- A team from The Wilderness Project has conducted a survey of hippos along Zambia’s Kafue River, one of the last strongholds of the species in Southern Africa.
- The expedition faced regular close encounters with hippos, which are highly territorial and can be dangerous to people, especially when traveling by boat.
- The team counted nearly 2,400 hippos, most of them along a 350-kilometer (217-mile) stretch of the Kafue within the Kafue National Park and adjoining protected areas, underscoring the park’s conservation importance.
- Despite their ecological significance as ecosystem engineers, hippos remain understudied and increasingly vulnerable to habitat loss and pollution, including upstream mining spills.
KAFUE NATIONAL PARK, Zambia — “Hippos ahead.”
The warning comes from the lead canoe. Hippos are dangerous — highly territorial and fiercely protective of their young. They are capable of capsizing boats and inflicting serious injuries.
The line of five canoes hugs the riverbank to avoid a group midstream, but suddenly a mother hippo (Hippopotamus amphibius) and her calf charge down from the bank where they’d been grazing unseen. The animals barrel toward the river and the flotilla, the mother braking at the last second. Changing course, she and her calf plunge into the water just behind the last canoe.
It’s only the second close encounter of the 34-day expedition, says Mike Ross, leader of The Wilderness Project (TWP) team. “You can’t really blame them — the water is their safe place.”
And this particular stretch of the Kafue River, in central Zambia, is especially so.
Over 82 days in 2024, Ross and a TWP team journeyed along the entire length of the Kafue, tracing its path from the river’s source near the Democratic Republic of Congo border, through the Copperbelt’s industrial zones, across the papyrus-lined dambo wetlands of the Kafue Flats and down to the steep Kafue Gorge — a topographical bottleneck that prevents some species, like African tigerfish (Hydrocynus vittatus), from moving upstream from the Zambezi.
“An ever-changing river,” is how Ross describes the Kafue — slouching across open floodplains in its upper reaches, surging in shallow rapids around rocky islands and onward through broad channels lined by riparian forest to where it’s held back in the vast expanse of the Itezhi-Tezhi hydroelectric dam’s reservoir.
The team counted nearly 2,400 hippos, most of them along a 350-kilometer (217-mile) stretch of river within Kafue National Park and adjoining protected areas, underscoring the park’s conservation importance. But beyond its boundaries, which sit roughly midway along the river’s length, hippos were hard to find.
Hippos are still occasionally seen around the Copperbelt city of Kitwe and beyond, and Ross concedes that the Kafue Flats may support more than TWP’s methods — which use the main river channel as a transect — have captured.
“I think they, to some degree, still have a widespread distribution. They just exist at very low densities outside Kafue National Park.”
By the time Mongabay joined this year’s expedition in late May, the team had already completed a 600-kilometer (373-mile) survey of the Lunga River, a major tributary. They counted just 285 hippos there.
Once widespread across sub-Saharan Africa, hippos have disappeared from much of their former range. Before the 1950s, they are believed to have occupied most areas with slow-flowing water, ample bankside grazing and deep pools. Today, their range is fragmented.
“It is most likely that human overexploitation [(]through illegal hunting] has driven the contraction of the hippos’ range,” says Mike Voysey, a South African ecologist and Harvard University Ph.D. candidate not involved with the expedition. “I am confident that hippos would have been abundant in other sections of the [Kafue] River in days gone by.”
Voysey says protected areas like Kafue National Park and the adjoining Mushingashi Conservancy now serve as critical refuges. “In a largely dry continent, people and hippos often compete for the same vital water resources, and hippos tend to lose out under intense human pressure.”
Water extraction and dam construction also threaten their habitats by altering water depth and flow. The Kafue is under particular strain. In 2018, the World Wide Fund for Nature estimated that annual demands on the river and its catchment exceeded 11 billion cubic meters (388.5 billion cubic feet), driven by needs for hydropower, agriculture, industry and urban centers. It warned then that a “business-as-usual” approach on the Kafue was not sustainable.
And even inside national parks, hippos aren’t completely safe. In February, a tailings dam at a Chinese-run mine in the Copperbelt failed, releasing more than 50 million liters (13.2 million gallons) of highly toxic acidic sludge into a tributary of the Kafue. The spill, though seemingly neutralized before it reached the park, killed fish, crocodiles, frogs, water monitor lizards and at least one hippo.
The incident, Voysey says, highlights the vulnerability of river systems.
“Hippos may be especially vulnerable to these external pressures compared to many terrestrial species,” he says. “Rivers flow freely in and out of protected zones, linking landscapes and exposing hippos to upstream impacts.”
