When I started this blog, waaaaaaay back in February, I did a post about the Okavango delta, a geomorphically/stratigraphically contentious little piece of real estate in Botswana. A comment under that post, by Brian of Clastic Detritus fame, mentioned his curiosity regarding any studies that had looked at the sedimentological and geomorphic effects of some of the charismatic megafauna in the region. Well, I recently ran across a paper that addresses just that, so I thought I’d share (there’s a full citation at the end of the post, and if anyone actually wants the paper, I’d be happy to e-mail them a pdf).
McCarthy et al. (1998) state that hippopotamus in the Okavango play an important part in the geomorphology of the distributary system, primarily for the following reasons:
1) Hippos are big
2) In order to get that big, Hippos eat a lot of grass, which they prefer to get in the form of short grasses that they keep cropped by their constant grazing.
3) Hippos are gregarious, so they’re a bunch of them in any one spot
4) Hippos hang out near channel, preferentially.
That’s a pretty comprehensive list of hippo characteristics, if you ask me. Because of these attributes, Hippos have the tendency to move around in search of food, forging paths through the vegetation that are maintained by continuous hippo traffic.
The authors point out that there are lots of big grass eating critters running around in groups in the Okavango (such as Elephants or Water Buffalo). The important difference between the trails of these critters and the trails of Hippos lies in their different ecologies. Elephants and Buffalo are primarily trying to get from island to island, so their paths through the swamps and backwaters of the Okavango tend to cut perpendicular to the regional gradient. Hippos like to stick close to the water, and so they tend to produce trails that parallel their favorite trunk channels and therefore, run parallel to the regional slope. The picture below is from page 49 of the Mcarthy et al. (1998) paper, and shows the short, parallel trails of more terrestrial vertebrates being cut be long trails running from left to right; these trails are hippo paths.
Because of the parallel-to-regional-slope nature of the hippo trails, the authors posit that these artificial channels, kept free of vegetation by hippo activity, could play an important role in channel avulsion. Their evidence here is a little sparse, though they cite a historical avulsion in the delta that they infer to be hippo-mediated, at least in part.
In addition to channels, Hippos also tend to produce cleared paths in the lakes and quiet-water regions of the Okavango. The picture below, form page 51 of McCarthy et al. (1998), shows a hippo track in a dry lakebed in the delta region.
It would be nice, of course, if someone would go through and survey in the cross-sections of these hippo-channels, both within the course of a single season as well as across multiple seasons. Detailed sedimentology in the channels would also be a nice touch, particularly if you could demonstrate a hippo channel getting utilized as a major sediment routing pathway in the delta region.
And I think it’s good to keep in mind the important lesson of this sort of work: the word Hippo-turbation is hilarious. And at the end of the day, hilarious jargon is all that really matters.
McCarthy, T.S., Ellery, W.N., and Bloem, A., 1998, Some observations on the geomorphological impact of hippopotamus (Hippopotamus amphibius L.) in the Okavango Delta, Botswana: African Journal of Ecology, v. 36, p. 44-56.