Dwarf hippos of Madagascar were forest dwellers, not grazers

Researchers at the University of Cincinnati have found compelling evidence that extinct dwarf hippos, once indigenous to the island of Madagascar, were forest dwellers as opposed to their mainland African counterparts that favored open grasslands. The study illuminates the significant role that forests played in Madagascar’s ecological past, challenging previous notions about the island’s landscape.

In the journal Plants, People, Planet, the experts report that the extensive grasslands that currently dominate Madagascar are more a result of recent human-driven changes than a naturally sustained habitat influenced by the island’s large vegetarian fauna.

Malegasy hippos

When Madagascar detached from mainland Africa around 150 million years ago, it evolved independently, nurturing a unique ecosystem secluded in the Indian Ocean. Unlike the mainland that was populated with elephants, giraffes, rhinos, and other large mammals, Madagascar had small hippos.

These dwarf or Malagasy hippos were much smaller than their four-ton relatives – the common hippos. Still yet, the dwarf hippos were one of the island’s largest land animals, along with Nile crocodiles and the gigantic flightless elephant bird. 

The Malagasy hippos, roughly the size of a cow, bore a striking resemblance to the endangered pygmy hippos presently inhabiting the forests and swamps of Liberia and Guinea in West Africa, noted study lead author Professor Brooke Crowley.

“Ecologically, we think the Malagasy dwarf hippos were pretty close to the pygmy hippos that live in forests in West Africa,” said Professor Crowley. This assertion is based on an isotopic analysis of stable carbon and nitrogen found in the bones of the extinct Malagasy dwarf hippos, providing a ‘fingerprint’ of the animals’ diet, thereby shedding light on their preferred habitats.

What the researchers discovered 

The researchers sampled bones from both museums and ones they collected on the island, and the analysis revealed that the dwarf hippos did not regularly graze on grass in dry, open habitats, even in regions presently dominated by grassland. 

On the contrary, they seemed to favor plants found in wetter, forested landscapes. This finding suggests that forests were more prevalent before human interference began to alter the landscape for agriculture, domestic livestock grazing, and obtaining firewood and building materials.

The common hippopotamus, found on the mainland, is a known lover of grass, earning it its name from the Greek words for “river horse.” As part of their nocturnal routine, they depart from the safety of rivers and waterholes to graze on fresh grass, returning only at dawn. 

However, the researchers found that grass represented only a small part of the diet of Malagasy dwarf hippos, who were more inclined to feed on sedges and leaves, behaving more like browsers than grazers.

Human impacts

“For years we’ve seen evidence that these animals were not grazers,” said study co-author Professor Laurie Godfrey of the University of Massachusetts Amherst. She argues that the extinction of hippos on the island was likely triggered by the advent of permanent human communities transitioning from hunting and gathering to pastoralism and crop cultivation

Professor Godfrey calls this theory the “Subsistence Shift Hypothesis,” an extension of a similar idea previously proposed by noted archaeologist Robert Dewar. “There is pretty compelling convergent evidence showing that many of the extinct animals disappeared in a short window of time coinciding with the transition of people from hunting and gathering to pastoralism,” said Professor Crowley.

Conservation actions

Given the importance of forests for Madagascar’s wildlife as underlined by the study, Professor Crowley advocates for the restoration of native forests on the island. She contests the idea that grasslands were a critical habitat, at least not for the island’s hippos. 

“Some colleagues argue that grasslands are ancient and that we need to protect and manage them like we do forest,” says Crowley, “I would argue that forests are far more important. We are not contending that grasses did not exist in the past, but pointing out that there is no evidence for large grasslands devoid of trees prior to about 1,000 years ago.”

Emphasizing the urgent need for novel conservation efforts, the researchers concluded: “It is clear that Madagascar faces a biodiversity crisis much greater than that which it has already endured. Preventing this crisis will demand new conservation actions.” 

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