What’s Warming the Cold-Blooded Iguana?

Now that dinosaurs no longer roam the Earth, scientists use modern animals to understand how their ancient ancestors reproduced, walked, and even how they regulated their body temperature. While researchers continue to debate about how dinosaurs regulated their body temperature, some are looking to reptiles as a potential model, because they are able to gain and lose heat rapidly and precisely by using their circulatory system to not only transport blood through their body, but also to control their body temperature.

The green iguana transfers heat from its head to its body and vice versa using known sites of thermal exchange – or areas where warmer or cooler blood is delivered to regulate temperature in some reptiles – in the blood vessels, located in their mouth, nose, and eyes. They do this without an advanced rete mirabile, or complex network of blood vessels, that regulates temperature in some vertebrates. The basic circulatory system seen in green iguanas led scientists to suspect that it may be similar to that used by dinosaurs. This is because extinct dinosaurs are also thought to have highly vascularized sites, which suggest that they were able to thermoregulate in a way that may be similar to the iguana.

The authors of a recently published PLOS ONE study mapped the circulatory system of green iguanas with the goal of identifying which blood vessels may contribute to heat transfer within the blood, as well as create a comprehensive map of the circulatory system for future reference. To do this, the authors obtained five green iguana cadavers and scanned each one using a series of sliced X-ray images, using a technique called CT or computerized tomography, to obtain detailed 3D pictures.  Then, they injected the carotid arteries and internal jugular veins with a colored latex solution. After the arteries and veins were injected with the latex solution, the authors scanned the green iguana cadavers again in the same manner as above.

The researchers combined the images from pre-injection and post-injection to provide a clear picture of the connection between the bone and the blood vessels in the iguana. They used the highest-quality images from all five green iguana cadavers to create an interactive 3D PDF. Once the skull in the PDF is clicked, the images “can be freely rotated and zoomed, structures can be made visible, invisible, or transparent, and the vessels can be identified simply by clicking on them and reading the bolded name at left in the Model Tree.”

Through this process, the authors created an incredibly comprehensive map of the circulatory system, as can be seen in the following figures from the paper.

Above is a left view showing the arteries of the head, and below displays the arteries of the palate.

Above shows the arteries in red and the veins in blue along the underside of the head. Below exhibits the veins in the head from a left view.

The authors found that the jugular constrictor muscle, or the muscle that restricts or allows blood flow into the jugular pathways, as well as the veins in the nasal cavity and the orbital sinus may be key in influencing heat transfer. Researchers think that the primitive vascular heat control exhibited in diapsids, and in particular these iguanas, could be an ancestral trait and may be similar to the way that dinosaurs heated and cooled themselves. It remains to be seen what more these maps can tell us, but perhaps if the path of blood flow is followed, it could lead us back into the past.

Citation: Porter WR, Witmer LM (2015) Vascular Patterns in Iguanas and Other Squamates: Blood Vessels and Sites of Thermal Exchange. PLoS ONE 10(10): e0139215. doi:10.1371/journal.pone.0139215

Images: Close-up of Green Iguana by William Warby, Flickr, Fig. 1, Fig. 3, Fig. 7, Fig. 9, Fig. 11