During their Mesozoic reign, dinosaurs occupied just about every corner of the planet, from the northern reaches of Alaska and Siberia all the way down to the Antarctic. Even in those warmer times, the polar parts of the planet were chilly, which poses a puzzling question: how did polar dinosaurs keep their eggs warm?

"Many reptiles today only live in warm environments, tropical or subtropical," said Kohei Tanaka of the University of Calgary while presenting his research last week at the Society of Vertebrate Paleontology conference. "I wanted to know how dinosaurs reproduced in high-latitude environments, in cool regions.”

Dinosaur eggs – and the remains of young dinosaurs – have been found as high as 76° North latitude, putting them in what would have been ancient Siberia, where even summer temperatures are estimated to have reached less than 20°C (68°F).

These eggs belonged to herbivorous hadrosaur dinosaurs. Fossilised eggs can often be linked to the dinosaurs that laid them if they are found nearby adult or baby skeletons, or if there are fossilised embryos inside. Image: Kohei Tanaka

Dinosaurs belong to a group of life called archosaurs, so to answer the egg-warming question, Tanaka looked for inspiration in the reproductive habits of the still-living members of this group: crocodilians, which are generally limited to warmer regions where they (and their eggs) can stay toasty; and birds, which often get around the problem of cold surroundings by placing their warm fluffy bodies over their nests and brooding their eggs.

Brooding is not just for the birds – there are dinosaurs that are known to have sat upon their nests. Amazing fossils of small feathery dinosaurs like Citipati reveal the animals crouched over their eggs, feathery arms spread out to the sides. But these were small and particularly bird-like dinosaurs, compared to the more famous giants of the time. "We don't think large dinosaurs brooded," Tanaka told me. "They could crush the eggs!"

Fortunately, there are other sources of warmth to be found in nature. "If you depend on the environment to incubate eggs, you have to get heat from solar radiation or geothermal activities or microbial respiration," Tanaka said. Could dinosaurs have used these strategies to keep eggs warm in cold regions?

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Solar heating is pretty straightforward: lots of birds and crocs build sandy sun-baked nests, but this strategy only works in warmer parts of the world, otherwise the cool air temperature counteracts the sun's warmth. Even more rare is the geothermal approach, used by animals like the maleo bird of Indonesia, which nests near active volcanoes! But a dinosaur would have to live in a lucky spot to count on this method for egg-warming.

Microbes, on the other hand, are everywhere. Many living crocodilians and megapode (or mound-builder) birds bury their nests in piles of soil and vegetation, rich in organic material, and some ancient dinosaurs are already suspected to have done this as well. As the plants are rotted away by hungry microscopic organisms, the decomposition process releases heat, which can keep the eggs significantly warmer than the surrounding environment. There are modern-day archosaurs that use this strategy to keep their nests warm in high latitudes, where the sun's heat wouldn't be enough.

Small, feathery dinosaurs like Citipati osmolskae (pictured) would have crouched over their eggs, feathery arms spread out to the sides. Image: Kumiko/Flickr

So, how do we know which heating strategy different dinosaurs used? The answer may be in the rocks. Fossil eggs are rare, but many nests are known from such extinct creatures as the duck-billed hadrosaurs, the huge long-necked sauropods as well as smaller, feathery theropods like oviraptorids. Tanaka surveyed nearly 200 ancient nests from these dinosaur groups, and found patterns in the sediments surrounding the eggs.

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Sauropod (long-necked dinosaur) eggs come in two varieties. One egg type was most commonly laid in coarse sandy sediment, the same type of material preferred by solar-nesters today. The second egg type was almost always found in fine soils associated with plant material, just like what we see in birds and crocs that lay microbe-fuelled nests. It seems the different sorts of sauropods employed different nesting strategies, and Tanaka noted that the vegetation-nesting sauropods are more widespread.

It looks like hadrosaurs ("duck-billed" herbivores) had a habit of nesting with plants as well, as their nests were mostly made of soil and vegetation. It may be no coincidence that hadrosaurs are known to have lived in Siberia and other high-latitude areas, as this habit of heating their eggs with decomposition could have been the key to their survival in those regions.

On the other hand, the brooding dinosaurs – oviraptorosaurs and troodontids – showed no sediment preference in their nests. They were split about fifty-fifty between sand and soil, which makes sense: if they were warming their eggs with their own body heat, the material of their nest wouldn't matter much.

These elongated eggs belonged to an oviraptorosaur, a group of dinosaurs known to have sat over their eggs and kept them warm with the feathers that covered adults' bodies. Image: Kohei Tanaka

These results are intriguing, but Thomas Holtz of the University of Maryland urged caution, given how much we still don't know about dinosaur nests. "Since nests are fairly rare," he said, "the lack of finding a nest isn't necessarily strong evidence that those nest types weren't there." There are many dinosaurs known from polar regions – including big meat-eaters and armoured ankylosaurs, for example – whose eggs have never been found. When we do finally make those discoveries, they might not meld so well with the data Tanaka has recovered.

"That said, there seems to be at least good theoretical bases behind his idea," Holtz continued. 

There are other potential strategies for reproducing in cold regions, but they seem unlikely for dinosaurs. Many lizards and snakes, and most mammals, have abandoned egg-laying altogether, but there's no evidence supporting live birth in any dinosaurs. Some researchers have even suggested polar dinosaurs may have migrated to warmer climes to lay eggs, but there is evidence that casts doubt on this as well, including the inferred long hatching times of some dinosaurs.

Tanaka and Holtz both expressed interest in finding more eggs and baby dinosaurs from cold regions. Understanding how these animals were hatching, growing and living in more challenging environments will add to our growing picture of how these incredible creatures were able to conquer habitats across the entire planet.



Tanaka presented this research at this year's meeting of the Society of Vertebrate Paleontology.