Placing the Platypus

Defying categorisation since its discovery, was the platypus a mammal, a reptile, or something else? 

An ornithorhynchus (duck billed platypus). Coloured etching by J. F. Cazenave after A.C. Vauthier (1790-c.1831). Wellcome Collection.

On 29 August 1884 William Hay Caldwell sent the most important telegram ever written about a platypus. Then just 25 years old, he had spent four months searching for its eggs on the banks of the Burnett River in North Queensland – with little success. After much splashing around, he and his Aborigine assistants had found lungfish eggs, echidna eggs – everything, in short, but what they were really after. He could have been forgiven for wanting to give up. But a few days earlier he had shot a female. To his amazement, he found that not only had she laid an egg, but had actually been laying another when he had caught her. When he cracked one open, his excitement only grew. He dashed off a message to the annual meeting of the British Association for the Advancement of Science in Montreal. He kept it short – only four words. And to the telegraph operator, it probably seemed like gibberish. But it contained the answer to a century-long puzzle and transformed the way naturalists understood animal classification forever. 


Sui generis?

The platypus had baffled European scientists ever since the first specimen had arrived in England in 1798. Sent by John Hunter, the governor of New South Wales, it consisted of a preserved skin rather than a complete body and had not fared well on the voyage. Although it was accompanied by a sketch, it was difficult to know quite what to make of it. Some naturalists struggled to believe it was real. Bemused by the duck-like bill, the thick fur and the webbed feet, they assumed that it was a hoax by Chinese taxidermists, whose fondness for producing ‘mermaids’ out of fish tails and monkey heads was already well known.

Even when – in 1799 –George Shaw proved that Hunter’s specimen was genuine the mystery remained. What sort of creature was this? Since naturalists had only a skin, with no internal organs to work from, there was a limit to what they could say about its anatomy. But the problem was really about classification. 

In principle, there shouldn’t have been a problem. Though taxonomy was as old as biology itself, the foundations of the modern science had been laid by Carl Linnaeus just 40 years before the arrival of Hunter’s platypus. Like most of his contemporaries, Linnaeus believed that all living things existed in the form which God had created them; but he broke new ground in attempting to introduce a system of classification based on observed similarities. He proposed five levels of classification: kingdom, class, order, genus, species. Within the animal kingdom there were six classes, including mammalia (mammals), aves (birds) and amphibia (reptiles and amphibians), for each of which he deduced a definition. 

According to Linnaeus, mammalia suckled their young by means of ‘lactiferous teats’ (mammary glands), had teeth, breathed with a pair of lungs, generally had four limbs, were often covered in hair and sometimes had tails – as well as a host of other characteristics. Aves, by contrast, laid eggs covered with a calcareous shell and had wings, but lacked external ears, external testes, wombs, diaphragms and bladders. Amphibia laid (usually) membranous eggs and also lacked a diaphragm; but occasionally had a ‘horrid poison’, too. 

But Linnneaus’ method was built on sand. Although the identification of similarities was sensible enough, his assumption that animals had always existed in their current forms proved a major weakness. Since this denied any causal affinity between species other than providence, it rendered his classifications arbitrary and made it impossible to determine which, if any, characteristics were essential to a class. This meant that any taxonomist trying to categorise an oddity like the platypus either had to adopt a pick ‘n’ mix approach to his definitions or just guess wildly about their place in Creation. 

Of course, the platypus looked as if it should have been a mammal. Based on its fur, beak and webbed feet, Shaw – who gave it the name Platypus anatius – felt confident enough to class it with sloths, anteaters and other toothless mammals. But even he had his doubts. So much about it seemed wrong. Given the state of Hunter’s skin, it was hard to say exactly what, but there was enough to defy easy categorisation. Even after seeing another, better preserved, specimen the engraver Thomas Bewick admitted that he was so puzzled that he didn’t know how to classify it at all. In A General History of Quadrupeds (1800), he described it as ‘an animal sui generis’, which appeared to have characteristics of ‘a fish, a bird, and a quadruped’ and seemed to be ‘related to nothing that we have hitherto seen’. 

Male platypus skin, held at the Natural History Museum.
Male platypus skin, held at the Natural History Museum © Natural History Museum, London/Bridgeman Images.

The more closely the platypus was studied, the deeper the mystery grew. In 1800 the German naturalist Johann Friedrich Blumenbach conducted the most detailed study to date. Since the name ‘platypus’ had already been given to a beetle, he rechristened it Ornithorhynchus paradoxus, literally, ‘bird-nosed paradox’. He was troubled by the females’ apparent lack of the characteristic which ‘had given rise to the name mammalia by which LINNEUS ha[d] distinguished them’ – namely, mammary glands. But he could not ignore the fact that they still had a number of other characteristics that did seem reminiscent of mammals. After rejigging Linneaus’ orders a bit, he therefore classed them with the web-footed Palmata

Two years later Sir Everard Home took a rather different view. He reported that the male platypus had internal testes like reptiles and a cloaca (a single orifice for the digestive, urinary and reproductive tracts). At the time it was believed that cloacae belonged only to reptiles, amphibians and birds, and was indicative of egg-laying. Since this appeared to set the platypus apart from mammals ‘in a very remarkable manner’, Home therefore suggested that in the Great Chain of Being, in which God had assigned all created beings a place according to their ‘perfection’, it stood below mammals, but above birds and reptiles/amphibians. Exactly where it belonged, however, was unclear. 



