Most mammals can maintain a relatively constant and high body temperature. This is considered to be a key adaptation for theses species, enabling them to successfully colonize new habitats and survive harsher environments. Scientists from the Institut des Sciences de l’Evolution de Montpellier (ISEM) investigate the possible correlation between the maxilloturbinal in the anterior nasal cavity and the body temperature maintenance by using Micro Tomography (or MicroCT) at the MRI core facility (FBI Montpellier node). This technique was essential in this study as it rebuilt the hypothesis around body temperature maintenance. Here is what they found.

MicroCT: Image in a non-destructive way

First of all, what is Micro Tomography? Micro Tomography, or Micro-CT, is a 3D imaging technique using X-rays to see inside biological material, at a small animal or body part level. Slice by slice, this technology scans the object in a series of 2D images that are reconstructed in a 3D model. Micro CT is, thus, non-destructive. This means that it can be used to image a sample without having to cut it! Not only your material is still in one piece but you can use it for further experiments.

Phylogenetic studies as an example of application

In this study examining the correlation between skull structure and the stabilization of body temperature, MicroCT was the key. The presence and the relative size of the maxilloturbinal has been proposed as a hypothesis that reflects the endothermic conditions and basal metabolic rate in extinct vertebrates. Among bony structures, respiratory turbinals (e.g., maxilloturbinal) are interesting anatomical structures that may offer important insights to the origins of endothermy, in other words to the origin of warm-blooded animals. Indeed, respiratory turbinals are highly vascularized, which amplifies the surface area and offers an effective mechanism to avoid loss of internally-produced and costly heat.

You probably figured it out: scientists needed to compare the structure of the maxilloturbinal in order to take conclusion. This is when Micro Tomography was very useful. They scanned 424 individuals from 310 mammal species using high-resolution X-ray micro-computed tomography, with approximatively half of the samples imaged at MRI, part of our Montpellier node. Using the obtained comparative 3D µCT dataset, they explored the anatomical diversity of the maxilloturbinal based on relative surface area, morphology and complexity. They specifically tested the relationship between multiple parameters such as the size-corrected basal metabolic rate (cBMR), the relative surface area of the maxilloturbinal (Maxillo RSA) or body temperature.

And the results surprisingly showed that…

…there is no evidence to relate the origin of endothermy and the development of some turbinal bones! Even though scientists used a comprehensive dataset of Micro CT-derived maxilloturbinals spanning most mammalian orders, they demonstrate that neither corrected basal metabolic rate nor body temperature significantly correlate with the relative surface area of the maxilloturbinal. These results challenge the hypothesis of thermal regulation being linked to respiratory bone structure.

So, what could be linked with the thermoregulation of mammals? Researchers proposed 3 more hypothesis. First of all, environmental conditions could have a bigger role: “the maxilloturbinal function could have a more prominent heat/moisture exchange role in species that face harsh environmental conditions, thus helping to limit spurious heat and moisture loss”. Another major role of the maxilloturbinal is water conservation. As an example, the naked mole-rat avoid breathing through the mouth when performing energy intensive digging because the lips close behind the digging incisors and this species has the lowest value of predicted Maxillo RSA of the entire sample. But most of all, the factor could be a multifactorial physiological question. What is the relation of the maxilloturbinal with the overall nasal cavity? Do other functions play a role in the evolution of this body part, such as its protective role against toxic elements? Is it linked with brain cooling?

Well, imaging will certainly give them an answer in the future!

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Martinez, Q., Okrouhlík, J., Šumbera, R. et al. Mammalian maxilloturbinal evolution does not reflect thermal biology. Nat Commun 14, 4425 (2023). https://doi.org/10.1038/s41467-023-39994-1