Floating meadows in South Africa, with particular reference to behavioural inversions in the hippopotamus

@magdastlucia @ludwig_muller @prairie_rambler @bobwardell @beartracker @tonyrebelo @goosiaczek @danielatha @brothernorbert

Floating meadow (https://en.wikipedia.org/wiki/Floating_island) is an important vegetation type in Amazonia. One way to think of this vegetation is as a grassland rooted in water instead of being rooted in soil.

Naturalists may not realise that floating meadow occurs in South Africa. Because its occurrence in this country is associated with the hippopotamus (Hippopotamus amphibius), it is relevant to mention other aspects of the ecology of this megaherbivore in the same region.

My source is Tinley K L (1976, Ecology of Tongaland).

On the dried mud along the edges of the pans, where short sparse growths of Echinochloa spp. occur, the vegetation is mainly Cyperus fastigiatus (https://www.inaturalist.org/taxa/583237-Cyperus-fastigiatus), Cyperus spp. (Zulu ‘iMizi’, suggesting that these cypes are useful in some way), Sesbania sesban, and Gomphocarpus physocarpus (https://www.inaturalist.org/taxa/281273-Gomphocarpus-physocarpus).

Behind the water edge vegetation are stands of scattered or closely growing large trees, commonly Ficus sycomorus (https://www.inaturalist.org/taxa/340164-Ficus-sycomorus), Vachellia xanthophloea (https://www.inaturalist.org/taxa/348763-Vachellia-xanthophloea), Trichilia emetica (https://www.inaturalist.org/taxa/595643-Trichilia-emetica), Faidherbia albida (https://www.inaturalist.org/taxa/343039-Faidherbia-albida), and Kigelia africana (https://www.inaturalist.org/taxa/81491-Kigelia-africana).

The tall trees are interspersed with clumps of Grewia caffra (https://www.inaturalist.org/taxa/340279-Grewia-caffra), Mimosa pigra (https://www.inaturalist.org/taxa/47445-Mimosa-pigra), Ficus capreifolia (https://www.inaturalist.org/taxa/507292-Ficus-capreifolia), Sesbania sesban (https://www.inaturalist.org/taxa/168865-Sesbania-sesban), and Phyllanthus reticulatus (https://www.inaturalist.org/taxa/340305-Phyllanthus-reticulatus), with a varying width of grass, some 2-5 feet high, of iSwani, whatever that means.

Flood plain vegetation: The areas of the flood plain which have no open stretches of water are covered mainly by Echinochloa pyramidalis interspersed with reedbeds (Phragmites), and with scattered growths of Cyperus fastigiatus and other Cyperus species on the ‘green mat’ of Echinochloa. Small patches of tangles (Grewia and Ficus spp.) are to be found, usually growing adjacent to the growths of Sesbania sesban.

Tinley also describes the catena thus:

At the edge of the flood plain Vachellia xanthophloea is dominant, interspersed with Trichilia emetica and occasional large Ficus sycomorus.
In places reeds Phragmites mauritianus and Echinochloa pyramidalis grow to the edge of stands of the V. xanthophloea.
Between the V. xanthophloea and the reeds, there are clumps of Ficus capreifolia and Grewia caffra growing over small shrub-like growths of V. xanthophloea and F. sycomorus.
In other areas there are fairly wide mud flats (during winter) between V. xanthophloea and the pans. Many of these mud-flat areas are covered with water-grasses (whatever that means) and sedges Cyperus spp.
In some areas thorn-bush grows to the water’s edge, and in others pans are surrounded by Echinochloa pyramidalis and reeds.
The majority of pans have lilies Nymphaea sp. and other aquatic plants growing in and along the edges.
Where pans do not occur, wide sections of the flood plain are covered with E. pyramidalis, interspersed with reed beds.
The Pongolo river forest is dominantly of fig trees (F. sycomorus), throughout the river’s length (now largely chopped down).
In the quieter stretches of the Pongolo River, submerged aquatic plants are able to flourish.

