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Velociraptors on air: the megapode complex.

October 10, 2011
Rahonavis

Rahonavis as appearing in Dinosaur Revolution. The model is also apropriate for the juveniles of it's closest relatives, specially Buitreraptor (the skull, for example, is a slightly less elongated version of Buitreraptor's skull, since Rahonavis' actual skull is incomplete)

Deinonychosaurs are, like most dinosaurs, depicted as engaging in extensive parental care. The idea that dinosaurs were good parents has overall become very prevalent in media, as it is the most heartwarming way to show how complex these animals were. And indeed, we do know of several dinosaurs that engaged in extensive parental care, like ornithopods and ceratopsians.

However, the truth is that parental care in dinosaurs was a more complex issue than most people give it credit for. Unlike modern birds, most non-avian dinosaurs had extremely slow growth rates; instead of reaching adult size and sexual maturity in the space of an year or less, the norm was to grow for several years, reaching sexual maturity several years later, and even then keep growing for a few more years. This style of growth rate is more remaniscient of that present in endothermic reptiles, except in that growth stops some time after sexual maturity instead of occuring through the whole of the animal’s life (this style of growth is obviously also present in hominids like us, but for completly different reasons).

This slow grow rate would be severely detrimental for animals engaging in parental care; most animals cannot afford placing so much energy to offspring that develop so slowly. This is why birds and mammals generally have fast growth rates: to kick their offspring out of the nest as quickly as possible. Primates and a few other intelligent mammals are the exception because we rely almost exclusively on the entire group to raise the young, which are not expected to leave until adulthood. This style of prolonged parental care is not observed in birds as equally intelligent such as corvids, which have a much different brain development process. Furthermore, non-avian dinosaurs were far inferior in terms of mental capacity, which would render such a state of extensive parental care a self-destruction mechanism

It is more likely, thus, that most dinosaurs only engaged in parental care through a very brief period of time, which the young leaving very early on their lives. Once independent, their energy would be spent more on day-to-day survival rather than growth, thus ending up with a very slow growth rate. Because they’d be much smaller than the adult animals, these young dinosaurs would occupy a very different ecological niche, not only reducing species diversity, but also ensuring differences between the young and the adults.

Evidence to support this

Maleo

Megapodes like the Maleo don't engage in parental care, instead leaving their young to fend for themselves. As a result, there is a much smaller diversity of Galliformes in Oceania than elsewhere in the world.

Modern examples of endothermic animals with similar growth rates and ecological implications are the birds known as megapodes. These primitive Galliformes are unique among Neornithes in that they don’t engage in parental care; while the parents do happen to incubate the eggs in a crocodyllian like fashion (and even then some species like the Maleo merely lay the eggs in volcanic sands and end their bond to the offspring right after), the young are left to fend for themselves. As a result, they are able to fly within days after birth, if not right after birth. The young are obviously much better flyers than the adults; some have been recorded being capable of flying for over 35 km without stopping , and in fact they might be responsible for the megapode success at colonising melanesian and micronesian islands (as well as the entirety of Indonesia).

As a side effect, there are very few megapode species co-existing in the same area. Most of their diversity is due to the fact that these birds colonised a very large number of islands, the vast majority of which with only one or two species residing on them. Their slow growth rates indicate that most ecological niches occupied by fowl are pretty much taken all through the lifetime of these birds; the small, partridge like niches are taken by juveniles, while larger niches akin to those of pheasants are taken by the adults. There’s little room for other species with all availiable niches taken.

A similar situation occurs with most dinosaurs, with a high diversity of large dinosaur species, but considerably fewer smaller species, even in lagerstattë fossil sites. The major exceptions seem to have been ornithischians like ornithopods and marginocephalians, as well as basal coelurosaurs, and even then their adult forms tended to have been medium sized animals. In the Late Cretaceous, this style of niche partitioning seems to have been taken to its logical conclusion; in most ecosystems, the vast majority of the present dinosaurs had gigantic adult sizes, with pratically no medium sized dinosaurs, with the exceptions of a few small ceratopsians, relic non-hadrosaur ornithpods and, of course, deinonychosaurs. For instance, in Maastrichtian North America, Tyrannosaurus rex occupied pratically all large carnivorous niches, leaving only room for species predating on small prey such as the dog sized dromaeosaur Saurornitholestes and the giraffe sized pterosaur Quetzalcoatlus. (Troodontids, being omnivores, were exempt from competition)

This can even be observed in deinonychosaurs. Few taxa actually share the same environments; despiste coming from the same formation, Sinornithosaurus and Microraptor were set apart by 10 million years, for example. And even small specimens like Bambiraptor were usually just the juveniles of larger species. Coupled with the slow growth rates recorded in all tested specimens, it is more likely than not that deinonychosaurs were superprecocial (the proper term for animals able to leave the nest within days or even right after birth), leaving the nest early and occupying various niches across lifetime.

