As our small boat wended its way up the Wami River in Saadani National Park, Tanzania, we passed a crocodile basking on the bank. Nothing unusual about that, but this croc only had three legs. I asked if one leg had been chopped off by a boat’s propeller? “No,” said our guide, Eliona Sabaya, “It was bitten off by a hippo.”

Here was dramatic, amputee-reptilian evidence to illustrate a well-known factlet about Africa: that hippos are the most dangerous (non-human) animals on the continent, responsible for the deaths of more people than lions. Actually, let’s say that hippos are the most dangerous mammals: Malaria-spreading mosquitoes cause many more deaths in Africa than hippos.

That observation was, however, of little comfort as we encountered a family of hippos wallowing in the center of the shallow river.

“The crocodile was very lucky,” Eliona was saying. “A hippo can easily bite a man in half.”

Eliona, a wildlife guide provided by Saadani Safari Lodge, told us more about these mighty creatures. Weighing over three tons, an adult male is a formidable beast, the third-largest land animal in the world after the elephant and the rhino. Groups are comprised of a bull male who controls up to 30 females and young animals. There were about 15 in the family we were approaching, including, we could see on the shore, a mother and baby.

Hippos have gigantic mouths and teeth. And they are highly territorial.

Then — just as Eliona had explained the “territorial” bit — by the side of our boat there was a sudden and noisy eruption of bubbles. Maybe it was a hippo, right next to us. If there was any doubt, it was dispelled in the next moment, as the animal heaved itself into the side of our boat from below, causing it to pitch violently and almost capsize.

If I hadn’t just sat down — I’d been standing to watch the mother and baby on the bank — I would have gone into the river.

“Wow, they are aggressive,” I said.

“He just wanted to come up for air and bumped into the boat by accident,” Eliona said.

I wasn’t so sure. The bull was probably telling us that we’d come close enough to his family. And though the river was cloudy with mud, hippos have a remarkable ability to see through it: echolocation. Not only would the hippo have been able to hear our boat, he would have known its precise position. I think he had attacked it deliberately.

Yes, echolocation — the biological sonar used by dolphins and whales, and also by bats. Hippos too? That hippos have this peculiar sense points to a fact that is, to my mind, more remarkable than their status as Africa’s top (non-human) mammalian killers: that their closest living relatives are whales.

It’s a wonderful illustration of evolution by natural selection, and of Darwin’s principle of “descent by modification.” The ancient Greeks thought hippos were related to horses, hence the name hippopotamus, which means “river horse.” After all, they can run at up to 30 kph on land. Even until fairly recently, modern scientists thought the hippos’ closest relatives were pigs. It wasn’t until about 20 years ago that molecular evidence suggested that link was in fact to whales.

While the DNA evidence was compelling, it was nonetheless surprising given the physical differences between whales and hippos.

Since then, though, fossils have been found that reveal intermediary forms of animals between whales and hippos. There was a water-loving group of animals that lived 50 to 60 million years ago and split into two groups: the early cetaceans, which eventually became totally aquatic; and a large group of four-legged animals called anthracotheres. This group mostly died out around 2 1/2 million years ago, but it left one descendent: the hippopotamus.

It is only relatively recently that echolocation has been confirmed in hippos. It makes sense. Though they feed mostly on grass they graze on land at night, echolocation would help them navigate in muddy waters, and perhaps communicate.

It is logical, too, that two groups of related animals — hippos and whales — share this sensory method.

Eliona told us that we were safe in the boat, and even the villagers who live near the hippos have never had any problems with them. However, I’d noticed there was a huge skull of a hippo on the river bank, and a man with an AK-47 at the jetty where we’d boarded our boat. When I asked Eliona why the man had been armed, he said that poachers will sometimes attempt to kill a hippo for meat, and that the man was a park ranger.

Hippos are classified as “vulnerable” on the IUCN Red List of Threatened Species, and it is illegal to hunt them in national parks. Still, men will sometimes shoot them, butcher the giant bodies into manageable chunks, and carry the meat back to their villages.

Hunting hippos is a practice that has been going on for hundreds of thousands, if not millions, of years. Anthropologists who have found fossilized, butchered hippo bones alongside stone tools even believe that the extra protein provided by a diet containing meat from animals such as hippos and crocodiles played a key role in the development of a larger, more humanlike brain.

So, despite not being as endangered as some other African mammals — the great apes, for example — hippos need protection. And that’s all the more so because nature, too, can take a heavy toll — as it did this year in Queen Elizabeth National Park, Uganda, where at least 83 hippos died of anthrax they contracted from bacteria in the mud around a lake in the park.

Rowan Hooper stayed at the Saadani Safari Lodge in Tanzania (www.saadanilodge.com). Follow Rowan Hooper on Twitter at twitter.com/rowhoop. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human).”

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