On the road to Dinosaur National Monument, along Vernal, Utah’s main drag, there stands a dinosaur wearing a bikini. The cartoon Diplodocus has always unsettled me a little. Dinosaurs did not have mammary glands, much less breasts – that’s only a feature of some placental mammals, such as primates, elephants, and sea cows – so I’m not entirely sure why Dinah had to be covered up. The top implies the existence of something that could not have existed. Yet, even though we can be certain that dinosaurs did not have monstrous mammaries, a new Journal of Experimental Biology paper proposes that the archosaurs might have been capable of nurturing their young with an alternative to mammalian milk.
Simply titled “Dinosaur lactation?”, the commentary by University of Wollongong health scientist Paul Else speculates that a peculiar form of nurturing seen among modern birds might have originated among non-avian dinosaurs. Birds such as doves, flamingos, penguins, and petrels can produce a milky substance in their crops or other parts of their upper digestive system. The fluid contains antibodies, fat, protein, and other nourishing elements. Perhaps, Else speculates, non-avian dinosaurs fed their young a similar substance.
There is no direct evidence that dinosaurs produced “crop milk.” Else makes his case based upon the evolutionary connection between birds and dinosaurs, as well as the hypothesis that the substance would have been one way for adult dinosaurs to feed their newly-hatched young.
Of all dinosaurs, Else suggests that hadrosaurs such as Maiasaura were the most likely to produce milk because their babies may not have been able to effectively break down plant food until they developed teeth and the proper gut flora. Else hypothesizes that the fluid could have been a starter substitute until the infants were able to consume and digest fibrous vegetation. Such a fortified liquid boost, Else also supposes, might account for why young dinosaurs grew incredibly rapidly.
I doubt that proud parental Parasaurolophus slobbered milk into the mouths of nestlings, though. As evocative as Else’s foray into speculative zoology is, his hypothesis is undermined by the relationships of birds that produce milk-like products.
All of the birds Else mentions in his study belong to a group called Neoaves, and each of the mentioned avian species produce the milk-like substance in different ways. Pigeons generate the fluid in their crop, for example, whereas emperor penguins secrete the liquid from the lining of their esophagus. This hints that the ability to produce such substances evolved multiple times within Neoaves, rather than being a shared feature that goes back to non-avian dinosaurs.
If alligators and crocodiles engaged in “secretory feeding”, as well as birds throughout the avian family tree, then we could build a case that dinosaurs might have done the same. This principle, known as the extant phylogenetic bracket, is quite simple. Birds are dinosaurs, and crocodylians are the closet living relatives to the Dinosauria as a whole. If both birds and crocodylians share a particular feature, then there’s a chance that the trait was present in non-avian dinosaurs, too. It’s the same logic behind the hypothesis that male dinosaurs had an “intromittent organ.” Male crocodylians do, and various species among the basal branches of the bird family tree do, as well, and so it’s reasonable to hypothesize that any given male dinosaur had a phallus even though fossil evidence of the organ has never been found. In the case of bird pseudo-milk, by contrast, the presence of the ability in some derived birds and the lack of the trait in crocodylians indicates that there’s no good reason to think that dinosaurs lactated in any sense of the word. Maiasaura milk only exists in the realm of imagination.
Else, P. 2013. Dinosaur lactation? Journal of Experimental Biology. 216: 347-351