Hungry Hippos

The hippopotamus (Hippopotamus amphibius) is an unusually large animal. Among land-dwelling mammals, only the elephants and some species of rhinoceros are larger. It is also, like elephants and rhinos, herbivorous and, since it prefers to eat plants that aren't especially nutritious, this means it needs to eat a lot. 

As in, it eats 35 to 50 kg (77 to 110 lbs) of food each day. 

Which is fine if the hippo happens to be out in the wild, far from human interference. But the reality is that there are fewer and fewer such places around these days. It's not so much the urban sprawl that humans bring, or even the roads and other infrastructure of an expanding African economy, but more the cropland that's required to feed us all. Although hippos are hunted for the ivory in their teeth, the number one threat to their survival is probably the expansion of farmland. Compared with many other animals, this is exacerbated by their reliance on large amounts of fresh water, so even if the farmland isn't near them, they suffer if water is diverted to where it is needed for crops. 

As a result, the hippopotamus is considered, if not yet endangered, a "vulnerable" or "threatened" species. They are found across most of Subsaharan Africa, but in scattered populations - essentially, wherever there are still large, reliably filled rivers. To some extent, this has probably always been the case, but the size of most of these populations has declined over the last several decades as pressure on the animals increases.

This makes it important to better understand hippos' feeding habits. Not just for the sake of the hippos, but also for humans. Hippos will raid croplands if they can, and when farmers try to protect those lands and drive them off, there's a good chance they will be injured in the process. You really don't want to get in a fight with an angry 1½-ton hippo.

Of course, we already know a fair amount about hippos and their feeding habits; they are hardly obscure animals. They belong to a family that includes only one other living species, the pygmy hippo, and that belongs to the same larger order as most other large herbivorous animals, from camels to cows. It used to be thought that their closest living relatives were the pigs, but we have known since the 1990s that they are actually closer to the whales and dolphins - basically, hippo-like animals that became so adept at living in the water they stopped needing legs.

Hippos' favourite food is grass. A hippo therefore needs two things to be comfortable: plenty of fresh water and plenty of grass. These aren't perhaps their only requirements, but they are the most important, and, when you're the size of a hippo, you need a fair amount of both. A hippo's daily routine consists of lounging about in the water during the day then, when the sun starts to go down, climbing out to visit suitable grazing areas. These are typically 3 to 5 km (2 to 3 miles) away from the riverbank, but sometimes can be up to twice that. They then spend up to five hours happily grazing before returning to the river for a pre-dawn rest.

Being so widespread, however, the exact details may differ across the hippo's range. To conserve hippos - and, hopefully, human croplands - in a particular place, we need to understand what the hippos are doing there, not what they're doing across the other side of the continent. Many such studies have been done, but Africa is a big place, and there are still gaps in our knowledge. One of these was recently filled in with a study on hippo diet and behaviour in Ethiopia.

The study was conducted at the Dhidessa Wildlife Sanctuary, which lies on the river of the same name, a tributary of the Blue Nile. This is in western Ethiopia, a highland region dominated by savannah grassland. While originally set up with the primary aim of protecting hippos, the sanctuary has proven ineffective in recent years, with substantial agricultural encroachment, doubtless because the Ethiopian government has had more important things to worry about.

The study showed that the hippos in the area, like those elsewhere, spend a lot of their time just lazing about and doing very little, especially during the hottest parts of the day when they partially submerge themselves in the water to keep cool. Males, it seems, are lazier than the females, which may well be because some of the females had young to care for or protect, which will take time out of anyone's day.

The hippos were most active during the early part of the rainy season, when temperatures are cooler and food is most abundant. That tailed off in the later half of the season, perhaps because by then, lush grass was growing near the rivers, and there was little point in travelling far to get to it. But the hippos were least active during the sunshine of the dry season, preferring to stay in their rivers away from the heat, rather than wandering off in search of food before they had to.

This is, as might be expected, not so different from what hippos do elsewhere. However, diet is inevitably influenced by what plants happen to be around, so we should expect there to be some variation in what hippos eat depending on where in the world they happen to be. In this case, analysis of the hippos' dung showed that they had been eating a wider variety of plants than a previous study in a similar habitat in Zambia, but less than in areas away from agricultural land.

Most of the diet does consist of grass, as we would expect. Bermuda grass is arguably the most favoured (this is, despite the name, not native to Bermuda) in the sense that it seems to be important for hippos everywhere we have bothered to look. But here, sedges and cockspur grass formed even more of the diet, along with a particularly large portion of bulrushes (formerly known as "reedmace", but for some reason, no longer).

The bulrushes are interesting because those are found only near water, so that the hippos aren't heading out to graze on them, but doing so from the comfort of the river. They are also, for what it is worth, not a form of grass (or, indeed, a form of rush), although they are related. Other non-grasses/sedges regularly eaten by the hippos include herbs such as joyweed, knotweed, and tropical spiderwort, aquatic plants such as pennywort, and some climbing legumes. But not, for example, any of the shrubs and trees that are locally common, such as bushwillow and myrrh. 

It was also possible to see how the hippos' diet changed through the year, from the relative richness and diversity of plants in the wet season, to a narrower range of available food when the weather became drier. Even so, grasses and sedges always formed around two-third of the food eaten. Hippos, it seems, can adapt to what's around them, and in the right circumstances, that is going to include the sugarcane and melons that are the main local agricultural crops. (Sugarcane is a grass; melons obviously aren't). But they are still limited, both by a need for plentiful grass and, of course, by the need for water - as all those bulrushes indicate.

Sure, this particular wildlife sanctuary contains some good habitat for hippos, but with the current problems elsewhere in Ethiopia, and the encroachment of agricultural land into places that are theoretically protected, it's by no means all plain sailing. Understanding exactly what they are doing and eating can perhaps help with that when and if things settle down.

[Photo by xorge, from Wikimedia Commons.]