It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Humans live in a world abundant in salt, but this everyday seasoning is a luxury for wild herbivores, and it’s far from clear how these animals get enough.
A new study published today in Nature Ecology and Evolution and authored by Northern Arizona University researchers and collaborators found the density and distribution of Earth’s largest land animals, including elephants, giraffes and rhinos, appear to be limited by this kitchen essential. There are only a few areas in the world where these large animals can get enough sodium from the local flora to survive.
“In Africa, sodium availability varies over a thousandfold in plants,” said Andrew Abraham, lead author of the study, a research associate at City University of New York and NAU alumnus. “This means that in many areas, wild herbivores simply cannot get enough salt in their diet.”
This is true to some extent for all herbivores—most plants don’t need salt and often contain trace amounts of it—but it’s especially pronounced for megaherbivores. Previous research had suggested that sodium deficiency increases with body size. Using a totally separate methodology, this study reached the same conclusion.
Mapping the missing megaherbivores
The authors combined their high-resolution maps of plant sodium with databases of animal dung and density measurements. Dung can tell scientists a lot about animals, including whether they’re getting enough salt. They connected areas with salt limitation to lower numbers of larger herbivores.
It’s not just about ability to survive, though. Salt limitation explains several interesting behaviors exhibited by wild animals.
“In Kenya, elephants enter caves to consume the sodium-rich rocks and in the Congo rainforest, they dig for salt in riverbeds,” Abraham said. “Gorillas are known to fight for the saltiest foods, while rhinos, wildebeest and zebra often gather at salt pans from the Kalahari Desert to the Maasai Mara.”
This study also offers a new explanation for the “missing” megaherbivores.
“West Africa is a very productive region, but there aren’t many megaherbivores there,” said Chris Doughty, a professor of ecoinformatics at NAU. “We think that a lack of sodium, likely combined with other factors such as overhunting and soil infertility, plays an important role in limiting their numbers.”
This research raises a number of conservation concerns. Many protected areas are located in low-sodium environments, and humans have created artificial sodium hotspots through various activities like borehole pumping and road salting.
“If animals can’t get enough sodium in their natural habitats, they may come into conflict with people on their quest to satisfy their salt hunger,” Abraham said.
Herbivores require a steady intake of sodium to keep their metabolism running smoothly. This is why farm animals have long been given salt or mineral licks. Animals in the wild, however, need to get their salt from sources in their habitats. In some areas, plants and other natural sources of salt provide sufficient sodium, while in others sodium levels are scarce. These differences can influence where certain species settle or how far they will migrate to find natural salt licks.
A new study conducted in collaboration with the University of Zurich now shows that in many places the largest herbivores in the wild – elephants, giraffes and rhinos – have limited access to sodium. The researchers combined high-resolution maps of plant sodium with data on the animals’ population density and with results of fecal analyses. Since sodium deficiency is directly detectable in the feces, they were able to draw conclusions about the species’ actual sodium intake.
The larger the body, the greater the risk
“Plant sodium availability varies by a factor of 1,000 across sub-Saharan Africa,” says Marcus Clauss, co-director at the University Animal Hospital at UZH and co-author of the study. “In some areas, wild herbivores are simply unable to get enough salt through their food.”
However, not all herbivores are equally affected. The researchers found that sodium scarcity is particularly common among larger-bodied species, or megaherbivores. Their study confirms previous findings that the risk of sodium scarcity increases with body size.
Sodium shortage affects habitat selection
The study also explains certain wildlife behaviors. “In Kenya, for example, elephants enter caves to reach sodium-rich rock, while in the Congo they dig for salt in riverbeds. And this behavior isn’t limited to elephants. Gorillas fight over particularly salty foods, and rhinos, wildebeest and zebras often congregate at salt pans in the Kalahari Desert,” says first author Andrew Abraham of Northern Arizona University.
The study also provides a new explanation for the scarcity of megaherbivores in West Africa, a region rich in vegetation and species but where megaherbivore numbers are low. The researchers suspect that sodium deficiency plays a central role in the low numbers observed, likely in combination with other factors such as overhunting or poor soil fertility.
Potential conflicts
The researchers also point to key issues for nature conservation. “In areas populated by humans, artificial sodium hotspots are created by boreholes or – in northern parts of the world – by road salting. However, since many protected areas are located in regions that are low in sodium, animals that travel long distances in search of salt could come into conflict with humans more frequently in the future,” explains Clauss.
Simulations showed that sound waves applied to the eardrum of Thrinaxodon (top) would have enabled it to hear much more effectively than through bone conduction alone (bottom). (Credit: April I. Neander, Alec Wilken)
One of the most important steps in the evolution of modern mammals was the development of highly sensitive hearing. The middle ear of mammals, with an eardrum and several small bones, allows us to hear a broad range of frequencies and volumes, which was a big help to early, mostly nocturnal mammal ancestors as they tried to survive alongside dinosaurs.
