How Did Dinosaurs Raise Their Families?

The Child-Rearing Behavior of Dinosaurs

Tyrannosaurus rex dinosaur

How difficult is it to figure out how dinosaurs parented their children? Well, consider this: until the 1920's, scientists weren't even sure if dinosaurs laid eggs (like modern reptiles and birds) or gave birth to live young (like mammals). Thanks to some spectacular dinosaur egg discoveries, we now know the former to be the case, but the evidence for child-rearing behavior is more elusive--consisting mainly of the tangled skeletons of individual dinosaurs of various ages, preserved nesting grounds, and analogies with the behavior of modern reptiles, birds, and mammals.

One thing is clear, though: different kinds of dinosaurs had different child-rearing regimens. Just as the babies of modern prey animals like zebras and gazelles are born with the ability to walk and run (so they can stick close to the herd and evade predators), one would reasonably expect that the eggs of large sauropods and titanosaurs produced "ready-to-run" hatchlings. And since modern birds care for their newborns in specially prepared nests, at least some feathered dinosaurs must have done the same--not high up in trees, necessarily, but in clearly marked-out birthing grounds.​

What Can Dinosaur Eggs Tell Us About Dinosaur Families?

One of the main difference between viviparous (live birthing) mammals and oviparous (egg laying) reptiles is that the former can only give birth to a limited number of live newborns at a time (one for large animals like elephants, seven or eight at a time for smaller animals like cats and pigs), while the latter can potentially lay dozens of eggs in a single sitting. A female Seismosaurus, for instance, may have laid as many as 20 or 30 eggs at a time (despite what you may think, the eggs of 50-ton sauropods weren't any bigger than bowling balls, and often significantly smaller).

Why did dinosaurs lay so many eggs? As a general rule, a given animal will only produce as many young as are necessary to assure the survival of the species). The gruesome fact is that out of a clutch of 20 or 30 newly hatched Stegosaurus babies, the vast majority would immediately be gobbled up by swarming tyrannosaurs and raptors--leaving just enough survivors to grow into adulthood and ensure the perpetuation of the Stegosaurus line.  And just as many modern reptiles, including turtles, leave their eggs unattended after they're laid, it's a good bet that many dinosaurs did too.

For decades, paleontologists assumed that all dinosaurs employed this drop-your-eggs-and-run strategy and that all hatchlings were left to struggle (or die) in a hostile environment. That changed in the 1970's when Jack Horner discovered the immense nesting grounds of a duck-billed dinosaur he named Maiasaura (Greek for "good mother lizard"). Each of the hundreds of Maisaura females that populated these grounds laid 30 or 40 eggs apiece in circular clutches; and Egg Mountain, as the site is now known, has yielded numerous fossils not only of Maiasaura eggs, but of hatchlings, juveniles, and adults as well.

Finding all these Maiasaura individuals tangled together, in different stages of development, was tantalizing enough. But further analysis demonstrated that newly hatched Maiasaura possessed immature leg muscles (and thus were probably incapable of walking, much less running), and their teeth had evidence of wear. What this implies is that adult Maiasaura brought food back to the nest and cared for their hatchlings until they were old enough to fend for themselves--the first clear evidence of dinosaur child-rearing behavior. Since then, similar behavior has been adduced for Psittacosaurus, an early ceratopsian, as well as another hadrosaur, Hypacrosaurus, and various other ornithischian dinosaurs.

However, one shouldn't conclude that all plant-eating dinosaurs treated their hatchlings with this degree of tender, loving care. Sauropods, for example, probably did not look after their young too closely, for the simple reason that a twelve-inch-long, newborn Apatosaurus would easily have been crushed by the lumbering feet of its own mother! In these circumstances, a newborn sauropod might stand a better chance of survival on its own--even as its siblings were picked off by hungry theropods. (Recently, evidence has come to light that some newly hatched sauropods and titanosaurs were capable of running on their hind legs, at least for brief periods of time, which helps to support this theory.)

The Parenting Behavior of Meat-Eating Dinosaurs

Because they were so populous and laid so many eggs, we know more about the parenting behavior of plant-eating dinosaurs than that of their meat-eating antagonists. When it comes to large predators like Allosaurus and Tyrannosaurus Rex, the fossil record yields a complete blank: in the absence of any evidence to the contrary, the going assumption is that these dinosaurs simply laid their eggs and forgot about them. (Presumably, a newly hatched Allosaurus would be just as vulnerable to predation as a newly hatched Ankylosaurus, which is why theropods laid multiple eggs at a time, just like their plant-eating cousins.)

To date, the poster genus for child-rearing theropods is the North American Troodon, which also has the reputation (deserved or not) of being the smartest dinosaur that ever lived. An analysis of the fossilized clutches laid by this dinosaur hints that the males, rather than the females, incubated the eggs--which may not be as surprising as you think, given that the males of many extant bird species are also expert brooders. We also have evidence of male brooding for two distantly related Troodon cousins, Oviraptor and Citipati, though it's still unknown whether any of these dinosaurs cared for their young after they hatched. (Oviraptor, by the way, was given its libelous name--Greek for "egg thief"--in the mistaken belief that it stole and ate the eggs of other dinosaurs; in fact, this particular individual was sitting on a clutch of its own eggs!)

How Avian and Marine Reptiles Raised their Young

Pterosaurs, the flying reptiles of the Mesozoic Era, are a black hole when it comes to evidence of child-rearing. To date, only a handful of fossilized pterosaur eggs have been discovered, the first as recently as 2004, hardly a large enough sample to draw any inferences about parental care. The current state of thinking, based on the analysis of fossilized pterosaur juveniles, is that chicks emerged from their eggs "fully cooked" and required little or no parental attention. There are also hints that some pterosaurs may have buried their immature eggs rather than incubating them inside their bodies, though the evidence is far from conclusive.

The real surprise comes when we turn to the marine reptiles that populated the lakes, rivers, and oceans of the Jurassic and Cretaceous periods. Compelling evidence (such as tiny embryos fossilized inside the bodies of their mothers) leads paleontologists to believe that most, if not all, ichthyosaurs gave birth to live young in the water rather than laying their eggs on land--the first, and as far as we know only, reptiles ever to have done so. As with pterosaurs, the evidence for later marine reptiles like plesiosaurs, pliosaurs, and mosasaurs is pretty much nonexistent; some of these sleek predators may well have been viviparous, but they may also have returned to land seasonally to lay their eggs.