Certainly nothing is unnatural that is not physically impossible.
—Richard Brinsley Sheridan
Grützner and his colleagues found that platypuses lack the gene SRY, which determines sex in mammals, and instead has a gene similar to DMRT1, which determines sex in birds. A draft version of the platypus genome (Warren et al. 2008) identified at least two genes otherwise known only in birds (see also Rens et al. 2007.). This genetic evidence, then, is consistent with the idea that platypuses are anciently derived from a cross between a mammal and a bird.
So is the fact that they both lay eggs and produce milk. Their strange morphology, too, combines birdlike traits with those of a mammal. Obvious and well-known features of this sort are the presence of both a duckbill and a hair coat. But they also have a variety of lesser-known traits that seem to connect them with birds (Access a list of such traits). Why?
Perhaps they are bird-mammal hybrids? A great deal of evidence suggests such may well be the case. For example, research appearing in the journal Nature (Grützner et al. 2004) demonstrated that the platypus genome contains sex chromosomes of both bird and mammal origin (read an article about this research >>). As the lead author of the study, Frank Grützner, specifically states: "The platypus actually links the bird sex chromosomes system with the mammalian sex chromosome systems.
Platypuses are classified as monotremes, as are echidnas. Like platypuses, echidnas lay eggs and produce milk. They also have a birdlike beak, but lack the feathers of a bird, and instead have a coat of spines like a hedgehog or porcupine. The name monotreme, meaning "single orifice," refers to the presence in both platypuses and echidnas of a cloaca, a common chamber into which the intestines, as well as the reproductive and urinary ducts, vent (Vaughan et al. 2011, p. 120). Though monotremes of both sexes do have a cloaca (as do birds, reptiles and amphibians), other mammals don't—with the exception of certain Malagasy insectivores known as tenrecs (Vaughan et al. 2011). In fact, the great French anatomist Georges Cuvier stated that the reproductive tracts of monotremes are "infinitely closer to those of oviparous animals than to those of mammals" (Recherches sur les ossemens fossile, vol. 5, p. 68).
It is because platypuses and echidnas have many features that are typical of birds, and not of mammals, that they are classified as separate from all other mammals. Since their genomes both contain genes otherwise found only in birds, and since they have so many other traits that connect them with birds (Read a list of such traits), is it not, perhaps, reasonable to consider the possibility that they are not merely birdlike mammals, but that they are actually rare examples of hybrids descended from ancient, unusual matings between mammals and birds?
One might suppose that such distant crosses would be absolutely impossible, which is the big sticking point for some people. Indeed, there are those who are quite vehement on this point, though it's hard to see why, since little experimentation has been carried out to test the question. Commercial investigations using artificial insemination demonstrate beyond doubt that in some fairly distant crosses among birds (e.g., turkey × chicken or quail × chicken), a small percentage of fertilized eggs actually develop into mature hybrids (McCarthy 2006). And extensive experiments involving the hybridization of distantly related fish, Newman (1915, 1917 and 1918) found that many such crosses did produce hybrids, even in the case of inter-ordinal crosses.
And such findings do not go entirely against expectation. After all, it's not as if a mammal spermatozoon that suddenly found itself in the cloaca of a bird would clasp its blushing cheeks and cry, “Oh my ears and whiskers! I should not be here! Whatever shall I do?” On the contrary, one might reasonably suppose that, at least in certain instances, any such mislaid gamete might swim merrily on, and that one or more of them might at last reach an ovum, and that sometimes, perhaps very rarely, fertilization might occur. And over geological time, among the many such zygotes formed, some few might well have reached maturity and produced offspring.
So it may well be that even in very distant crosses, such as one between a bird and a mammal, after many thousands of matings, a few hybrids would be produced. Of course, without assiduous experimentation the frequency of such events must remain a matter of speculation.
One would expect scientific reports of bird-mammal hybrids to be few and far between, if indeed one could find them at all. It would be necessary to search through the old literature as astronomers do when they search out ancient records of supernovas and great comets in Chinese and Babylonian annals. And this sort of approach has, in fact, thus far revealed seven reports, all except one from the 1700s. One concerns an ostensible dog-turkey hybrid. Another is about an apparent hybrid between a pigeon and a rabbit. A third involves a bitch crossing with a cock parrot. And a fourth is about what seems to have been a human-chicken hybrid. The fifth is an account about three supposed hybrids between cat and duck, which appeared in a medical journal in 1778. Sixth, there is a brief report about a cat-chicken hybrid hatched on a Pennsylvania farm in 1880. Seventh, there is a very old and dubious report about a dog-hawk hybrid. And finally, there is a seemingly fanciful reports about women giving birth to goose-human hybrids.
