Reconstructing extinct species' sense of smell

[Music] Welcome to Science Sessions, the podcast of the proceedings of the National Academy of Sciences, where we connect you with Academy members, researchers, and policymakers. Join us as we explore the stories behind the science. I'm Paul Gabrielson. How good was the sense of smell of the extinct saber-toothed tiger, or the giant ground sloth, or the ancestors of today's cetaceans? When fossilization preserves only the bony parts of an animal's body, it's challenging to reconstruct a brain function like olfaction. Unless that is, there are clues in the preserved skull that can help. In a recent PNAS study, Kanton Martinez of the Natural History Museum in Stuttgart, Germany, and colleagues, estimated the olfactory capacities of five extinct species based on the volumes of the part of the fossilized skull containing the olfactory bulb. They did this using a correlation in modern mammals between the number of their olfactory genes and the size of the olfactory bulb. Estimating extinct animals' olfaction can help answer questions about behavior, ecology, and evolution. Kanton, tell us more about why we want to reconstruct olfaction in extinct animals. Yes, olfaction is a key sense for animals, and it is involved in food detection and food evaluation, so also predator avoidance, and also in many social interaction, for example finding a potential mate for our productions. And it is also well known that olfaction can vary with ecological adaptation of species. So, for example, species living in aquatic environments, like marine mammals, are known to have reduced olfactory capabilities. And in contrast, for example, subterranean species, meaning species living fully underground, are hypothesized to have an end olfactory capabilities. And this is because their vision is reduced, and they need to rely more on other senses for their behavior. So, olfaction is closely related to species ecology, and being able to reconstruct olfactory capabilities in extinct mammals can strongly inform us about their ecology and behaviors. And also from an evolutionary biology perspective, knowing the morphology and the relative volume of different brain structures, such as the olfactory bulbs, it's a low-hust to better understand the mechanisms of evolution, from an ancestral condition to the condition observed now in living species. Tell us about the olfactory bulb endocast, or space within the skull that contained the olfactory bulb. Why is it important in evaluating olfaction in extinct animals? In most living mammals, the most entire part of the brain is called the olfactory bulb. So, this part of the brain mainly processes information related to the sense of smell. And in living mammals, general hypothesis is that when species has a large olfactory bulb, it relies strongly on olfaction for short reviles, and likely as good olfactory capabilities. And the problem with extinct species is that the brain are rarely preserved in the fossil records, and especially in deep time. So therefore, we need to rely on the proxies, meaning bonnie proxies or bonnestructures that fossilize and can inform us about the brain. So in mammals, we are lucky because the volume of the brain case, which is the bonnestructures that surround the brain, it's highly correlated with the volume of the actual brain. So the volume of the brain case calls the brain endocast, can therefore be used to infer the brain volume, and especially olfactory bulb volume in extinct species. But this needs to be proven. So, other bonnestructures have also been studied as potential proxies for mammalian olfaction, such as bonnestructures in the nasal cavity called turbinals, or the cribry form plays, which is a perforated bonnil plate through which olfactory nerve pass. And the problem is that in extinct species, these structures are rarely preserved, or they are often damaged. And in contrast, the brain case is often preserved in fossil, and therefore if we can prove that the olfactory bulb endocast is a good proxy for olfaction, we can therefore study olfaction in hundreds of extinct mammals. Why did you study the genomics of chemoreceptor genes in addition to the bonnestructure? So, for the olfactory bulb endocast to be a good proxy, its relative volume need to correlate with olfactory capabilities in living species. This means that species with good olfactory capabilities should have large olfactory bulb endocast, and species with reduced olfactory capabilities should have a small one. And in 2024, we published an article showing that in mammals, there is a relationship between olfactory capabilities and the genomic proxy of olfactions called the functional olfactory receptors genes. So, species with many functional olfactory receptors genes usually have good olfactory capabilities. And therefore, to test if the olfactory bulb endocast is a good proxy for mammalian olfactions, we need to test whether there is a significant correlation between the relative volume of the olfactory bulb endocast and the number of genes related to olfactions. What correlation did you find? So, to test this potential correlation, we first needed to acquire morphological data, meaning the relative volume of the olfactory bulb endocast in living mammals. And for this, we perform compute tomography scans of mammalian skulls, and these scans allow us to reconstruct the brain endocast in three dimensions and measure its volumes. And we also collected genomic data for the same species, and in total, we still dies about 70 living mammal species, representing all mammalian order. So, to come up with this morphological data, we need, of course, to first make the whole data acquisition that was quite fun. Then we need to see to scan the skulls of those species that are museum-preserved skulls. And it was quite a lot of work because we need to get in various international institutions. It was also a technical challenge for big skulls, like cetacean skulls, elephant skulls and so on, because generally, a city facilities can only allow small objects to be cetacean. And when you have a elephant skull that is very, very big, it's quite challenging. So, for all the big skulls, we get access to peculiar facilities, or made cetacean facilities in big and situate in Germany. But it was a lot of work because it was a small scan that need to be stitched, altogether, to make huge data, and then there is plenty of post-processing that are quite challenging. And then, using BUS, genomic and morphological data, we found that there is a significant correlation between the relative volume of the olfactory bulb on the cast, and the number of functional olfactory receptor genes. So, this means that species with a large olfactory bulb on the cast tend to have more olfactory receptor genes, and therefore potentially good olfactory capabilities. We were able to use these relationships to estimate olfactory receptor genes, numbers in extinct species. And for this old extinct species, genomic data cannot be recovered. So, using the measured olfactory bulb on the cast volume, and the correlation we identified, we can mathematically estimate the number of olfactory receptor genes. What insights can we get from your reconstruction? Yes, so this study is mainly a first step, as it validates the use of the olfactory bulb on the cast as a proxy to study olfaction. However, we were already able to gain interesting insight, especially for extinct cetaceans. So, modern cetacean are fully aquatic mammals, but their common ancestor was terrestrial. And during the transition from long to water, the general hypothesis is a cetacean progressively reduced their olfactory capabilities. And in this study, we analyzed a next thing to us that lived about 45 million years ago, and we found that despite being at least partially aquatic, this early whale still had a very large olfactory bulb on the cast, and suggesting that it's retained a strong olfactory capabilities. And more generally, we can apply this approach to many groups of extinct mammals and many political contexts, such as subterranean species or species with specialized diets or species living in peculiar environments. What are the caveats or limitations of the study? The correlation we found is still indirect. We did not directly test the relationships between olfactory bulb on the cast volume and olfactory capabilities. Instead, we tested the relationship with the olfactory genes, which have previously been shown to correlate with olfactory capabilities. And the main reasons is that olfactory capabilities have been experimentally tested in only a few species using behavior experiments, often in zoo. And with so few species, statistical power is very low and strong correlation or conclusion are quite difficult. Also, another limitation is that some species deviate from the general trends. For example, some species have a large olfactory bulb on the cast, but few olfactory genes. And we also have the opposite patterns, such as the rhinocerosis. This means that not all species follow the general patterns. But this is all about the end. also very interesting because these species are good candidates to better understand or all-faction walks. And indeed, compared to other senses, like vision or hearing, all-faction and the mechanism of outdoor detections are still not fully understood. Thanks for tuning into science sessions. You can subscribe to science sessions on iTunes, Spotify, or wherever you get your podcasts. If you like this episode, please consider leaving a review and helping us spread the word.