Different dinosaur groups independently evolved gigantic body sizes, but they all faced the same problems of overheating and damaging their brains.

«The brain and sense organs like the eye are very sensitive to temperature,» said Ruger Porter, Assistant Professor of Anatomical Instruction and lead author of the study. «Animals today often have elaborate thermoregulatory strategies to protect these tissues by shuttling hot and cool blood around various networks of blood vessels. We wanted to see if dinosaurs were doing the same things.»

Many of the famous gigantic dinosaurs — such as the long-necked sauropods or armored ankylosaurs — actually evolved those big bodies independently from smaller-bodied ancestors. «Small dinosaurs could have just run into the shade to cool off,» said study co-author Professor Lawrence Witmer, «but for those giant dinosaurs, the potential for overheating was literally inescapable. They must have had special mechanisms to control brain temperature, but what were they?»

The answer turned out to be based in physics, but still part of our everyday experience. «One of the best ways to cool things down is with evaporation,» Porter said. «The air-conditioning units in buildings and cars use evaporation, and it’s the evaporative cooling of sweat that keeps us comfortable in summer. To cool the brain, we looked to the anatomical places where there’s moisture to allow evaporative cooling, such as the eyes and especially the nasal cavity and mouth.»

To test that idea, the team looked to the modern-day relatives of dinosaurs — birds and reptiles — where studies indeed showed that evaporation of moisture in the nose, mouth, and eyes cooled the blood on its way to the brain.

Porter and Witmer obtained carcasses of birds and reptiles that had died of natural causes from zoos and wildlife rehabilitation facilities. Using a technique developed in Witmer’s lab that allows arteries and veins to show up in CT scans, they were able to trace blood flow from the sites of evaporative cooling to the brain. They also precisely measured the bony canals and grooves that conveyed the blood vessels.

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