Africa’s long decline in giant herbivores did not follow the pattern scientists might expect from a simple size-driven extinction crisis. A reconstruction spanning 23 million years shows that species weighing at least 1,000 kilograms generally had lower extinction rates than smaller species, while their ability to produce new species was intrinsically low and, in some lineages, declined further as Africa became more arid. The result was a long-term loss of megaherbivore diversity that began well before the final Pleistocene extinction phase.
The decline began long before the final extinctions
Modern African ecosystems contain far fewer megaherbivores than the continent once supported. In this study, megaherbivores are defined as herbivores weighing at least 1,000 kilograms.
The decline was not a single event. Local diversity and abundance of Africa’s largest herbivores had already begun decreasing by about 4 million years ago, substantially before the emergence of hominins capable of hunting or significantly modifying their environments. The researchers therefore examined the decline as a long evolutionary process rather than focusing only on the relatively recent extinctions of the Late Pleistocene.
The central question was whether large herbivores disappeared mainly because they were unusually prone to extinction, or whether another process was reducing their diversity.
That other process was speciation, the formation of new species.
Over long periods, the number of species in a group depends on both processes. Extinction removes species, while speciation adds them. A lineage can therefore lose diversity even without having an unusually high extinction rate if it produces new species too slowly to replace those that disappear.
That distinction proved important for Africa’s megaherbivores.
A 23-million-year reconstruction
Researchers reconstructed diversification among 396 African ungulate species using 3,327 fossil occurrences covering the last 23 million years. They used a Bayesian neural-network diversification model that could examine speciation and extinction together with body mass, tooth crown height, evolutionary relationships and environmental change.
The analysis divided body size into several categories, with the largest groups including animals weighing 1,000 to 5,000 kilograms and more than 5,000 kilograms. Tooth crown height was classified as brachydont, mesodont or hypsodont. The researchers also incorporated a family-level phylogeny so that differences associated with evolutionary history could be considered.
For environmental change, the researchers examined two long-term indicators. One was dust deposition used as a proxy for aridification. The other was a carbon-isotope signal from long-chain compounds used as an indicator of the expansion of C₄ grasses. The model using dust flux explained more variation in diversification rates than the model based on the C₄ grass indicator. When both environmental measures were included, aridity still had greater explanatory power.
The analysis revealed a major change in diversification beginning around 7.2 million years ago.
Speciation slowed while extinction later accelerated
Around 7.2 million years ago, several African herbivore lineages experienced increases in both speciation and extinction. Speciation then plateaued after about 3.6 million years ago.
Extinction followed a different trajectory. At the beginning of the Pleistocene, about 2.58 million years ago, extinction rates rose sharply and reached roughly three times their Miocene baseline. This increase produced widespread diversity losses across different herbivore groups.
The timing matters because the largest herbivores had already been experiencing weak speciation before this major extinction phase.
The researchers found that larger species generally had lower rates of both speciation and extinction than smaller species. At the peak of aridification, species weighing less than 45 kilograms had extinction rates only about 15% higher than species weighing more than 1,000 kilograms. But the smaller species were producing new species 2.2 times faster.
That difference changed the balance between the creation and loss of species.
For the largest herbivores, the problem was therefore not an unusually high inherent extinction rate. Their low rate of speciation left them with less capacity to replenish diversity, and environmental change further suppressed speciation in some large-bodied lineages.
Aridification changed diversification in different ways
Aridification was the strongest environmental predictor of extinction dynamics in the analysis. It was associated with an approximately threefold increase in extinction rates among African ungulates. During the most arid conditions, predicted extinction rates exceeded speciation rates by about twofold. After 2.58 million years ago, aridification reached levels associated with widespread net diversity losses.
Its effect on speciation was more complicated.
Aridification did not have a single uniform effect across all species. Instead, its influence depended on body size, evolutionary relationships and ecological traits.
Among species weighing less than 180 kilograms, environmental change was associated with about a 50% increase in speciation rates. In megaherbivores, by contrast, speciation rates were reduced by about 20%. The researchers found this suppression particularly in giraffids and hippopotamids weighing more than one tonne. Proboscideans and rhinocerotids did not show the same response.
The response also varied with tooth structure.
Species with high-crowned teeth, known as hypsodont teeth, generally had higher diversification rates. These teeth are more durable under hard, abrasive diets. Hypsodont taxa showed increased environmentally driven speciation under moderate aridity, with the effect strongest between about 7.2 and 2.58 million years ago. But as aridification intensified during the Pleistocene, speciation among hypsodont lineages slowed.
This produced a changing evolutionary landscape rather than a simple pattern in which drying conditions affected every herbivore in the same way.
Large size was not linked to higher extinction risk
The relationship between body size and extinction was one of the study’s clearest results.
Larger species had lower overall turnover. They both originated and disappeared at lower rates than smaller species. The researchers associate this pattern with the broader geographic ranges and wider habitat distributions of large species, which can make them less sensitive to local variation in resources and environmental conditions.
The result runs against the idea that the long-term African megaherbivore decline can be explained simply by large animals being preferentially eliminated.
