Slow Emergence of New Species Drove the Decline of African Megaherbivores, Study Finds

A new study by Spain's National Museum of Natural Sciences (MNCN-CSIC), the National Research Center on Human Evolution (CENIEH) and the University of Alcalá offers a new explanation for the long-term decline of African megaherbivores — animals weighing more than a tonne, including today's elephants and hippopotamuses, that act as ecosystem engineers. Published in Nature Communications, the study finds that their loss of diversity was not because they were more vulnerable to extinction, as previously proposed, but because new megaherbivore species emerged at particularly low rates.

To reach that conclusion, the team reconstructed the evolutionary dynamics of herbivorous mammals — including hippopotamuses, antelopes and giraffes — over the past 23 million years, analyzing 3,327 fossil records covering 396 species and incorporating data on body mass, tooth crown height, evolutionary relationships and environmental change. The analysis shows that Africa's climate began growing drier around 7.2 million years ago. Both the emergence of new species and the disappearance of others increased at first, but around 3.6 million years ago new species stopped emerging at the same pace even as extinctions kept rising, accelerating further at the start of the Pleistocene 2.58 million years ago — long before humans had the technological capacity to hunt animals of that size.

"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 periods of peak aridity, animals under 45 kilograms went extinct only 15% faster than those over a tonne, but generated new species at a rate 2.2 times higher, allowing smaller and medium-sized herbivores to better offset their losses.

"Large lineages, by contrast, kept losing diversity because not enough new species emerged to replace those that disappeared," said Óscar Sanisidro of the University of Alcalá de Henares, adding that aridification reduced new-species formation among megaherbivores by nearly 20%. Species affected by the decline include Deinotherium, an extinct elephant relative with downward-curving tusks, and the Anthracotheriidae, hippopotamus relatives that resembled a large pig. "Our results suggest that, 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.