Did dinosaurs produce milk for their young?
A controversial study suggests dinosaurs may have produced a milk-like substance to feed rapidly growing hatchlings.
A THEORY THAT SOME dinosaurs fed their young using a process similar to lactation, a function mostly associated with mammals, may change the way we look at dinosaur babies says an Australian academic.
The idea, published this week in The Journal of Experimental Biology is little more than a hypothesis at this stage. Nevertheless, the idea could redefine how we understand the way dinosaurs fed and grew, says author Professor Paul Else, a comparative physiologist at the University of Wollongong, NSW.
Traditionally, many dinosaurs were thought to apply a 'set-and-forget' approach, in which food is collected and then left for the young in nests. It's as if the parents were saying "here's a nice stick of green stuff - go for it," says Paul.
However, evidence now suggest that some dinosaurs may have regurgitated food for their young, just as modern birds do. But for young dinosaurs, even half-digested food required a lot more work, and this would have limited the rate at which they could grow.
"A lot of dinosaurs were believed to be quite fast growing, so it doesn't fit very well," says Paul, who wonders how dinosaurs grew to such huge sizes.
Dinosaurs may have 'lactated' like birds
To explain this fast growth, Paul has proposed that some dinosaurs used a process similar to lactation seen in mammals, and employed by some modern birds, including pigeons, doves and flamingos.
'While not strictly mammalian 'lactation', these birds produce a milk-like substance in the stomach and throat that is mixed with regurgitated food and fed to their young.
As researchers at Deakin University found in 2011, the bird 'milk' – which can be in the form of pellets or an "oily, yellowish cheese-like substance" – contains protein and fat as well as immune-enhancing factors and antioxidants which allows chicks to grow faster than if receiving only regurgitated food.
As bird are their living descendants, Paul questions whether some dinosaurs used a similar mechanism. "I think it is the type of mechanism which would be very easy for different groups to evolve independently," he says.
Fossil evidence hard to find
The dinosaur milk could have been loaded with growth hormone, antioxidants, calcium and minerals to boost growth. As a bonus, both parents can do it, says Paul. The problem now is finding fossil evidence for this theory, which "is going to be hard," he says.
The parts of the body responsible for secreting the 'milk' would have been made from soft tissue that is unlikely to be preserved in the fossil record.
One source of evidence, however, turns up in an organ called the crop, which is responsible for the secretion of the milk-like substance in modern birds. Recently it has emerged that ancient birds may have had this organ, and so this may mean that it was found in bird-like dinosaurs.
The idea is "definitely controversial," comments Professor Frank Seebacher, a biologist at the University of Sydney, who agrees that it will be next to impossible to find any evidence for the theory. "I think, in my opinion, it will remain an idea, but that's what science is made out of," he says.
Another link in chain between dinosaurs and birds
Dr Steve Salisbury, one of Australia's leading dinosaur experts, who is based at the University of Queensland, says "the use of the term lactation is a bit misleading," because the process is so different to mammals.
Nevetherless, he argues that Paul's idea is a logical progression in our understanding of the links between birds and dinosaurs. "I think rather than adding some amazing new insight, it's just altering what we are already coming to accept about dinosaurs," Steve told Australian Geographic.
In recent years many brooding and nesting behaviours known in birds, have been found to have originated in their dinosaurian ancestors. Duck-billed hadrosaurs and their relatives in particular are known to have formed vast nesting colonies where they cared for their young as seabirds do today.
*This story was updated on 12/02/13 to include a comment from Steve Salisbury.
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