Truly, as Weird Al Yankovic once sang, “Jurassic Park is frightening in the dark.” Stumbling around after nightfall when there are voracious Velociraptor and Tyrannosaurus about is not advisable under any circumstances. The dinosaurs will know you’re there long before that sharp split second when you find out they knew exactly where to find you. But did dinosaurs really stalk the night? A speculative Journal of Experimental Biology paper suggests that at least some of the big ones did, although the argument has more to do with keeping cool than hunting.
We’re 66 million years too late, at minimum, to directly observe non-avian dinosaurs. (And, much as I’m pained by saying so, cloning the long-lost creatures is impossible.) To investigate dinosaur lives, then, researchers often turn to living animals to examine the problems dinosaurs must have faced and possible solutions to common biological hurdles. How large dinosaurs coped with excess body heat is one of these puzzles.
Think of a relatively small dinosaur – let’s say the feathery, pigeon-sized Anchiornis – standing next to a fuzzy, 40-foot-long Tyrannosaurus. Compared to surface area, the larger dinosaur holds a greater volume. This means that the Tyrannosaurus will gain and lose body heat much more slowly than the little Anchiornis. On the positive side, this consequence of large size can help Tyrannosaurus maintain a high, constant body temperature in a phenomenon called gigantothermy. But the cost comes with exercise – a running Tyrannosaurus will quickly build up body heat that the dinosaur must somehow shed before hitting lethal internal temperatures. How did large dinosaurs manage to be the active animals we think they were without overheating?
Biologist Michael Rowe and coauthors suggest that living elephants might hold a few clues. While not at all closely related to dinosaurs, the big mammals still face some of the same problems related to body size and heat management. As detailed in their Journal of Experimental Biology study, Rowe and coauthors measured how two captive Asian elephants stored exercise-generated heat inside their bodies after walking outside under full sun in temperatures ranging from about 46 to 94 degrees Fahrenheit.
The amount of heat the elephants stored inside varied with the seasons. During November and February trials, the elephants were able to dump around 40% of the excess heat into the surrounding environment by changing bloodflow around the periphery of their bodies. But in the heat of the June trials, the skin of the elephants was too hot for this strategy to work. The elephants couldn’t shed any of the exercise-generated heat in the June trial. Since elephants can’t sweat, it’s no wonder that one of the test elephants – Panya – shuffled right into her pool to cool down for a few hours after some of the trials.
Based on the temperature fluctuations recorded in the captive animals, Rowe and coauthors calculated that an elephant could build up a lethal amount of internal heat if the behemoth walked continuously for four hours in an ambient temperature of about 88 degrees Fahrenheit. That’s problematic for animals that regularly travel long distances in the wild. To circumvent the heat problem, Rowe and collaborators suggest, elephants rely on behavioral solutions. Wild elephants can bathe and wallow in water sources along their travel routes to cool off, and walking at night would reduce heat stress for the huge mammals. There are other reasons for elephants to roam at night – avoiding humans, for one thing – but heat management is a possible benefit.
Hefty dinosaurs, Rowe and coauthors hypothesize, might have employed similar strategies to keep their internal temperatures within safe limits. When the researchers applied the heat storage model they developed for elephants to the shovel-beaked hadrosaur Edmontosaurus – a roughly 40-foot-long, four-ton herbivore – they proposed that the dinosaur would have probably suffered a fatal amount of heat buildup after three and a half to four hours of continuous exercise. The variation in the estimate is based on different expectations about dinosaur physiology.
An endothermic Edmontosaurus – which would internally maintain a near-constant body temperature – would overheat in about three and a half hours, while an Edmontosaurus with a body temperature that varied according to the outside environment would only last about four hours. For such a big animal, there would hardly be any difference between the two strategies, and this undermines the idea – still favored by some dinosaur traditionalists – that big dinosaurs must have been ecothermic to prevent themselves from overheating. If the model proposed by Rowe and colleagues is correct, having an ectothermic metabolism doesn’t seem to provide very much benefit to big, active animals.
The geographic context of some Edmontosaurus also demonstrates that these dinosaurs were not just big lizards. As Rowe and coauthors point out, Edmontosaurus skeletons have been found within the Cretaceous Arctic Circle – an environment that would have been slightly warmer than today, but which still experienced snowfall and several months of total darkness every year. Rowe and colleagues state that these dinosaurs migrated out of the Arctic during these cold times, thus making the herbivores ancient analogs of far-traveling elephants, but evidence extracted from the inside the bones of Edmontosaurus indicates that these dinosaurs remained in the Arctic all year round. The dinosaurs were not restricted to the warm, lush habitats we so often imagine them in, but were capable of surviving in chillier habitats. If you were to travel back to the time of Arctic Edmontosaurus, you may very well have seen the dinosaurs trodding through the snow. To persist in such a place, the dinosaurs would have required a physiology more like that of modern birds and mammals than reptiles.