New research by paleontologists from the University of Chicago shows that this modern mode of hearing evolved much earlier than previously thought. Working with detailed CT scans of the skull and jawbones of Thrinaxodon liorhinus, a 250-million-year-old mammal predecessor, they used engineering methods to simulate the effects of different sound pressures and frequencies on its anatomy. Their models show that the creature likely had an eardrum large enough to hear airborne sound effectively, nearly 50 million years before scientists previously thought this evolved in early mammals.
“For almost a century, scientists have been trying to figure out how these animals could hear. These ideas have captivated the imagination of paleontologists who work in mammal evolution, but until now we haven’t had very strong biomechanical tests,” said Alec Wilken, a graduate student who led the study, which was published this week in PNAS. “Now, with our advances in computational biomechanics, we can start to say smart things about what the anatomy means for how this animal could hear.”
Testing a 50-year-old hypothesis
Thrinaxodon was a cynodont, a group of animals from the early Triassic period with features beginning to transition from reptiles to mammals, like specialized teeth, changes to the palate and diaphragm to improve breathing and metabolism, and probably warm-bloodedness and fur. In early cynodonts, including Thrinaxodon, the ear bones (malleus, incus, stapes) were attached to their jawbones; later, these bones separated from the jaw to form a distinct middle ear, considered a key development in the evolution of modern mammals.
Fifty years ago, Edgar Allin, a paleontologist at the University of Illinois Chicago, first speculated that cynodonts like Thrinaxodon had a membrane suspended across a hooked structure on the jawbone that was a precursor to the modern eardrum. Until then, scientists who studied mammal evolution mostly believed that early cynodonts heard through bone conduction, or via so-called “jaw listening” where they set their mandibles on the ground to pick up vibrations. While the eardrum idea was fascinating, there was no way to definitively test if such a structure could work to hear airborne sounds.
Turning fossils into an engineering problem
Modern imaging tools like CT scanning have revolutionized the field of paleontology, allowing scientists to unlock a wealth of information that wouldn’t have been possible through studying physical specimens alone. Wilken and his advisors, Zhe-Xi Luo, PhD, and Callum Ross, PhD, both Professors of Organismal Biology and Anatomy, took a well-known Thrinaxodon specimen from the University of California Berkeley Museum of Paleontology and scanned it in UChicago’s PaleoCT Laboratory. The resulting 3D model gave them a highly detailed reconstruction of its skull and jawbones, with all the dimensions, shapes, angles and curves they needed to determine how a potential eardrum might function.
Next, they used a software tool called Strand7 to perform finite element analysis, an approach that breaks down a system into smaller parts with different physical characteristics. Such tools are usually used for complex engineering problems, like predicting stresses on bridges, aircraft, and buildings, or analyzing heat distribution in engines. The team used the software to simulate how the anatomy of Thrinaxodon would respond to different sound pressures and frequencies, using a library of known properties about the thickness, density, and flexibility of bones, ligaments, muscles, and skin from living animals.
The results were loud and clear: Thrinaxodon, with an eardrum tucked into a crook on its jawbone, could definitely hear that way much more effectively than through bone conduction. The size and shape of its eardrum would have produced the right vibrations to move the ear bones and generate enough pressure to stimulate its auditory nerves and detect sound frequencies. While it still would have relied on some jaw listening, the eardrum was already responsible for most of its hearing.
“Once we have the CT model from the fossil, we can take material properties from extant animals and make it as if our Thrinaxodon came alive,” Luo said. “That hasn’t been possible before, and this software simulation showed us that vibration through sound is essentially the way this animal could hear.”
Wilken said the new technology allowed them to answer an old question by turning it into an engineering problem. “That’s why this is such a cool problem to study,” he said. “We took a high concept problem—that is, ‘how do ear bones wiggle in a 250-million-year-old fossil?’--and tested a simple hypothesis using these sophisticated tools. And it turns out in Thrinaxodon, the eardrum does just fine all by itself.”
The study, “Biomechanics of the mandibular middle ear of the cynodont Thrinaxodon and the evolution of mammal hearing,” was supported by UChicago, the National Institutes of Health, and the National Science Foundation. Chelsie C. G. Snipes from UChicago was an additional author.
3D models of the jaw and associated middle ear bones of the Triassic mammal ancestor Thrinaxodon show that the switch to mammal-like hearing with an eardrum evolved much earlier than previously thought.
(Credit: April I. Neander, Alec Wilken)
Zhe-Xi Luo (left) holds the fossil specimen of Thrinaxodon, while Alec Wilken (right) holds a 3D printed model of the inner ear of a modern opossum for comparison. (Credit: Matt Wood)
Olfaction written in bones: New insights into the evolution of the sense of smell in mammals
By combining anatomical skull analyses and genetic studies, researchers have succeeded in assessing the sense of smell in both living and fossil mammals. The research results were published in the journal ‘Proceedings of the National Academy of Sciences
Quentin Martinez and Eli Amson holding mammalian skulls they CT-scanned for this study (a La Plata dolphin and a West Indian manatee) within the mammalian collection of the SMNS, standing next to a mammoth skull.
