Animals invest tremendous time and effort into their offspring, even before the youngsters take their first breaths. An elephant is typically pregnant for 23 months. Depending on the altitude, black alpine salamanders will gestate for two to three years before giving birth to live young. Sharks stay pregnant for up to 3.5 years. Even the humble velvet worm can carry its offspring for up to 15 months, and komodo dragon eggs take 7 to 8 months to incubate and hatch. All in all, building even a tiny organism is a huge time commitment.
And birds? Birds have it relatively easy. Their eggs are ready to hatch after 11-85 days of incubation. Even the immense ostrich needs only 42 days – six weeks – of incubation time for its chicks to emerge and meet the world. This short developmental period is advantageous – rapid development minimizes time spent at especially vulnerable stages, and makes each individual offspring a less risky investment to its parents.
For example, if a predator kills an infant elephant on its first day of life, its mother just wasted two years of her life (and an enormous amount of energy) creating that individual, and won’t be able to have another one for over two more years. On the other hand, if something eats an ostrich chick, much less time has been lost, and a new one can be produced relatively quickly.
Birds are living dinosaurs, so many scientists assumed that bird and dinosaur eggs incubated at similar rates, after adjusting for size and other physical factors . If that were the case, dinosaur eggs would take 45-80 days[*] to hatch (Carpenter 1999). That was all just an assumption based on extrapolating data, however – until recently, no one had actually tried to measure incubation times of embryonic dinosaurs.
How would you even going about such a study? First, you need some embryonic dinosaurs. While these aren’t incredibly common, they do exist. To be careful, you would probably want samples of dinosaurs covering as broad a size range as possible. You would also need a way to determine daily growth rate of the embryos.
While all of that may seem like a tall order, it is precisely what a group of scientists from the American Museum of Natural History, Florida State University, and the University of Calgary have done. They acquired fossils of embryonic young from two species spanning a good portion of dinosaur egg size spectrum: Protoceratops, with eggs weighing in at less than half a pound (0.23 kg), and Hypacrosaurus, with eggs that can weigh almost 9 pounds (4 kg). (For reference, an ostrich egg weighs around 3 pounds, or 1.4 kg).
These embryonic specimens had emerging teeth that featured “lines of von Ebner“, which function essentially like the growth rings on a tree. Each line is a record of the daily mineralization of a developing embryo’s teeth. If you count the number of these lines on a dinosaur’s tooth, you have a record of the number of days that tooth had been developing. Teeth don’t erupt at the very start of incubation, so the researchers used information from tooth emergence time in other reptiles to estimate how long the egg had been developing before tooth mineralization began.
The results of Erickson and colleagues’ analyses showed that dinosaur eggs developed at less than half the rate of bird eggs. The smaller Protoceratops egg would take around 40 days to hatch if it developed at the same rate as a bird, but fossil evidence suggests that it took around 83 days. Similarly, the much larger Hypacrosaurus would only need 82 days to hatch at a birdlike rate, but the dental analysis showed that it actually took a whopping 171 days – over five and a half months.
This study has overturned a long-held assumption about one of dinosaur’s most fundamental life history traits: developmental time. So, what does that mean for our understanding of dinosaurs, of how they lived, of what they were? The implications are both broad and deep. With an empirical way to estimate developmental time, scientists will be able to move forward with much more accurate estimates of dinosaur generation times, reproductive investment, lifespans, metabolism, and even nonavian dinosaur extinction.
Today, many of the world’s most endangered animals are those with relatively long generation times, because they require sustained periods of time to recover from population declines. Could the ponderous incubation period required by dinosaurs have put them at a similar disadvantage? Eggs are incredibly vulnerable objects, and if nonavian dinosaurs required significantly longer periods of time in egg form than avian dinosaurs, that could also help to explain why the latter group survived to the present day, while the others died out at the end of the Cretaceous. Much more research and many more insights and discoveries will likely stem from this work, but for now, we know that dinosaurs had to protect and watch over their eggs for far longer than previously thought.
Carpenter, K. 1996. Dinosaur eggs and babies. Cambridge University Press. New York.
Erickson, G. M., Zelenitsky, D. K., Kay, D. I., and Norell, M. A. 2017. Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth how reptilian-grade development. Proceedings of the National Academy of Scientists. 114(3):540-545. doi: 10.1073/pnas.1613716114
[*] If that number seems small, keep in mind that dinosaur egg size wasn’t necessarily proportional to dinosaur body size. There are physiological constraints on how big an egg can be while still diffusing gases between the membranes and shell efficiently. Dinosaurs laid large clutches of small eggs, and the largest eggs known are less than five times the size of an ostrich egg. Even the one hundred ton Titanosaur laid eggs less than two meters in diameter.