo’s life has remained a mystery until now.

Scientists studying remains of the extinct avians say they have managed to put flesh on the bones of the dodo’s existence, revealing aspects of their life from when they laid eggs to how quickly they reached adulthood, and even that they shed and regrew their plumage each year.

The dodo’s annual cycle, including moulting period and egg laying.
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 The dodo’s annual cycle, including moulting, egg laying and a period of arrested growth during the summer months, when cyclones and poor weather are common in Mauritius. Photograph: D. Angst et al/Scientific Reports

“Before our study the only things we knew about the ecology of these birds was that they were a big pigeon [with a body mass of] about 10 kilos,” said Delphine Angst, a palaeontologist and first author of the research from the University of Cape Town.

Native to the island of Mauritius in the Indian Ocean, the dodo was wiped out in the 17th century after the arrival of Dutch sailors and the animals that came with them, with hunting, competition for food and habitat destruction all contributing to the bird’s demise.

But, despite contemporary records – including those from one Thomas Herbert who after dropping by Mauritius described the dodo in 1634 as having eyes like diamonds, “her clothing downy feathers, her train three small plumes, short and inproportionable” – little is known about the birds, with most records far from scientific.

Now researchers have managed to fill in some of the gaps.

Thomas Herbert, who visited Mauritius in the mid-17th century, described the dodo as having eyes like diamonds.
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 Thomas Herbert, who visited Mauritius in the mid-17th century, described the dodo as having eyes like diamonds. Photograph: The Natural History Museum, Lond

Writing in the journal Scientific Reports, Angst and colleagues from the Natural History Museum in London and Tring describe how they examined under a microscope thin cross-sections of 22 leg and wing bones, thought to be from 22 different dodos.

The results reveal that, like the majority of modern birds, dodo bones have three layers of tissue. However, previous research in modern birds has shown that the outermost tissue is only found in adults.

“As soon as they achieve sexual maturity [the] bone tissue [develops] very slowly – and we can see that in the cross-sections,” said Angst.

The finding proved illuminating. “For the first time we can say that for sure these specimens are juvenile, even if it looks like it is almost an adult in terms of size,” said Angst, adding that the results showed that the chicks grew rapidly after hatching.

And there’s more. Female birds, notes Angst, lay down a special type of tissue inside their bones when they ovulate, with the tissue providing a supply of calcium for egg production. “In our samples we found several specimens with this specific kind of central bone and then we [can tell] for sure that this specimen is a female and it is a female during ovulation, which is quite cool,” said Angst, adding that for the other bones, without the extra tissue, it was unclear to which sex they belonged.

The study also shed light on the birds’ plumage.

When birds moult they use calcium from the inside layers of their bones to build new feathers, leaving telltale holes in the internal bone walls – a feature, the authors note, seen in the bones of birds ranging from penguins to pigeons. “It is exactly what we see for the dodo – for these specimens we can say that they died when they were actually moulting,” said Angst.

The researchers believe that differing descriptions of the dodo are not contradictory or wrong, but describe the birds at different phases of the moulting cycle.
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 The researchers believe that differing historical descriptions of the dodo are not contradictory or wrong, but describe the birds at different phases of the moulting cycle. Photograph: Agnès Angst

That could help explain why contemporary descriptions of dodos differed significantly.

“It was usually believed that the descriptions are different because they were wrong. But the descriptions were not wrong. Actually they describe the dodo in different states of moulting.” said Angst. The authors propose that mariners who described the birds as having a downy plumage probably saw them just after moulting began, with those describing dodos as sporting grey or black feathers seeing them between periods of moulting.

Further tell-tale signs within the bones allowed the team to unpick just when such events occurred.

When resources are scarce, the outer layer of bone stops growing, leaving a line. In the dodos, these lines recur regularly, suggesting arrested growth was a seasonal event – most likely, the authors say, reflecting the summer months from November to March when cyclones and other poor weather is common in Mauritius.

By looking at thickness of the bone deposited between such lines, the researchers were then able to determine the time of year that the dodos ovulated or moulted.

“In the specimens that are moulting, just after the [line] a tiny, tiny amount of bone is deposed – that means that the moulting happened just after the last summer,” said Angst.

Transverse sections of hindlimb bones of the dodo showing resorption cavities, which are interpreted as evidence of moult.
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 Transverse sections of hindlimb bones of the dodo showing resorption cavities, which are interpreted as evidence of moult. Photograph: D. Angst et al/Scientific Reports

The upshot, says Angst, is that dodos probably moulted between March and July – a suggestion that fits with the historical accounts of the bird’s plumage – with ovulation likely occurring early in August and chicks reaching full size by November, before the bad weather began.

Daniel Field, an avian palaeobiologist from the University of Bath said that new research was a fascinating illustration of how creative scientific techniques can unpick the biology of long-extinct creatures.

“The authors of this study have done a wonderful job filling important gaps in our understanding of how the dodo lived over 300 years after the last dodo died,” he said.