Certainly, it is not as if such matings never occur. In fact, it has long been known that mammals and birds will voluntarily mate (a few such cases are described below). For example, buck rabbits have been observed voluntarily mating with domestic chickens (see below). And it is extremely well documented that a wide variety of hybrid matings occur in a natural setting. So odd matings, even between birds and mammals, must occur now and then in nature as well. Therefore such matings, even if very rare when measured on the human timescale, must have occurred millions upon million over geological time.
If platypuses and echidnas were bird-mammal hybrids it would explain why they, unlike other mammals have so many avian traits. As has already been mentioned, such traits are enumerated on a separate page of this website. As to the specific type of mammal and bird proposed as parents, for a platypus a likely choice would seem to be some sort of duck mated with an otter- or perhaps beaver-like mammal. In the case of the echidna, a reasonable combination would be a hedgehog mated with a kiwi (an echidna actually looks much like a hedgehog body with a kiwi head attached). Such crosses would, of course, be far outside what conventional biological thought would accept as possible. But, then again, perhaps convention is mistaken.
Though few biologists seem to want to come out and actually say so, these facts are consistent with the idea that birds can, at least on rare occasion, successfully hybridize with mammals. That is, judging from the morphology and genetic make-up of a platypus, it would seem that an otter-like mammal at some point crossed with a duck-like bird. For how could the platypus exist today with such a combination of chromosomes, genes and physical traits, if a bird did not at sometime in the distant past hybridize with a mammal and not only produce a hybrid, but also a hybrid that was itself capable of producing offspring?
The platypus and echidna are perhaps the best evidence suggesting that bird-mammal hybrids can sometimes occur. But another question is whether a bird and a mammal would actually mate in a natural setting. Various authors have reported matings between birds and mammals. That is, they describe coitus without production of offspring. For example, on Marion Island in the southern Indian Ocean, there have been two separate cases recently, in 2008 and 2013, of male Antarctic fur seals (Arctocephalus gazella) mating with king penguins (Aptenodytes patagonicus). In the 2008 encounter, the seal overcame the penguin by lying on top of it, while the penguin frantically flapped its flippers. The penguin, however, "showed no outward signs of injury" during or after the encounter. "The seal alternated between resting on the penguin and bouts of pelvic thrusting copulatory behaviour, attempting to penetrate the cloaca of the penguin" (de Bruyn et al. 2008). The more recent case (2013) was described in the New York Times (the BBC recently published another story on this topic, citing additional incidents). Since this behavior has been observed twice in a five-year period, one can only imagine how many times it has happened over evolutionary time.
Paul Broca (1824-1880), the renowned French physician, anatomist, and anthropologist who first discovered the brain region governing speech (Broca’s area), states the following (Broca 1859, p. 224):
Mating of buck European rabbits (Oryctolagus cuniculus) in captivity with domestic hens (Gallus gallus) has been described by various authors and actually captured in videos on Youtube (e.g., Video #1, Video #2). Johann Friedrich Blumenbach (1752-1840), the German physician, naturalist and physiologist, says one of his professors, Christoph Gottlieb Büttner, had "often" observed such behavior (Blumenbach 1776, p. 13).
And the eighteenth-century French scientist René-Antoine Ferchault de Réaumur (1683-1757) provides a detailed description of copulation in captivity between a buck rabbit and a domestic hen (De Réaumur 1750, pp. 450-456; 1751, pp. 339-357; Zirkle 1935, p. 171). In fact, Haartman (1751, published in Linnaeus 1756) and Dahlberg (1755), two students of Linnaeus, incorrectly claimed that de Réaumur actually reported the successful production of this hybrid. Both claimed it was like a cock, but had hair instead of feathers. The following is from the original account (de Réaumur 1750, pp. 450-453:
The whole town of Paris was some years ago full of amours far stranger than those of a duck and a cock, viz., of the amours of a hen and a rabbit. The Abbe de Fontenu…informed the academy of sciences of what passed in the house of his brother, where he lives, between a hen and a rabbit: he informed them of the strong inclination which this ill-sorted couple of animals had contracted for each other, and which was such that the rabbit behaved with the hen as he would have done with his doe, and the hen let him take as much liberty with her as she would have allowed to a cock. The academy judged this fact to be one of those which ought never to be credited, unless it is certain that they have been seen by a discerning eye. The Abbe de Fontenu is one of that sort: but he did not assert the copulation of the rabbit with the hen upon the testimony of his own eyes, he only attested it upon the report of all the servants in the house; as the coachman, the footmen, the cook, and others, all of them not much used to make observations, who pretended they daily saw in a yard where the rabbit and the hen lived together, things which the Abbe de Fontenu was not near enough to observe from the window of his study. I no sooner had hinted to him that I would be very curious to have the hen and the rabbit in my possession to assure myself with my own eyes of the reality and the degree of the liking they had taken to one another, but his kind disposition towards me, of which I am very proud and of which he has given me many proofs, made him promise me without the least hesitation that the two animals would be at my house the very next day.