My commentary:

Floating meadow suffers from a failure to recognise it as a vegetation type in Africa, and Tinley’s description is limited accordingly.

The main problem with his description is that he does not state the depth of water in which Echinochloa grows. I infer from elsewhere that it is deep enough, at least in places, for the hippopotamus to hide in, i.e. > 1 m deep. It is also unclear what Tinley means by Cyperus growing ‘on’ the mat of floating grasses; do the roots of these sedges fail to reach the ground?

My impression is that, during the dry season, one would never notice Echinochloa as anything special in the Pongolo River area. I certainly did not, during my visit to Ndumo in 2013. This grass is present during the dry season but can be overlooked as just another grass on the edges of pans. However, during the wet season Echinochloa grows widely on the inundated floodplain, floating on the surface (I am unsure if the roots actually float as well; I presume so). This, I infer, becomes seasonally the dominant grass in summer over most of the floodplain. As far as I can make out, it is ungrazed by any large mammal during this season, except occasionally by the hippopotamus (foraging while submerged and visible only by the movements of the floating mat of grass). Surely all of this is rather extraordinary?

It is well-known that the hippopotamus rests in the water. However, as Tinley knew when he was only 21 years old, it rests on land during the

night (after its grazing activity but before dawn),
winter (when the harrassment by biting flies, namely Glossina (https://www.inaturalist.org/observations?place_id=any&taxon_id=67370&view=species) and Tabanus (https://www.inaturalist.org/observations?place_id=any&taxon_id=49704&view=species), abates seasonally), and
summer (when the floods extend so far that even the riparian forest is flooded, and the hippopotamus rests by night in dense stands of spinescent Vachellia, or even sand forest).

An aspect clear in Tinley’s report is how important parasites, including large-bodied biting flies, are for the behaviour of the hippopotamus.

The hippopotamus must rest under cover by day, but not necessarily in water. "Leeches are common throughout the waters of the flood plain and are to be found on the animals in quantities". In winter, when no biting flies are active, the hippopotamus often rests in riparian fig forest.

This means that even vegetation types we would not associate with the hippo, such as dense Vachellia and sand forest (where there is neither standing water at any time of the year nor any significant grazing available) do provide resting sites for the hippopotamus. The megaherbivore is hard to observe because the animals are under cover of vegetation and, partly, the darkness of night.

It is well-known that the hippopotamus prefers short grass on dry land (including regeneration after fire). Tinley states: “Watching a hippo grazing one notices that all the grass is cropped with the lips, which cover the incisors completely. The upper lip does the cutting, as the head is nodded downward with each gathering. On close inspection of the lips, it is found that the entire fore-edge of the upper lip has a sharpish, hard ridge, serrated in parts. When out, it has the appearance of the hard callouses which form on the hands and feet of humans.”

However, a particular grass eaten by the hippopotamus at the edge of riparian forest is Setaria megaphylla (https://www.inaturalist.org/taxa/552642-Setaria-megaphylla), which is actually called ‘hippo grass’. It is interesting that, when the floodplain is flooded shallowly (including the zone of Setaria megaphylla), the hippopotamus does not graze on the shallowly submerged grass. ‘In many places they wander as much as six miles in search of suitable grazing.’ I also infer something else that is seldom explicitly stated: that domestic Bos taurus X indicus also avoids shallowly submerged pastures. One does not see this domestic bovine grazing with its muzzle in the water.

