Nesting Times – Dinosaur Brood Parasites?

In the great evolutionary arms race to survive, thrive and reproduce many animals cheat. Consider the cuckoo for example. They are famous for laying their eggs in the nests of other birds and in fact this strategy, called brood parasitism, is found in many animal groups from beetles to fish. Its a clever behavior, foistering all the hard work onto other, unsuspecting parents and ensuring their own offspring the best possible start in life. Although we might find it somehow unsporting in the struggle of natural selection it can be a highly successful tactic.

Cuculus_canorus_vogelartinfo_chris_romeiks_CHR0791_cropped
European cuckoo. Source: Wikimedia Commons

So why are we talking about cuckoos in a blog about palaeopathology? The answer is an intriguing find from a dinosaur nest site discovered in 1994. The nest itself belonged to the much-maligned species Oviraptor, which was thought to be an egg thief until the discovery of this particular nest. Unlike other, earlier finds these eggs actually preserved tiny embryos inside them revealing that they in fact belonged to Oviraptor. Since then a cavalcade of various dinosaurs entombed whilst incubating their eggs have been found, rehabilitating Oviraptor’s reputation, and revealing that dinosaurs shared even more in common with their avian descendents then anyone had realised. That in of itself makes the 1994 nest a fascinating and seminal discovery but there is an intriguing footnote to this find. Alongside the clutch of Oviraptor eggs there were two, partial skulls belonging to quite a different species. They were tiny, hatchlings or embryos it wasn’t possible to be sure, and they were unmistakably dromaeosaurs. This is a group of small, predatory theropods made famous by Velociraptor. So what were they doing in an Oviraptor nest?

The most obvious answer is that the adult Oviraptor wasn’t above raiding the nests of other species after all. We know that they ate lizards, so hatchling dinosaurs could easily have been on the menu as well. Another logical possibility is that these tiny predators were searching for their own opportunistic dinner and came across the Oviraptor nest with its small, nutritious eggs. Finally they might have been washed into the nest after death and all buried together. There is another suggestion though, albeit a very tentative and speculative one, that is brood parasitism. Could a mother Velociraptor, or other dromaeosaur, have deposited a couple of her eggs into an unguarded nest? Naturally fossil evidence for this scenario is poor. There is no way of knowing from this one nest site what was really going on during that day in the late Cretaceous but can we draw any conclusions from what we know of modern birds?

Maiasaurusnest
Maiasaura were apparently devoted parents. Source: Wikimedia Commons

Firstly is there any reason to bother being a brood parasite if you are a dinosaur? Modern brood parasites gain an advantage because their young are taken care of and fed by the surrogate parents. In fact many have evolved to hatch early so they can get rid of the competition in the nest and commandeer all their host’s attention. There would be no reason to do this is your host wouldn’t feed you so did Oviraptor take care of their young after they hatched? Well evidence for postnatal care in dinosaurs is not unheard of. Perhaps the most famous example is Maiasaura, the ‘good-mother’ dinosaur, a nine-metre herbivore from the fossil beds of Montana. They nested in large colonies and their young were born underdeveloped, unable to walk. This would have required the parents to bring them food at least for their first few months of life and strongly suggests prolonged parental care. We know many other dinosaurs also nested in large groups and seem to have moved in herds later along with adolescents of the same species. There has even been one speculative suggestion that some species may have fed their young using milk, similar to the crop milk produced by modern pigeons. There is no direct evidence of whether Oviraptor explicitly fed its young after they were born but such care is far from out of the question. This means that brood parasitism would have potentially been a useful strategy, but was it actually possible?

Of all the modern bird species found today only 1% are obligate brood parasites so it seems to be a relatively rare occurrence but that being said it has evolved independently at least seven times. Apparently then it is no huge leap to develop the behaviour. The problem is though that all known brood parasites fall into one of two categories. Either they are obligate brood parasites, like cuckoos, which don’t make a nest of their own ever and entirely co-opt those of other birds, or they are non-obligate parasites. These lay their eggs in the nests of other members of their own species as well as building and protecting their own nest. In the bird world it very much seems to be an either/or behaviour with no known examples of animals laying their eggs in other species and their own nests. If we take this as a rule of thumb then whichever dromaeosaur laid its eggs in Oviraptor’s nest would have been an obligate parasite. So far very few nests from any dromaeosaurs have been found but given how rare the fossilisation of eggs is this shouldn’t really surprise us. It also makes it almost impossible to evaluate how these animals really cared for their young, if at all. Hopefully new fossils will come to light because any information about the parental care of these ancient creatures would be fascinating, but its a long way from saying they were all slyly depositing their eggs in someone else’s basket. Also so far, from all the other known fossil egg sites, none seem to have held any apparent interlopers. Ironically the second type of brood parasitism we mentioned, non-obligatory, is a much more plausible scenario, especially for colonially nesting dinosaurs like Maiasaura but it is also totally impossible to identify from fossils alone. It would take a DNA test to confirm this particular behaviour and of course that’s never going to be possible.

Personally I like a good bit of rampant speculation now and again but we must always be constrained by the facts when we come to draw conclusions. Sadly there is absolutely no evidence whatsoever of brood parasitism in dinosaurs and unless dramatic new evidence comes to light this is one idea that demands too many assumptions. Still dinosaurs inhabited the Earth for millions of years and I would be more surprised if in all that time none ever evolved with peculiar but highly successful trick. Only time will tell.

 

Reference: Norell, M.A., et al. 1994. A Theropod Dinosaur Embryo and the Affinities of the Flaming Cliffs Dinosaur Eggs. Science. 266(5186). pp. 779-782.
DOI: 10.1126/science.266.5186.779

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