Study Reveals the Unexpected Factor Restricting Earth's Biggest Land Animals

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. This means that in many areas, wild herbivores simply cannot get enough salt in their diet." 

Andrew Abraham, lead author of the study, research associate at City University of New York and NAU alumnus

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.