The rabbit and the hen were accordingly brought to me the day after, which was the twenty-fourth of June: I lodged them in a place where I could conveniently often observe them without interrupting them: it was in a wardrobe which is behind my study and in which I could see all what passed without entering into it. This wardrobe has a large, pretty low window which looks into my garden. They seemed to me perfectly indifferent to each other to the twenty-eighth of June, and it is likely the change of place had disconcerted them and taken up all their thoughts: on that very day at about half an hour after five in the morning, I saw the rabbit draw near the hen, placing his body along one of her sides a moment after, having his hinder paws on the ground, he jumped with the agility of a rabbit, and put the fore-part of his body on the back of the hen, part whom he laid hold of with his fore-paws, at the root of both her wings; he next drew the lower part of his belly as near as possibly he could to the hen’s hinder parts, and gave that part of his body small quick motions, the intention of which was plain enough. He was not allowed to remain in that posture but a very short time. The hen was not disposed to answer his caresses: his forelegs were but a little folded, she went forward and escaped from him.
On the twenty-ninth, I observed no courting on the part of the rabbit, I say of the rabbit, for the hen always behaved like a modest hen; but on the thirtieth, and at half an hour after five in the morning as before, I began to observe the same scene I had beheld on the twenty-eighth which was repeated several times. The rabbit had just eaten a lettuce leaf after having rubbed his paws one against the other, and wiped his chops with them. He went to his hen, and was no sooner by her, but he swiftly jumped with the fore-part of his body on her back; but before he could have time enough to hook himself upon it, the hen escaped the embrace: she was just then in the middle of the small room and ran under a chair The amorous rabbit followed her thither and notwithstanding the cross-bars of the chair-frame, which rendered the compass which the hen was within very narrow, he found means to lay his fore-paws on her back; she escaped a second time from him, and kept quite close to the wall of the window clapping one of her sides against it. The rabbit directly took advantage of a position in which it was become more difficult for the hen to escape him, he quickly jumped upon her: she shewing the same desire to resist him, and the projection of the window not leaving her liberty enough to fly away, she had recourse to force, she turned her head to peck the lips of the rabbit three or four times, which he indeed took not for caresses, it having quite another aspect, so that he directly quitted his hold and went off: he nevertheless paid a little more for his coaxing, and the hen pecked his forehead three or four times more. Her attacks calmed his passion. He remained quiet by the hen, and even let her go from him without following her, and went himself about the room. About half an hour after, the rabbit having eaten a lettuce leaf seemed to have forgot the ill-usage he had met with; he drew near the hen again, and repeated his tender pursuits, which she seemed not more disposed to comply with than before. He in vain was able to mount her three times running, she never would permit his continuance in that position: to get rid of him she got upon a pretty low stool, he was there almost as soon as she, and made new attempts that were not more successful than those he had made when she was on the ground.
The rabbit gave over pursuits which had availed him so little, and remained quiet for about an hour after which his tender fits came on him again; about seven o’clock, he went up again to the hen who had clucked a little before but not very loud: she was in the middle of the room when he placed the fore-part of his body upon her back with a new ardour; he perhaps found means then to hook himself better there than he had done hitherto, or perhaps the hen had no longer the same disposition to run from him: she nevertheless made a few steps forward, but slowly, and with her legs somewhat bent. The amorous rabbit kept his hold and remained where he was, but then the moment of her being conquered was come; she squatted herself as all hens do, which after having run away from the cock, consent at last to admit his caresses; she let the rabbit put himself in what position he pleased; he left his two hinder-paws on the ground, and laid his body all along the back of the hen, whose tail was removed to the left side by the pressure of the thighs of the rabbit; the hen in short became a perfect doe to him; he remained active upon her four or five times longer than a cock would have done.
But was the copulation as complete as that of a hen with a cock, or of a rabbit with a doe? This I indeed know not: all I know is that what passed between them proved sufficient to cool the rabbit: he not only let the hen alone for above two hours together, but he even seemed to want nothing but rest. He passed these two hours upon the above-mentioned low stool, perfectly motionless, and without changing his place.
There had been much speculation in Paris as to the potential nature of the offspring of such a cross, but de Réaumur inspected the hen’s eggs for progeny and found none.
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