Among the ingredients in the recipe for life, such as carbon, hydrogen or oxygen, the most abundant, nitrogen, phosphorus or potassium, there is one, for some, the seventh of the macronutrients, as essential in cooking as in nature: sodium (symbol Na). It is rare, representing only 3% of the earth’s cortex, and the vast majority is not bioassimilable. In the big cities, he killed everything, because the Romans were trying to cover Carthage with salt. But their role is disconcerting for scientists: often absent from plants, herbivores (and the rest of the animals behind them) die without the slightest element they need for their metabolism and cellular physiology. Today, a map of African soil shows that where there is greater availability, that is where large animals, such as giraffes, rhinos or elephants, thrive. And where they are rare, they are also rare.
“West Africa and central regions have high productivity, which means there is an abundant amount of forage available for herbivores,” recalls Andrew Abraham, researcher at the City University of New York and first author of this salt map, research published in Nature ecology and evolution. “However, for a long time there has been a mystery as to why there are no more megaherbivores in these areas,” he added. Indeed, unlike what happens on the southern fringe of the Sahel, in the eastern and southern savannahs of the continent, where there is a high density of large herbivores despite less plant availability, there are other areas of sub-Saharan Africa where there are no elephants, giraffes or rhinoceroses. There are also populations of large ungulates, such as brains. This seems absurd.
A group of scientists, led by Abraham, found meaning in this idea. They created a map with a resolution of approximately 10 kilometers by 10 kilometers with data on the sodium concentration in the hundreds of plant species found in each quadrat. To do this, they took 4,258 samples in 268 locations and, using machine learning, they extended their results to the entire subcontinent. Although it is on the coasts that the contribution of marine aerosols is the greatest, in inland areas, the quantity of sodium present in the leaves is, for the authors of the study, a reliable indicator of the quantity of sodium present in the surroundings.
Then they mined this salt map of Africa with one of the best known, the distribution of herbivorous mammal species weighing more than two kilograms. To connect the two maps, they analyzed the amount of sodium present in 1,356 samples from 28 different species. It then appeared that in areas where plants with the highest Na concentrations dominate, large herbivores are more abundant. Among these areas are those which may appear in the documents: the Okavango delta, the Turkana basin, the plains of Tanzania or Kenya of the Serengeti and Masai Mara conglomerate…
“Based on our investigation, we believe that salt, probably in conjunction with other factors such as excess caza and soil infertility, plays an important role in limiting the population,” says Abraham, who is currently working in the field in South Africa. “On the African continent, the availability of sodium varies more than a thousand times depending on the plants,” specifies the American scientist. Concentrations as low as a dozen milligrams per kilogram today have been measured, ranging up to 85,000 mg in halophilic species, adapted to very saline environments such as marshes, salt deserts or coastal areas. “This means that, in many areas, wild herbivores simply cannot get enough salt in their diet. »
Taking all herbivorous species together, the link was very weak. But when it came to isolating the larger animals, the relationship was much more obvious. “In previous investigations, our colleagues and I have demonstrated that sodium requirements increase disproportionately with body mass; in other words, larger animals appear to need relatively more salt in each bite to satisfy their requirements,” says Abraham. Indeed, in 2023, they published a work that showed how, as much as possible, as many approaches as possible could seek out alternative sources of sodium, such as bathing in salt pans or sucking up sales-rich rocks as if they were caramel (see image).
“In Kenya, elephants enter caves to consume sodium-rich rocks, and in the tropical jungle of Congo they dig for salt in river banks,” explains Abraham. “Gorillas are known to fight for the richest food, even as rhinos, ñúes and suelen brains congregate in salt flats, from the Kalahari Desert to the Masái Mara,” he adds.
Without any role in plant physiology, in animals sodium forms a pair with potassium in the most fundamental part of life, opening and closing the passage through cell membranes. It is key to fluid balance, nerve signaling or muscle response. Other data show its relevance: female mammals meet their salt needs both during pregnancy and during breastfeeding. Furthermore, in the case of herbivores, there is growing evidence that lack of sodium can lead to starvation of the intestinal flora. And that’s the key to the complete digestion of this group of animals, extracting everything from something as nutrient-poor as hojas or cattle.
Elizabeth Borer is an ecologist at the University of Minnesota (USA) who has studied how sodium is distributed around the world. Their work demonstrated that although sodium is toxic to plants, there is a wide range of concentrations in plants. To the intriguing phenomenon that plants do not need sodium, which is vital to the animals that eat them, Borer offers some explanations. It’s a biochemical problem. “Sodium in the soil can make it difficult for plants to absorb water through their roots, creating tree-like conditions. Once inside a plant, it can be toxic, altering cellular processes and damaging photosynthesis,” says Borer, who was not involved in the new study.
The other explanation is speculative recognition, but it is even more intriguing and concerns natural selection: “My second thought is that since animals need sodium in their diet, plants with high sodium content would probably be more attractive to herbivores. This attraction would be disadvantageous to them, provided that low sodium content is protective.”
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