The study found that extinction responses to aridification were delayed and weaker in larger size classes in 88% of posterior samples. Even though aridification raised extinction rates across African ungulates, its direction was relatively consistent across body sizes rather than strongly targeting the largest animals.
The final Pleistocene phase was also not size-selective in the model. From the Middle Pleistocene onward, extinction rates increased across the evolutionary tree and ecological space. Horses and bovids joined the broader extinction pattern later, and there was no evidence of size-selective extinction during the final phase covering the last 130,000 years.
Tooth structure helped shape which herbivores persisted
Body size was only one part of the pattern.
The analysis found important interactions among evolutionary history, body mass and tooth crown height. Phylogeny was an important influence on diversification, especially speciation. Artiodactyls, including hippopotamids, bovids and giraffids, showed relatively high speciation rates when the effect of phylogeny was isolated. Perissodactyls, including horses, rhinoceroses and chalicotheres, had the lowest inherent speciation and extinction rates in that analysis.
Hypsodonty was associated overall with higher diversification. The highest intrinsic diversification rates occurred among hypsodont elephantids, equids, suids and bovids.
The researchers interpret the pattern as a form of species-level sorting. Forms with dental adaptations that allowed more durable and effective processing of nutritionally poor, abrasive food tended to have lower extinction rates and higher speciation rates. In elephantids and suids, for example, hypsodont forms showed milder extinction risk than their lower-crowned or moderately high-crowned relatives.
The result was not simply that every large herbivore suffered equally. Different combinations of body size, dental traits and evolutionary history produced very different diversification trajectories.
Some giant herbivores were still diversifying
The decline of megaherbivores was not continuous across every lineage.
Between about 5.33 and 3.6 million years ago, elephantid diversification accelerated. This period produced the first hypsodont herbivores weighing several tonnes. At the same time, speciation increased among bovids and suids, while extinction rates were suppressed in bovids and in some mesodont and hypsodont suids and elephantids.
The expanding savannah biome provided a larger ecological zone, while habitat heterogeneity increased the carrying capacity of the system. Species able to graze or mix different food sources had higher intrinsic speciation and lower extinction rates. The researchers associate dietary flexibility with greater population persistence and with the survival of diverging populations that could eventually form new species.
Some ecological groups therefore expanded even as other parts of the megaherbivore niche began to empty.
Large browsing forms followed a different trajectory. Browsers weighing more than 45 kilograms experienced sustained net diversity losses after about 7 million years ago. Their extinction rates increased more strongly than their speciation rates, a pattern the researchers associate with shrinking suitable habitat for browse specialists.
The Pleistocene changed the balance across the continent
The Late Miocene and Pliocene were still periods of net positive diversification across much of the ecological space occupied by African ungulates. The Pleistocene marked a major reversal.
After 2.58 million years ago, speciation remained around Pliocene levels while extinction increased. Between 2.58 and 0.78 million years ago, several groups experienced major increases in extinction, including suids, hippopotamids, mammutids, gomphotheres and the remaining deinothere. Other large groups, including elephantids, chalicotheres and rhinoceroses, had more moderate extinction rates.
Bovids were unusual in maintaining overall positive diversification through the Early Pleistocene.
From about 780,000 years ago onward, speciation increased among some smaller suids and bovids, including browsing forms. At the same time, extinction rates increased across the broader ecological and evolutionary space. Horses and bovids experienced this increase later than some other groups, but eventually joined an increasingly widespread extinction pattern.
The researchers associate this later phase with increasingly seasonal and less productive habitats. Resources became more variable and patchily distributed, conditions that they argue could no longer support the former abundance and diversity of wide-ranging, multi-ton grazers. As the carrying capacity of ecosystems declined, extinction rates increased across clades.
The decline was a long evolutionary process
The reconstruction changes the emphasis from a simple story of large animals being killed off more readily than small ones.
Large-bodied lineages generally had lower extinction rates, yet their low speciation rates meant that they generated fewer new species. In some of these lineages, increasing aridification further suppressed speciation. When extinction rates eventually rose broadly across African ungulates during the Pleistocene, those large lineages had less capacity to offset the losses through the formation of new species.
The researchers therefore identify the loss of large species and the shift in biomass toward medium- and small-sized herbivores primarily with low, and in some cases environmentally suppressed, speciation among large-bodied lineages rather than with unusually high extinction rates in those lineages.
The findings also distinguish the long-term restructuring from the final Late Pleistocene extinction phase. Other factors, including the arrival of new taxa and human predation, could have contributed to individual extinction events, according to the researchers, but they were not identified as the cause of the widespread loss of entire ecological niches. The model instead links the broad restructuring to the long-term balance between speciation and extinction under changing environmental conditions.
The researchers caution that these results are constrained by the African fossil record. The Neogene and Quaternary sites used in the analysis mainly represent savanna environments, including woodlands, wetlands and grasslands. The processes identified here should therefore not be straightforwardly extrapolated to tropical forests, where the evolutionary dynamics may have differed.
The study was published in Nature Communications.