No real arguments against

Kiwi

Like megapodes, kiwis are also superprecocial, the young fending for themselves. Unlike megapodes, kiwis incubate their eggs, just like deinonychosaurs.

It has occasionally been stated that deinonychosaurs were not superprecocial because we have evidence of brooding behaviour among troodontids. However, this is a rather poor excuse, considering that incubation =/= parental care.

Like mentioned before, megapodes take care of their eggs (sans for a few examples like the Maleo). The breeding pair builds a mound where the eggs are placed, and for the remainder of the incubation period both parents pay close attention to the temperature of the nest, meeting all requirements for the sake of their offspring. Yet, the parents do not raise the chicks, leaving before the eggs hatch. Thus, we have an obvious example where parental care does not extend beyond incubation.

A more obvious example occurs in ratites.  We know that the examples of brooding troodontids are all male, implying that, much like ratites, male deinonychosaurs were the ones that raised the young. Baby ratites are generally not superprecocial, but they are very independent from the parents, relying on them for protection alone; they can find food on their own, thus the parents don’t waste energy bringing nourishment. There is, however, a ratite clade that is, in fact, superprecocial: Apteryx, the kiwis. Like all ratites, the male is responsible for incubating the eggs, but does not waste energy protecting the chick; the baby kiwi is every bit as solitary as the adult animals. In comparation to the other kiwis, this is not something revolutionary, since parental care is kept to a minimum anyway, but it shows that animals that invest a lot of energy in ensuring that the eggs survive will not spend the same amount raising the subsequent chicks.

Thus, parental care in deinonychosaurs was at most very relaxed. The known troodontid embryos show that the brain was considerably well developed, indicating that it was at the very least precocial. If juvenile deinonychosaurs were capable of flight, the need for parents would vanish completly. And even then both scenarios are not exclusive; modern small birds like bee-eaters use larger birds like bustards as perches from which they have good spots to hunt aerial insects by hawking. In species where the ecological niches of the adults would have been different from those of the babies, there’s enough room for symbiotic weirdness.

8 Comments leave one →
  1. June 10, 2016 9:39 am

    It is good that you stressed the fact that dinosaurs were not the dedicated parents most people think of. In fact many birds show minimal parental care, because they have precocial young. Most galloanserae are not so different from ratites. Chickens for example don’t do extreme things for their young. The only additional think they do for them is to show them the food, but they don’t feed them. Most people are biased by thinking that the passerin mode of parental care is the norm for birds. The fact is birds are ancestrally precocial, while mammals are ancestrally altricial.
    As for kiwis, they are probably an exception. Despite of being precocial, the apparently have quite large brains
    https://www.researchgate.net/publication/5814501_Evolution_of_Brain_Size_in_the_Palaeognath_Lineage_with_an_Emphasis_on_New_Zealand_Ratites

    • June 10, 2016 2:11 pm

      Indeed. Got into quite the fights with rabid dinosaur fans over superprecociality.

      Interesting. This would be a game changer in our understanding of dinosaur intelligence

  2. December 29, 2016 4:24 pm

    Also, is the megapod style of parental care unsuccessful in the modern world? They don’t seem to be able to coexist with placental mammals or substantial human populations. The vastness of Australia might be protective to them in the short run, but in many other smaller islands, like in New Caledonia, they went extinct after humans arrived.

    • December 29, 2016 6:38 pm

      Australia has a vast rodent fauna, so yes, they can co-exist with placentals.

      Island birds becoming extinct is not really surprising

  3. December 30, 2016 8:54 pm

    Ok, I must have narrowed down the competitors. I mean mostly small carnivorans like mustelids. Most Australian rodents restrict their diet to seeds, grasses and a few insects. There must be an animal that commonly and consistently attacks the nests or the young to be a problem. Is there any megapod above the Wallace line?

    • December 30, 2016 9:16 pm

      Yes, the Maleo and a few species in other “main” indonesian islands.

      Also some australian rodents have animalivorous diets

  4. January 7, 2017 2:03 am

    Ok then, some of them can establish themselves in more placental-dominated ecosystems. We must then acertain if they just hang out in a few protected refugia in some corner of the islands or really participate in the ecosystem, or even have expanding distribution.
    What do you mean by animalivorous? Most rodents eat insects, some eat small lizards, and some scavenge. Are they capable of weasel-like precision moves to capture much larger prey?

    • January 7, 2017 2:34 am

      I would say yes, megapodes seem to be dispersing westwards well and evidently can survive in ecosystems dominated by placentals.

      Various australian rodents eat small birds and other vertebrates

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