In the middle of Tsavo West National Park lies the Mzima Springs: a series of four natural springs sourced from a water reservoir 50 km north. The water spends 25 years filtering underground, running through volcanic lava rock and ash. The pristine waters of Mzima flow through a series of pools and rapids before being blocked by a solidified lava flow, forcing the water to disappear underground again.
Mzima Springs has several resident populations of hippopotamus and Nile crocodiles, both of which are isolated from other water sources. The hippopotamus sustain an aquatic food chain by grazing on the nearby savannah during the night and returning to the springs during the day. By defecating in the springs they bring essential nutrients utilized by freshwater algae as well as fruiting trees and shrubs. The fruit produced by these trees sustains monkey and bird populations while the algae forms the foundation of an aquatic food web.
It isn't just a poop-based economy. Several species of fish consume algae and parasites off the bodies of these hippos. During the day, the hippos of Mzima Springs get a true spa treatment. Groups of fish, at least four different species, attend these spas, specializing on cleaning particular parts of the hippopotamus. A carp of the genus Labeo (meaning lips) cleans the large flat surfaces of the hippopotamus. A species of cichlid cleans the bristly hair on the tail, removing unwanted material, while barbles clean the cracks in the soles of the feet. Tiny fish of genus Garra “clean” wounds by picking at dead cells, but this has yet to be shown to be beneficial for the hippos. In fact, some relationships where one species “cleans” the wounds of another species have been shown to be negative for the animal with the wounds (see my post on the Red-billed Oxpecker for an example); the “cleaner” actually slows down the healing process by continually opening up the wound. A close relative of the Garra in the Mzima Springs, Garra rufa, aka the doctor fish, are used in spas in Asia and Europe to remove dead skin of people that suffer from psoriasis.
Hippopotamus are far from passive recipients in this system. They deliberately splay their toes and spread their legs to entice fish. They open their mouth and allow fish to swim in to clean their jaws and tongues. The fishes congregate in cleaning stations and the hippos seek out these stations, much like how we visit spas to get a pampered treatment.
This relationship is facultative; both the fishes and the hippopotamus can survive without the other. For example, the carp that feed on the algae and parasites on the back of the hippos also graze on aufwuchs (algae and invertebrates encrusting rocks and other hard substrata).
For some great video of the interaction check the video below. With a little anthropomorphizing (attributing human characteristics on something non-human), you can really see how much they enjoy their treatment.
Don’t forget to check out more mutualisms of the month and all the other great articles on FTDM! Stay hungry!