But hippos were once common in Europe and southern Asia. In the case of Europe, for example, we have fossils from Portugal to Greece, and as far north as the UK. Nevertheless, hippos have never been especially diverse, at least in comparison to groups such as deer or pigs, and they are grouped into no more than around six widely accepted genera. This includes the two that are still with us, and we don't have a large amount of information on three of the others.
While some others were isolated on islands, there are only two widely accepted species of hippo that lived in mainland Europe, both doing so in the warm gaps between the Ice Ages. One of these is the common hippo of today, which ranged much further north than that it does now. The other is the "ancient hippo" (Hippopotamus antiquus), which was very widely ranging, being found across both western and southeastern Europe, as well as in Turkey, and perhaps as far east as the Caucasus.
This is not to say that other fossil species of European hippo have not been named down the years. Some of these were on the basis of fragmentary material that is now thought insufficient to identify anything, while others have been determined to be too similar to either of the known species to be separated out. One possible exception is the "Tiberian" hippo (Hippopotamus tiberinus) which was named in 1991 based on a fossil from Italy. More material was found in Germany in later years, so there's little doubt that it existed as something identifiable, but today it's generally regarded as a relatively late example of H. antiquus, rather than a distinct species.
The oldest known fossils of the ancient hippo date back about 2 million years, to roughly the start of the Ice Ages. At this time, it lived in the south, with fossils known from Spain, Italy, and Greece. So far as we can tell, it first headed north of the Alps just over 1 million years ago, passing through France and Germany to reach the Netherlands and southern England.
As Ice Ages came and went, it would have been forced back south from time to time, and the very last known remains are from Portugal and Italy. These both date to around 400,000 years ago, slightly later than the oldest known European fossils of the modern species, so that the two may have briefly lived side by side. The modern species then replaced it, finally leaving Europe either at the start of, or perhaps even during, the Last Ice Age.
Physically, the ancient hippo would have looked similar to the modern sort, but it had a narrower head and longer snout, and moreover, was much bigger, perhaps around twice the weight of the modern species at just over 3 tons - similar to a female Asian elephant or the largest male rhinos. This is roughly tied with the African species H. gorgops for the honour of "largest hippo ever", although there's enough uncertainty in estimating the weight of an animal for which you only have a skeleton that it's hard to be more precise.
Most fossils of the species, however, date back to the Early Pleistocene, and less is known about how it might have changed later on, which might have a bearing on the exact status of the Tiberian hippo (which might arguably be a subspecies, although it's never been formally named as such). A recently published study, however, takes a detailed look at remains from a site in Italy that may help to fill in some of this gap.
The fossil in question consists of a jawbone and part of the leg and was discovered in a cliffside at Ortona, on the Adriatic coast. It has been dated to between 790,000 and 680,000 years ago, and was once a sand bar along one edge of a lagoon. The same site has also provided fossils of red deer and the straight-tusked elephant (an extinct relative of the modern African species). The jawbone is perhaps the most significant part, its slender shape confirming that it belongs to H. antiquus, as we would expect from the age.
However, by comparing it with other fossils uncovered elsewhere, the researchers were able to identify two distinct forms of the species, an early one, and a larger one that lived later on and that matches the so-called Tiberian hippo.
As to how these forms related to each other, there are a couple of theories. The more popular is also the simplest: the later forms evolved directly from the earlier ones. However, an alternative, first proposed in 2015, keeps the idea that the ancient and Tiberian hippos are distinct species. Under this proposal, the "real" H. antiquus evolved directly from the oldest known species of the genus, H. kaisensis, from the African Rift Valley. It then died out 1.7 million years ago to be replaced by a new species that evolved from H. gorgops. If this is true, then the Tiberian hippo would either have to be restored as a species or described as a variant form of its African relative; either way, we would have three hippo species in Ice Age mainland Europe, not two.
Male hippos today have larger tusks than females, although that's at least partially because they are larger overall. This new analysis suggests that the same must have been true of the "Tiberian" hippo too, but to a much greater extent, so that it would surely have been obvious in life. This is far less true of the early forms of the ancient hippo, suggesting a change that became exaggerated as it evolved.
The teeth of an animal can also provide us with information about its diet. The general shape of the teeth is much the same in all of the Hippopotamus species, or they wouldn't be assigned to the same genus in the first place. But a closer look can show the presence of microscopic scratches and pitting that can give us more detail about things such as food toughness, while analysis of isotope ratios can also separate some food types from others.
The modern hippopotamus eats mainly grass, an abrasive foodstuff that leaves distinctive scratches on the tooth surface. In contrast, the ancient hippopotamus has a different pattern that suggests softer food, which, together with isotope and other evidence, is thought most likely to indicate lush water plants, soft leaves, and fruit. This may also explain the narrower snout, since grazing animals tend to have wider mouths to shovel more food in, rather than picking out the best bits or focusing on particular plants.
The new study shows a shift here, too, with the later "Tiberian" hippos lying somewhere between the older ancient hippos and the modern species. H. antiquus, it seems, may originally have evolved to be even more aquatic than the living species, not bothering to graze much beyond the riverbank, but it shifted towards a diet closer to the modern species with time.
Perhaps it didn't have much choice, as the Ice Ages changed the flora of Europe even beyond the tundra zone, where ice-covered rivers would have presented a more intractable obstacle. At the time the Tiberian hippo was alive, central Italy had a relatively dry, cool, climate, with an open landscape. The dominant vegetation consisted of plants like sagebrush and heather, which might have forced water-dwelling hippos to consume a wider diet than they had previously. Indeed, the fossil analysed in the study had unusually thin enamel, which might indicate that the animal in question had struggled to find enough nutritious food to keep itself fit through the winter.
All of this indicates that, even if the minority theory about the Tiberian hippo having evolved separately from its earlier counterparts is wrong, the ancient hippo changed over the million-plus years of its history, probably in response to the worsening climate. The authors point out that, if we find more complete remains of the animal, there may be enough differences in the bits we haven't seen yet to restore it to its former status as a distinct species. That's pure speculation on their part, of course, since we don't know what we don't know.
But could what we have already found be enough to designate it as an identifiable subspecies? Well, that, at least, is a plausible proposal.
[Photo by "FunkMonk" from Wikimedia Commons. Cladogram adapted from Pandolfi et al. 2020.]
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