Why are there so few elephants and giraffes in Africa? Researchers point to salt deficiency

A new study published in Nature Ecology & Evolution by scientists at Northern Arizona University and their colleagues has found that the density and distribution of the largest land animals - elephants, giraffes, rhinos and other mega herbivores - are largely limited by the availability of salt. According to their findings, there are relatively few places on the planet where such animals can get enough sodium from native vegetation to maintain viable populations.

"In Africa, the sodium content of plants can vary by more than a thousand times," explains the paper's lead author, Andrew Abraham, a researcher at the City University of New York and a graduate student at Northern Arizona University. - This means that in a huge number of regions, wild herbivores simply cannot physically get enough salt from their food."

Why giants are particularly affected

The problem, to one degree or another, is common to all herbivores: most plants don't need salt, so they are low in sodium. However, in large animals, the so-called mega-herbivores, the deficiency is particularly acute. It has previously been hypothesised that sodium deficiency increases with body size. A new study using a different approach and independent data has reached the same conclusion.

The scientists created highly detailed maps of sodium content in vegetation and compared them to databases of densities of large herbivores, as well as analyses of their faeces. According to the authors, manure can be used to judge, among other things, whether animals are getting enough salt. As a result, areas with low sodium availability coincided, as expected, with areas where large herbivores are significantly less abundant.

But it's not just a matter of numbers. Salt 'limitation' also explains a range of unusual animal behavioural strategies.

Salt as a cause of unusual behaviour

"In Kenya, elephants go into caves to gnaw on sodium-containing rocks," says Abraham. - In the rainforests of Congo, they dig up salt in riverbeds. Gorillas can fight over the saltiest plants, and rhinos, gnu and zebras gather en masse at salt marshes from the Kalahari Desert to the Masai Mara".

The authors of the paper also offer a new explanation for the phenomenon of "missing" mega herbivores.

"West Africa is a very productive region, there's a lot of biomass," says study co-author Chris Doughty, a professor of ecoinformatics at Northern Arizona University. - Yet large herbivores are relatively scarce there. We think sodium deficiency, probably in combination with other factors - overhunting and poor soils - plays an important role in limiting their numbers."

Risks to conservation and conflicts with humans

The findings raise serious questions for conservation policy. Many protected areas where large herbivores live are in low-sodium zones. Meanwhile, human activities create artificial "salt points " - for example, around water wells, road salting areas, pastures and farmland.

"If animals can't get enough sodium in their natural habitats, they will go out to humans in search of salt," Abraham warns. - This increases the risk of conflict, from trampled fields to dangerous encounters with large animals."

The authors emphasise that understanding the role of salt in megaherbivore ecology is important to consider when planning reserves, managing populations and developing measures to reduce human-wildlife conflict.