Everything changed with the publication of Jean-Baptiste Lamarck’s Zoologie philosophique in 1809. Lamarck argued that, rather than being static, all living things were in a constant state of change. Not only could they ‘acquire’ new attributes in response to environmental factors but, he claimed, they could also transmit those attributes to their offspring. As Charles Darwin later showed, this had obvious flaws: giraffes didn’t become giraffes just by stretching their necks to reach higher up the tree. But it clearly suggested that, over time, one variety of animal could transform into another. The implications for taxonomy were immense. Rather than classifying animals according to a random hodge podge of characteristics, it was now possible not only to trace the ‘transformation’ of animals over time, but also to determine more easily which characteristics were distinctive of a class or order and which were not. According to Lamarck, this made it easier to deal with ‘paradoxes’ like platypuses. Based on his work, the French naturalist Étienne Geoffroy Saint-Hilaire argued that, since the platypus didn’t have mammary glands, it ‘clearly’ wasn’t a mammal and should therefore be included in a completely new class of vertebrates, which he charmingly dubbed the Monotremata (‘one holed’). 

But rather than settling matters, this only seemed to make things worse. Influenced by Lamarck’s theories, the German anatomist Johann Meckel started looking more closely at some of the platypus’ odder features, perhaps in the expectation of solidifying Geoffroy Saint-Hilaire’s claims. What he found came as a surprise. While he established that male platypuses had a poison gland – usually associated with reptiles and amphibians – he also discovered that the females did have mammary glands after all. Clearly, the picture was too confused for any definite conclusions to be reached; but suddenly, the platypus was looking a lot more like a mammal than a short time before. 

In 1832 Lauderdale Maule, a young Scottish soldier, published two papers which bolstered this idea further. Although he acknowledged that platypuses had some reptilian characteristics, and even identified certain bones in the pelvic girdle that were found only in mammal-like reptiles, he drew attention to several features that were distinctively mammalian. True, the mammary glands were atypical, but he left no doubt that the females had nipples capable of expressing milk; and he confirmed that both sexes had warm blood and true diaphragms. 

Ornithorhynchus anatinus. 19th-century coloured engraving. Alamy.
Ornithorhynchus anatinus. 19th-century coloured engraving. Alamy.

To some, these discoveries might have been enough to grant platypuses a place among ‘proper’ mammals. But most naturalists quickly realised that they actually made classification more difficult. What they had revealed was an apparently irresolvable contradiction. On the one hand, it was obvious that, if an animal had mammary glands, its young were born in need of being suckled. This suggested that it either gave birth to live offspring (like humans and whales) or produced eggs, which hatched within the body (like seahorses). But on the other hand, it was thought that, if an animal had a cloaca, it laid eggs (like chickens and lizards) and that most embryonic development took place outside the body. In this case, the embryos were nourished from a yolk and – presumably – needed no suckling. It was almost impossible to imagine that any creature could have functioning mammary glands and lay eggs. It had to be one or the other. And, depending on which, the platypus would either be a mammal or something else. 

Everyone took sides. Whereas Meckel was convinced that the platypus gave birth to live young, the English naturalist Richard Owen argued that the eggs remained inside the body and Geoffroy Saint-Hilaire doggedly maintained that eggs were laid. What they needed was evidence. There were plenty of reports, of course. For years Aborigines and Australian colonists had claimed to have seen platypuses incubating eggs. But naturalists put little faith in hearsay. Nor were they inclined to trust the ‘samples’ that were sent to them. They had been gulled one too many times to fall for that one. In 1829 Saint-Hilaire had been tricked into believing that a tortoise egg had come from a platypus; and Owen had wasted hours dissecting two eggs which turned out to belong to a lizard and a snake. 


The Egg Man

The only way of resolving the puzzle for certain was either to identify a genuine egg or, better, to capture and kill a pregnant female. To this end, Owen asked George Bennet, the Curator of the Australian Museum, to start shooting likely animals. But no matter how many platypuses Bennet bagged, his quarry eluded him. Indeed, by 1860, he was worried that if he carried on he might actually drive the platypus to extinction. 

It fell to William Caldwell to settle things. And when he did, his telegram sent shockwaves through the biological world. It read: ‘Monotremes oviparous, ovum meroblastic.’ In other words, platypuses laid eggs, with large yolks – just like birds and reptiles. But crucially, the embryos inside were already highly developed, with rudimentary vertebrae and differentiated brains – suggesting that they had also been retained inside the mother for a certain period and required nurturing after hatching. 

Confounding all expectations, this proved that, while the platypus was indubitably a mammal, it belonged to an order all of its own, far more primitive than all the others. Long ago, they must have branched off from the last common ancestor of mammals and reptiles, developing some distinctive new characteristics, but without fully shedding their simpler past. At a stroke, this redrew evolutionary history – and obliged naturalists to rethink the development not just of mammals, but of all animal life. 

For all the trouble it caused 19th-century biologists, the platypus remains a delightfully strange creature. Indeed, its very strangeness is perhaps the source of its charm. 


Alexander Lee is a fellow in the Centre for the Study of the Renaissance at the University of Warwick. His latest book, Machiavelli: His Life and Times, is now available in paperback.