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.

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.