I’ve only seen hippopotami in the wild once in my life, and then only at a distance in the Yankari game reserve in northern Nigeria. I took some photos, but these were taken with only a moderately-powerful telephoto lens and crocodiles basking a few metres from where our Land Rover was parked were a more pressing concern. In any case, the prints from that holiday (years before digital cameras) are now lost. The photo at the top of this post is, in fact, a pygmy hippopotamus – a different genus to the common hippopotamus (Hexaprotodon liberiensis rather than Hippopotamous amphibious) – taken at smaller wildlife park (a glorified zoo, really) near Jos but it will do for my purposes.
A couple of years ago, I wrote about the role that bears may play in the transfer of essential nutrients from the ocean to the forests of north-western North America (see “Ecology’s bear necessities”). I recently came across a paper that described a situation where hippos were responsible for a significant movement of nutrients in the opposite direction: from land to water. We think of hippopotami as beasts that wallow in muddy water, but that is because they are filmed and photographed when there is enough light. Hippos are actually nocturnal animals, coming out of the water to feed on the savannah grasslands when it is dark, and resting in pools during the day. As they rest in their pools, the grass that they eat during the day is slowly digested (hippos are “pseudoruminants”) and, eventually, passes out of their colons as faeces. That is an important source of carbon, nitrogen and phosphorus for the river, but also of silicon, an essential nutrient for diatoms. Diatoms form the base of the food chain in Lake Victoria so, consequently, depend upon a constant supply of silica from the surrounding catchment, and the hippos are inadvertent vectors for this.
There is plenty of silicon in the natural environment (it is the second most abundant element on earth, after oxygen) but most is tightly-bound in particles and so is not in a form that is accessible to other organisms. Some plants, especially grasses, however, use silicon as a means of strengthening and supporting their cells. In the process, they also provide a measure of protection (as anyone who has been cut by the sharp edge of a grass leaf will know). The silicon is taken up by the plant’s roots, but is then laid down in the cells as structures called “phytoliths”. When these phytoliths are released back into the environment via a tortuous path through the hippo’s digestive system, the silicon they contain is in a much more accessible form than when it was trapped into minerals in the savannah soil.
The phytoliths released by the hippos form about three quarters of all the biologically-available silicon in the hippo pools and, when these have made their way down the stream, may also have an effect on the ecology of Lake Victoria. At this point, the paper gets rather speculative, but noting that there has already been a significant decline in hippo numbers in recent decades, the authors suggest that this may have had an impact on the ability of diatoms to compete with other algae, contributing to the greater dominance of cyanobacteria that has been observed in recent years.
Even allowing for a little academic hyperbole, this is a useful reminder that trying to keep ecology neatly compartmentalised is never a good idea. Everything is connected to everything else: lakes, rivers, terrestrial systems. We sort of know this instinctively but, at the same time, scientists spend so much time absorbed by their specialisms that they often forget this too. The hippopotamus seems to be an unlikely benefactor of tiny diatoms, but maybe that is the fault of our imagination rather than of nature.
Schoelynck, J., Subalusky, A. L., Struyf, E., Dutton, C. L., Unzué-Belmonte, D., Van De Vijver, B., Post, D.M., Rosi, E.J., Meire, P. & Frings, P. (2019). Hippos (Hippopotamus amphibius): The animal silicon pump. Science Advances 5: https://doi.org/10.1126/sciadv.aav0395
A baboon, photographed at Yankari game reserve in 1990. The photograph at the end of the post shows a waterbuck, photographed on the same visit.
And, once again, some notes on what else I have been up to this week:
Wrote this whilst listening to: Keith Jarrett’s Köln Concert
Cultural highlight: Keith Jarrett’s Köln Concert is so good that I am prepared to enter it under two headings. Worth listening to Tim Harford’s Cautionary Tales Podcast (Episode 7: Bowie, jazz and the unplayable piano) to learn more about this remarkable piece of music.
Currently reading: Raynor Winn’s The Salt Path. I’m in south-west England so a book about walking the South West Coast Path seems appropriate.
Culinary highlight: yet to happen. I’m at a conference at the University of Plymouth, subsisting on breakfast from a chain hotel and lunch from a university catering service that offers few options for those who are lactose-intolerant.