By the way, it is noteworthy that the hippopotamus forages on the tubers of Nymphaea. Baboons (Papio spp.) are more similar trophically to the hippopotamus than is usually realised, because these monkeys graze, eat the fruits of Kigelia, and also eat the tubers of Nymphaea (https://www.alamy.com/adult-male-chacma-baboon-papio-ursinus-eating-a-water-lily-tuber-kruger-image68361882.html and https://www.alamy.com/stock-photo-chacma-baboon-papio-ursinus-eating-waterlily-tubers-kruger-national-48783036.html and https://www.agefotostock.com/age/en/details-photo/chacma-baboon-papio-ursinus-eating-water-lily-tuber-kruger-natl-park-south-africa/NHP-NDE002115A and https://www.offset.com/photos/adult-male-chacma-baboon-papio-ursinus-eating-a-water-lily-tuber-92234 and https://www.dreamstime.com/stock-photo-baboon-eating-water-lily-river-image86193779 and https://www.afripics.com/image/detail/a-chacma-baboon-eating-water-lily-bulbs-retrieved-from-the-water).

Most interesting of all:
The hippopotamus grazes on floating meadows. Tinley (1976) is the only clear reference that I have found: “At Ntonto Pan a herd of 15 hippos live in both the open pan waters as well as in amongst the marsh area of the pan. This marsh area is covered with a ‘floating’ blanket of iMbuku grass [Echinochloa pyramidalis], interspersed here and there with bullrushes, Typha latifolia and other water grasses, Cyperus spp. Many of the animals cannot be seen, their presence being revealed by the occasional heaving of iMbuku grass into the air and continued loud munching noises. Flocks of cattle egrets, Bubulcus ibis wait tensely for such movements, which send insects scattering everywhere. Sometimes a hippo stands up out of the marsh with matted bunches of iMbuku grass hanging from it.”

Many naturalists may assume that the hippopotamus prefers to rest in the water and, if leaving the water to rest, ventures only on to the immediate banks, or trampled areas/sandbanks in reedbeds. However, this species is bothered by leeches, and most of the immature males are killed by mature males each winter when the water bodies are concentrated.

So there are incentives - including the coldness of the water on winter nights - for the hippopotamus to rest out of the water at some distance from the familiar concentrations of the species. As Tinley knew already in his early twenties, the hippopotamus (at least on the Pongolo floodplain) rests out of water whenever it can, to avoid its large parasites, and to avoid being killed by its own kind.

In the winter, it rests out of the water, both by day and by night (after grazing enough grass on the now drained floodplain to satiate itself), in riparian forest where the ground is dry and there is no grass in the first place. In the summer, it rests out of the water only by night because of the bits of large biting flies (including Glossina), and tends to do in vegetation types which most naturalists would assume never to be visited by this megaherbivore (e.g. thorn thickets and dense woodlands beyond the floodplain, which are never flooded and offer little grass at any season).

Complementing this refutation of the idea that ‘the hippo rests exclusively in or immediately adjacent to water’ is a refutation of the even more common assumption that the hippopotamus grazes only on land. In fact, it does sometimes

excavate the tubers of Nymphaea in shallow water, and
graze on floating meadow (Echinochloa, a genus shared with e.g. the far more extensive floating meadows of the Amazon, where grazed by the manatee).

Indeed, it seems possible that the hippopotamus acts, in part, as a counterpart for manatees (https://en.wikipedia.org/wiki/Manatee) elsewhere. Both animals graze floating meadow in the same way: immersed in water and approaching the grass from below by day, so that it can be heard foraging while invisible owing to the cover of the floating grass.

So, while the hippopotamus is in one sense more terrestrial/diurnal than many naturalists think, it may in another sense also be more aquatic/nocturnal than they think.

Can we picture these scenes on the Pongolo floodplain? In summer, the hippopotamus ‘grazes’ by day on floating meadow from below the grass (protected from its seasonal bane, the large biting flies), then moves right beyond the floodplain to rest under dense non-riparian woodland and thorn thicket by night.

The information for all of this was published by Tinley more half a century ago. However, I have found no mention of some of these behaviours in e.g. Smithers and Chimimba (2005, https://www.cambridge.org/core/books/mammals-of-the-southern-african-subregion/5C2E1D1A9BD464C8A0F3D5A550595AFA).

Publicado el 28 de junio de 2022 por

milewski

Floating meadows in South Africa, with particular reference to behavioural inversions in the hippopotamus

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