Sabre-toothed tiger’s sense of smell reconstructed using skull scans

Smilodon was a ferocious sabre-toothed tiger with teeth up to 28 centimetres long. But when it comes to its sense of smell, the fearsome predator was on a par with a mouse.

This is one of the findings of a new study examining the sense of smell of a variety of living and extinct mammals. By linking the internal space of a mammal’s skull, known as an endocast, to the number of genes devoted to smell, researchers were able to estimate how well-developed this sense was in extinct species.

Their results suggest that Smilodon’s sense of smell wasn’t as good as a wolf’s, while the extinct thylacine had a much more sensitive nose than either. At the other end of the scale, the ancient armadillo Glyptodon had a fairly weak sense of smell similar to some living bats.

Dr Quentin Martinez, the study’s lead author and award-winning wildlife photographer, says that the team’s research will help scientists to better understand how these long-lost animals lived.

“For decades, people have looked at the endocast to try and infer an animal’s sense of smell, but there just wasn’t enough evidence to connect it to their olfactory capabilities,” Quentin says. “We were able to link these characteristics together, meaning we’re now able to infer the sense of smell in a variety of extinct mammals.”

“Now we know that we can use the endocast to understand their sense of smell, there’s so much we can discover about these extinct species. The sense of smell is influenced by an animal’s diet, lifestyle, and habitat, so it can open many new windows into their lives.”

The findings of the study were published in the journal Proceedings of the National Academy of Sciences.

From skull to smell

Smell is an important sense for mammals, helping them to find food, avoid predators and search for mates. As a result, mammals have many genes that are used to code for the olfactory receptors that detect smells in the air.

These receptors then send signals to a part of the brain known as the olfactory bulb for processing. In most mammals, this structure’s outline is reflected in the shape of the surrounding skull, meaning endocasts can be used to reconstruct what the olfactory bulb looked like.

Using CT scans from 66 museum specimens of living animals, the researchers created detailed endocasts of the olfactory bulb for each species. These were then compared to the number of olfactory receptor genes in each animal, as mammals with more genes tend to have a better sense of smell.

In general, species with relatively large endocasts for their size had more olfactory receptor genes, and therefore a better sense of smell.

The African elephant, for example, has both a relatively large endocast and plenty of olfactory receptor genes. As such, it’s no surprise that these animals have an extraordinarily developed sense of smell and can tell how much food is in a sealed container using just its nose.

Macaques, meanwhile, have a relatively small olfactory bulb endocast and a low number of smell genes. This means that, like many other primates including humans, they have a pretty poor sense of smell.

What happened to the sense of smell in whales?

Using the trends they’d seen in living mammals, the team were then able to use skull scans from five extinct species to estimate their sense of smell. Their findings revealed a few surprises, especially in an ancient relative of whales known as Protocetus.

Marine mammals such as seals and manatees generally have a pretty limited sense of smell, as it’s not as useful in the oceans as it is on land. So, the team thought that Protocetus would have lost much of its ability to smell – but that wasn’t the case.

“Protocetus’ olfactory bulb is still quite developed, even though it’s already quite adapted to aquatic life,” Quentin remarks. “This is a surprise, because we’d have expected its sense of smell would have started to degrade already.”

The findings suggests that the widespread loss of smell in dolphins and whales must have happened later in their evolution than was initially expected. Whether this was the result of a specific selective pressure on these animals, or a more passive part of evolution, is hard to say.

“We know that the sense of smell in mammals is intimately linked with breathing, so attempting to smell underwater could lead to a marine mammal drowning,” Quentin speculates.

“It’s also possible that, in the trade-off for space within the marine mammal skull, smell just wasn’t as useful for survival as other senses and so lost out.”

Quentin hopes to dive deeper into this topic by examining the skulls of larger whales and dolphins to see how their sense of smell changed over time. He’s also planning on looking at why certain mammals, such as rhinos, don’t fit into the overall trend.

“This paper is a proof of concept that’s only the very beginning of what we can discover,” Quentin says. “Knowing that the endocast is a good proxy for a mammal’s sense of smell opens so many doors into research that we couldn’t do before.”

“As someone who’s been fascinated by the evolution of smell in mammals for a long time, I’m very excited to find out what comes next.”