Edmontosaurus was not restricted to the Arctic. Fossils of this Late Cretaceous dinosaur are found from Alaska through Wyoming, and possibly even as far south as Texas. How Edmontosaurus and other dinosaurs in warmer climes dealt with heat is still an open question. Perhaps, like elephants, the dinosaurs got around the problem by roaming at night. Rowe and coauthors calculate that an adult Edmontosaurus would have accrued internal heat much more slowly in the cool of the evening, allowing the dinosaurs to move about four to five hours longer than during the day. And Edmontosaurus may not even represent the full range of options open to dinosaurs.
Birds are living dinosaurs. But many of the traits we think of as being peculiar to birds actually originated tens of millions of years ago, among the non-avian dinosaurs. The system of air sacs that extend from a bird’s respiratory system into bone – helping breathing efficiency, but also making the skeleton lighten without sacrificing strengths – appears to have evolved among Triassic dinosaurs. More specifically, air sacs seem to be a common feature of saurishcian dinosaurs – that big branch of the dinosaur family tree that contains the theropods, the sauropods, and their closest early kin. (Edmontosaurus, an ornithischian, was on the other side of the dinosaur split, and no ornithischian has yet been found with evidence of air sacs.) As P. Martin Sander and coauthors speculated in a major review of why sauropod dinosaurs such as Supersaurus got to be so huge, the air sac system might have provided a large surface area over which to shed heat during exhalation. The internal soft tissues of sauropods and theropods, at least, might have given them an effective way to rid themselves off heat generated from running around.
What Edmontosaurus could do, and what the animal actually did, are two different things. Testing hypotheses about how dinosaurs behaved is frustrated by the fact that most evidence of dinosaur behavior comes from trace fossils – tracks, body impressions, and similar petrified monuments. But in 2011 paleontologists Ryosuke Motani and Lars Schmitz hypothesized that the bony rings that supported dinosaur eyes might hint at what time of day particular dinosaurs might have beene active. A dinosaur with a large scleral ring and a big aperature in the middle would appear to have eyes suited to low-light conditions, for example, and so might have been more active at dusk or at night. Large hadrosaurs were among those that Motani and Schmitz studied. The researchers concluded that the dinosaurs Corythosaurus and Saurolophus fit expectations for “cathemeral” animals that would have been most active around dawn and dusk.
Yet the bony rings might not be as accurate an indicator of behavior as Motani and Schmitz proposed. In a response paper, Margaret Hall and colleagues pointed out that the scleral rings of birds and lizards active during the day are similar to nocturnal species. The connection between bone and behavior isn’t so clear cut. Motani and Schmitz defended their analysis in a further reply, but the debate remains open.
Given that there were undoubtedly thousands of non-avian dinosaur species between 245 and 66 million years ago, we can have every expectation that there were dinosaurs which were active at dawn, midday, dusk, and night. But establishing which dinosaurs were most active at what time is still quite difficult. We can easily envision a herd of Edmontosaurus walking through the Cretaceous night, and even calculate some of the benefits of such a strategy, but these visions still exist at the very edge of the clues we can draw from living animals and what remains of the great dinosaurs.
Top Image: Edmontosaurus regalis by John Conway.
Chinsamy, A., Thomas, D., Tumarkin-Deratzian, A., Fiorillo, A. 2012. Hadrosaurs were perennial polar residents. The Anatomical Record. 295, 4: 610-614
Hall, M., Kirk, E., Kamilar, J., Carrano, M. 2011. Comment on “Nocturnality in dinosaurs inferred from scleral ring and orbit morphology.” Science. 334, 6063: 1641-1641
Rowe, M., Bakken, G., Ratliff, J., Langman, V. 2013. Heat storage in Asian elephants during submaximal exercise: behavioral regulation of thermoregulatory constrains on activity in endothermic gigantotherms. Journal of Experimental Biology. 216: 1774-1785
Sander, P., Christian, A., Clauss, M., Fechner, R., Gee, C., Griebeler, E. Gunga, H., Hummel, J., Mallison, H., Perry, S., Preuschoft, H., Rauhut, O., Remes, K., Tütken, T., Wings, O., Witzel, U. 2011. Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews. 86, 1: 117-155.
Schmitz, L., Motani, R. 2011. Nocturnality in dinosaurs inferred from scleral ring and orbit morphology. Science 322, 6030: 705-708
Schmitz, L., Motani, R. 2011. Response to comment on “Nocturnality in dinosaurs inferred from scleral ring and orbit morphology.” Science. 334, 6063: 1641-1641