Africa's Giant Herbivores Declined Because New Species Evolved Too Slowly, Study Finds

A new study suggests Africa's giant herbivores — including elephants, hippopotamuses and their extinct relatives — declined mainly because new species evolved too slowly to replace the ones that vanished, not because their huge size made them especially vulnerable to extinction. The findings, published in the journal Nature Communications, come from researchers at Spain's National Museum of Natural Sciences (MNCN-CSIC), the National Research Center on Human Evolution (CENIEH) and the University of Alcalá.

The team examined 3,327 fossil records covering 396 species of large plant-eating mammals — including hippopotamuses, antelopes and giraffes — that lived in Africa over the past 23 million years. They used neural network models to estimate how extinction and speciation (the formation of new species) rates changed through time, factoring in body mass, tooth height and environmental change.

"We found that, contrary to what was previously thought, larger species did not go extinct faster than smaller ones. In fact, their extinction rates were somewhat lower. The key is that megaherbivore species gave rise to new species much more slowly," said MNCN researcher Juan López Cantalapiedra. During Africa's driest periods, animals under 45 kilograms died out only 15% faster than those weighing more than a metric ton — but they formed new species at 2.2 times the rate.

As Africa grew drier from around 7.2 million years ago, forests gave way to open habitats. Speciation among giant herbivores stopped keeping pace around 3.6 million years ago, and extinctions then climbed sharply at the start of the Pleistocene epoch, 2.58 million years ago — long before humans had the technology to hunt animals of that size. Casualties included Deinotherium, an elephant relative with tusks that curved downward from its lower jaw, and the piglike Anthracotheriidae, relatives of modern hippos.

"Although human activity may have contributed to some recent disappearances, it was not the main cause of the widespread loss of these large herbivores," said Ignacio A. Lazagabaster of CENIEH.