Hippos are the most underrated of African megafauna. While they're completely herbivorous (well, mostly herbivorous), their faces essentially come with built-in, perpetually growing knives. Their chompers, which can reach lengths of some 50 centimetres (20 inches), never stop getting longer. Hippos are more deadly than sharks, killing more folks each year than all the lions, leopards, elephants, buffaloes and rhinos combined. They can hold their own in a tug-of-war with a croc (yes, that's a wildebeest playing the role of 'rope'). And they're fast, capable of reaching speeds over 30 km/h (20 mph). 

But hippos aren't only impressive; they also help to maintain Africa's aquatic ecosystems. They spend most of the day, up to sixteen hours, in the comfort of their pools. Every night, they lumber onto dry land where they gobble up loads of grass. Once they've eaten their fill, they return to the rivers, streams, lakes and ponds of Africa to digest their meals. And then – there's no inoffensive way to put this, so I'm just going to say it – they crap it all out. Magnificently.

Hippo Poop _2015_04_17
Image: Kevin Trotman, Flickr

If Hippopotamus amphibius makes you think of Disney's Fantastia, then you've never witnessed the spectacle of hippopotamus defecation. With their tails acting like high-speed propellers, they push the poo out of their back end, flinging it wildly about in a torrent of grassy, stinky, greenish-brown stuff. And because a single hippo can swallow 40-50 kilograms (88-110 pounds) of the grass each night, millions of tons of hippo dung gets propelled into Africa's rivers each year. 

It's a hippo's world, and the fish just live in it. And it's a good thing too, because all that hippo dung brings quite a lot of nutrition into the water with it. In fact, hippos are responsible for depositing literally tons of nutrients from terrestrial African ecosystems into aquatic ones. At least, that's the hypothesis. The question facing UC Santa Barbara biologist Douglas J. McCauley, in a study overseen by UC Berkeley researcher Justin S. Brashares, was just how critical hippos are, from an ecological perspective, in transporting organic materials into African watersheds.

The first thing that McCauley and his colleagues did was find two pools along Kenya's Ewaso Ng’iro River, one with hippos and one without. The hippo pool was continuously used by a pod of hippos for at least sixty years; the reference pool, located less than two kilometres (1.2 miles) away, was used as a control. Locals verified that except for one brief time in 2005, in which a female hippo and a single offspring briefly used it, the reference pool had been unused by hippos for at least sixty years.

The team caught some guppies, Poecilia reticulata, from the reference pool (the omnivorous guppies are an introduced species in the Ewaso Ng’iro). In a lab, some of the guppies were fed a 100% hippo dung diet ... Wondering how you prepare such a diet for a guppy? You go to the hippo pool and scoop up a bunch of hippo dung from four different hippos. You mix it together into one big homogeneous pile of hippo crap and you freeze it. Then, you grind it into bits and drop it into your fish tanks. Bon apetit!

The guppies (Poecilia reticulata) gobbled up the hippo dung with gusto. Image: Sphoenix Sphoenix, Flickr

Despite the fact that they came from the hippo-free reference pool, the fish dined on the dung with gusto. You can tell what kind of plants an animal has eaten by looking at certain chemical markers inside of its dung – and if the markers from the kinds of grass eaten by hippos show up inside of a fish, then it's reasonable to assume they got there because that fish ate hippo crap.

So after three to six months of captivity, the researchers analysed their fish for those chemical markers and compared them to other fish that were tested immediately upon capture. The results? The researchers were indeed able to distinguish dung-eating fish from those that had not feasted on hippo faeces.

Having established that they could identify whether a fish had been, er, eating shit, the researchers turned their attention to the critters living in both the hippo pool and the reference pool: an omnivorous fish called Labeobarbus oxyrhynchus, and the larvae of Trithemis dragonflies.

Their testing didn't work for fish captured in the wet season, possibly because fast-moving water dilutes or carries away hippo dung. But when the fish and larvae were sampled in the dry season when the water is lower and slower, those from the hippo pool tested positive for the same chemical markers as those fed hippo dung in captivity ... which means that for at least part of the year, aquatic fish and invertebrates rely upon hippo dung for their nutritional needs. 

This wasn't a purely academic exercise, of course. Labeobarbus is a commercially important group of fish in East Africa; hundreds of tons are harvested each year, making it a key source of protein for human populations in the region. Meanwhile, hippos are declining, and both climate change and development are altering African watersheds. Which leaves us with questions about what these broad-scale ecological trends might mean for the sustainability of the Labeobarbus fishery and the ability of East Africans to have enough fish to eat.