Beyond hippo counts, the expedition’s 12 members conducted a broader ecological assessment as part of TWP’s Great Spine of Africa initiative, which surveys the health of the continent’s major rivers. They documented other wildlife — Goliath herons (Ardea goliath), African fish eagles (Icthyophaga vocifer), elephants (Loxodonta africana), antelope like the puku (Kobus vardonii) — and measured water quality, including pH and salinity, to establish a baseline of river health.
The presence of so many hippos, while a hopeful sign for biodiversity, also brings risk. Soon after entering the national park’s northern boundary, the team begins seeing them regularly — basking onshore with red-billed oxpeckers (Buphagus erythrorhynchus) cleaning their hides, snorting as they surface from the depths or stampeding into the water in a flurry of spray.
Anticipating hippos’ movements is crucial, says Dominic Mwahamubi, a veteran canoe guide from Livingstone and one of the expedition’s boat captains. He reads the river like a map of hippo behavior.
Observe the moon — they like moonlit parts of the river and may still be there in the morning, he advises. Cold weather? They’ll be onshore, and if your boat hugs the bank, they might charge down. In shallow water, they’ll want to move to deeper spots — don’t block their way.
“You have to give them space; when you give them space, you’re OK, they can just be looking at you,” he adds.
Despite their ecological importance, hippos remain understudied. Their size and semiaquatic lifestyle make them hard to tranquilize and fit with tracking collars. Yet their influence on ecosystems may be greater than previously thought.
Recent work by Voysey and colleagues suggests hippos may be Africa’s most impactful mega-herbivores, potentially surpassing even elephants. Their trampling and wallowing reshape riverbanks, carve channels and create gullies. Their grazing creates short-grass “lawns” that benefit other grazers, like the puku, and their dung fuels aquatic food chains.
One study found that hippos ingest silica particles from grasses and deposit them into rivers via dung. Silicates are used up by diatoms — microscopic algae — which in turn nourish macroinvertebrates, forming the foundation of aquatic ecosystems.
“These effects are not all that easy to observe but likely play an important role in maintaining the vitality of aquatic life,” Voysey says.
Some of this is evident to those who live and work along the river.
“While we’re guiding on the [Zambezi] River, I’ve seen even the fishermen will be fishing next to the hippos just because the fish would like to eat all those particles, which are not well digested by the hippos,” Mwahamubi says.
He adds that Kafue’s hippos tend to be more tolerant than their Zambezi counterparts, though he notes their temperaments may change later in the year when water levels fall.
Back on the river, not long after the encounter with the hippo mother and calf, the team faces another challenge. The river splits around an island, and both channels are blocked by hippos. The boats stall, uncertain.
Ross points to a tree on the island. “We’re going to head across to that tree on the left, then cut across.”
The canoes move cautiously, skirting the island. A burst of movement distracts them — water birds taking flight: black-crowned night herons (Nycticorax nycticorax), water thick-knees (Burhinus vermiculatus), Egyptian geese (Alopochen aegyptiaca). The researchers document the birds quickly, then regroup.
One captain taps his paddle against the boat’s side — a warning signal to the hippos. All five boats slip through the blockade.
“Sorry guys, that was a bit chaotic,” Ross calls back.
Behind them, one of the hippos surfaces. Twitching its ears, it releases a sound like deep, booming laughter.
Mongabay joined an expedition of The Wilderness Project traveling the length of the Kafue River, part of TWP’s wider project to gather data in Africa’s major river basins. Read Ryan Truscott’s other articles from this journey here.
Banner image: While navigating the Kafue River inside the national park, the team regularly pauses to deliberate on the best route to take around hippos that block their way. Image by Ryan Truscott for Mongabay.
Wildlife & tourists on the up in Zambia’s Kafue Park: Q&A with manager Craig Reid
Citations:
Voysey, M. D., De Bruyn, P. J., & Davies, A. B. (2023). Are hippos Africa’s most influential megaherbivore? A review of ecosystem engineering by the semi‐aquatic common hippopotamus. Biological Reviews, 98(5), 1509-1529. doi:10.1111/brv.12960
Schoelynck, J., Subalusky, A. L., Struyf, E., Dutton, C. L., Unzué-Belmonte, D., Van de Vijver, B., … Frings, P. (2019). Hippos ( Hippopotamus amphibius ): The animal silicon pump. Science Advances, 5(5). doi:10.1126/sciadv.aav0395
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