The sense of smell is vital for animals, as it helps them find food, protect themselves from predators and interact socially. An international research team led by Dr Quentin Martinez and Dr Eli Amson from State Museum of Natural History Stuttgart has now discovered that certain areas of the brain skull allow conclusions to be drawn about the sense of smell in mammals. Particularly significant is the volume of the endocast of the olfactory bulb, a bony structure in the skull that is often well preserved even in very old fossils. This volume is closely related to the number of intact odour receptor genes – an important indicator of olfactory ability. This allows the sense of smell to be estimated even in extinct species such as early whales, sabre-toothed cats or the Tasmanian tiger, also known as the thylacine. The study, which provides a reliable method for reconstructing the sense of smell in extinct mammals, was published in the journal‘Proceedings of the National Academy of Sciences’ (PNAS).
From the brain to the genes – the connection between anatomy and genomics It is a major challenge for scientists to understand the development of the sense of smell, especially in long-extinct animals whose behaviour can no longer be observed today. In mammals, the volume of the braincase cortex roughly corresponds to the volume of the brain. The present study shows that the larger the front part of the braincase, which contains the olfactory bulb, the more functional odour receptor genes the animal has – an important indication of the development of the sense of smell. Since the bony braincase is well preserved in many fossils, researchers can reconstruct the development of the sense of smell even in long-extinct species.
‘Our approach – from the brain to the genes – combines the anatomy of the skull with genetic information. This helps us to better understand the evolution of the sense of smell in mammals,’ explains Dr Quentin Martinez, scientist at the State Museum of Natural History Stuttgart and lead author of the study.
Extensive skull analysis: from shrews to elephants For this comprehensive study, the research team examined skulls from all mammalian orders using computed tomography (CT).
"Our samples ranged from the ten-gram shrew to the five-ton African bush elephant and included the endocranium of elephants, whales, rhinos, primates and many other species. Scanning extremely large skulls in particular required unusual CT scanning equipment and was a technical challenge. Trying to scan an elephant or whale skull can be quite an adventure," says Dr Eli Amson, palaeontologist at the State Museum of Natural History Stuttgart and expert on fossil mammals.
What could extinct mammals smell? With the help of comprehensive anatomical and genetic studies and detailed analyses of fossils and bones, researchers have been able to assess the olfactory abilities of a wide variety of extinct mammals.
"Among other things, we examined fossils of early whales from the Eocene, sabre-toothed cats and the Tasmanian tiger, as well as other extinct species. We found it particularly exciting that some of the early whales still had a clearly pronounced olfactory bulb. This suggests that they had a good sense of smell – in contrast to today's toothed whales such as dolphins, whose olfactory bulb has shrunk considerably in the course of evolution. Early whales from the Eocene therefore probably had a very good sense of smell," says Dr Quentin Martinez.
A new window on the evolution of the senses By linking anatomical features of the skull with genetic information, the study provides a better understanding of the development of the sense of smell over the course of evolution. It yields new insights into the lifestyle and ecological adaptations of today's mammals and those that lived millions of years ago. These findings are an important basis for research into sensory evolution and open up new perspectives for interpreting the palaeoecology and behaviour of extinct mammals.
Background:
State Museum of Natural History Stuttgart: The State Museum of Natural History Stuttgart is a future-oriented research and communication institution. Its research collections, the archives of diversity, comprise over 12 million objects. The museum researches the evolution of life, analyses the biodiversity of various ecosystems and communicates research findings to the public. http://www.naturkundemuseum-bw.de
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The olfactory bulb endocast as a proxy for mammalian olfaction
Article Publication Date
12-Dec-2025
Photo of a mouse (Mus musculus), a species whose brain endocast was examined in the current article by Martinez et al.
Photo of the semi-aquatic nutria (Myocastor coypus), a species whose brain endocast was examined in the current article by Martinez et al.
CrediT
Quentin Martinez
Photo of a Guiana squirrel monkey (Saimiri sciureus), a species whose brain endocast was examined in the current article by Martinez et al. and which, like most primates, has a reduced sense of smell compared to other land vertebrates.
A mammoth skull from the collections of the State Museum of Natural History Stuttgart.
Differences in the endocast of the olfactory bulb in five iconic extinct mammals. The endocast of the olfactory bulb is shown in yellow, while the rest of the brain endocast is shown in dark grey. The skulls are not shown to scale.
Credit
Martinez et al.
The teams from the State Museum of Natural History Stuttgart (SMNS) and the Karlsruhe Institute of Technology (KIT) with a box containing the skull of an early whale for CT scanning.
