Africa's giant herbivores declined from a birth deficit, not a death rate

A study in Nature Communications by researchers at the National Museum of Natural Sciences (MNCN-CSIC), the National Research Center on Human Evolution (CENIEH) and the University of Alcala concludes that African megaherbivores lost diversity because new large species stopped emerging, not because large species were more extinction-prone ^[1][2]. The paper's title states the finding directly: lower speciation, not higher extinction, drove the collapse ^[3].

That reverses the standard reading, in which large body size is treated as an extinction liability ^[2]. Megaherbivores here means animals over a tonne, a group that today includes elephants and hippopotamuses and is described as acting as ecosystem engineers ^[4].

The team reconstructed evolutionary dynamics for herbivorous mammals over the past 23 million years using 3,327 fossil records covering 396 species, including hippopotamuses, antelopes and giraffes ^[5]. That is an average of roughly eight records per species, which is worth holding in mind when reading rate estimates ^[6]. The model incorporated body mass, tooth crown height, evolutionary relationships among families, and environmental change on the continent ^[7].

The sequence matters. Around 7.2 million years ago Africa began drying, shifting landscapes and vegetation and reconfiguring ungulate communities; at first both origination and extinction rose together ^[8][9]. Then, 3.6 million years ago, new species stopped appearing at the same pace while extinctions kept climbing, accelerating at the onset of the Pleistocene 2.58 million years ago ^[10]. The gap between the origination stall and the Pleistocene acceleration is about a million years ^[11].

The size comparison is the load-bearing number. During periods of peak aridity, animals under 45 kilograms went extinct only 15% faster than those over a tonne, but produced new species at 2.2 times the rate ^[12]. "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," says MNCN researcher Juan Lopez Cantalapiedra, who adds that megaherbivores gave rise to new species much more slowly ^[13]. Oscar Sanisidro of the University of Alcala de Henares says aridification worked in both directions at once, favouring speciation among smaller ungulates while cutting it by nearly 20% among megaherbivores ^[14]. Casualties of the pattern include Deinotherium, an elephant relative with downward-curving tusks, and the Anthracotheriidae, hippo relatives that resembled large pigs ^[15][16].

The authors note the diversity loss was already visible long before humans had the technology to hunt animals of that size ^[17]. Ignacio A. Lazagabaster of CENIEH says human activity may have contributed to some recent disappearances but was not the main cause of the broad loss ^[18].

The operational consequence is unflattering for how large-mammal conservation is usually framed. If the deficit sits in origination rather than survival, there is no evolutionary replacement queue: the dynamics described here play out over intervals of a million years and more ^[10][11], while management plans run in decades. On this account, a lost megaherbivore lineage is a permanent line item, not a slot that refills.

Worth watching: whether the speciation asymmetry holds on other continents and under different fossil sampling densities, given roughly eight records per species here ^[6], and whether the 45-kilogram threshold used for the small-bodied comparison ^[12] is a real biological break or an artefact of how the size bins were drawn.