Four species, of which the Eurasian otter occurs only along the fringe of the African continent north of the Sahara; it has been described under Europe.
Cape clawless otter, Aonyx capensis (Fig. 2.22)
Just below a large dam in South Africa, the Olifants River runs as a beautiful clear, fast flowing stream through arid country, with bushy vegetation and trees along its banks. When, on a sunny early morning, I creep up to one of its rather stagnant channels, there are two clawless otters foraging in the submerged, grassy vegetation. I manage to get quite close without disturbing them; both are large animals as otters go, and their huge whiskers glisten in the sun when they surface. Obviously they cannot see much in the densely matted grass underwater, but they are both poking around in it with their forefeet, moving fast along the surface, and frequently plunging under. Every 20 seconds or so, one emerges with a
small crab and audibly crunches it with its mouth pointing skywards, then goes down again.
This is the usual way in which the Cape clawless otter acquires its calories, feeding on the very common freshwater crabs, from dense vegetation or from under stones. The animals often catch their prey in shallow water, but they may also use deeper sites, for instance when foraging in 3-metres deep places in the sea, with average dive lengths of 32 seconds.
The Cape clawless otter is common in suitable habitat in large areas anywhere in Africa south of the Sahara, except in the Congo basin (Fig. 2.23). The animal ranges widely in all kinds of wetlands (rivers, lakes, marshes, tiny streams—wherever there are crabs). It also occurs frequently along suitable sea coasts, where it needs access to fresh water for washing and drinking like the Eurasian otter, but where it feeds on marine prey. Remarkably, these otters make long trips overland to isolated dams, and are quite capable of moving far and fast when out of the water.
The Cape clawless otter grows large, in some inland areas almost as big as the South American giant otter (up to 1.8 m in length), weighing up to 18 kg, but smaller along sea coasts, and with only a small difference between male and female. It is brown on top with a rather round head, a clear white throat, and especially conspicuous large, abundant whiskers. The forefeet are without webs between the
Figure 2.23 Geographical range of the Cape clawless otter: coasts and inland Africa south of the Sahara, not in the Congo basin, and in desert areas.
long fingers, with rounded fingertips and no nails, obviously important during foraging. Once caught, crabs are crunched with large, broad molars in the otter's massive skull.
Like its much smaller counterpart in Asia (the small-clawed otter), the Cape clawless' mainstay food in freshwater is crabs of various species, Potamon or Potamonautes, which are abundant in African fresh-waters, especially in nitrogen-rich rivers and lakes. Along the coast numerous crab species are taken, such as Plagusia and Cyclographus, and eaten in their entirety. The diet also includes fish (in some places it is the main food), frogs and even some molluscs; the fish are mostly slow bottom dwellers such as eels and catfish. All of these are usually detected with the forefeet, the otter poking around in vegetation and under stones. The whiskers are probably used especially to detect escaping prey.
Once caught, a prey is held either in the mouth or in the forepaws, and, when eating, the long-fingered forepaws are used almost like the hands of primates; for instance, an otter floating in the water and eating a crab guides the food with its hands. Larger prey are taken ashore. There is no evidence of cooperative prey-catching by the Cape clawless, although it is by no means a totally solitary species. These animals often forage on their own, but one sees groups of up to eight, family parties of mother and (even adult-sized) cubs, or gangs of males only. As yet we know very little about these social arrangements, and what their function is.
The home ranges of individual otters overlap, with males covering stretches of river of more than 50 km (but usually less) and including the areas of several females, and female ranges probably including offspring but excluding non-related females. Along sea coasts, clawless otters have group territories with distinct borders between them, of four to six animals in each, whilst within these group territories individuals may be solitary, or form temporary coalitions. Each group territory contains several active holts. There is evidence that the size of the range is related to the distances between suitable crab sites, for instance reed beds in freshwater areas. The social system appears rather similar to that of the Eurasian otter, but the Cape clawless is somewhat more gregarious, including the small gangs of males as for the North American river otter.
Each individual within a home range uses a number of 'couches' (sleeping sites), usually above ground in thick vegetation or between rocks, but the otters also dig dens themselves (burrows up to 3 m long, with grass-lined nests) or they live in small cavities between rocks. Breeding occurs at all times of year, with 63 days' gestation (no delayed implantation), one to three cubs per litter, and young becoming independent of the mother after about 1 year. Otter densities along sea coasts have been estimated at about seven animals per 10 km of coast in one study, one otter per 2 km of coast in another, and elsewhere three groups inhabited a 5-km section of river. Usually, however, densities are much lower.
The tracks of this species are unmistakable, with large footprints showing the long, clawless fingers. Similarly characteristic are the faeces, which are large (often >3 cm in diameter) and full of bits of crab, deposited on sites that are usually some distance from the water's edge, and often close to a den or couch. Clawless otters often spraint (defaecate) immediately after foraging. As for the Eurasian otter, spraint sites are distributed throughout the range, and probably serve as communication between animals using the same area. In addition, scent marking is also one of the functions of the otters' urinating, rolling, scraping and rubbing. Another means of communication is vocalization: Cape clawless otters have a range of whistles, huffs, growls and screams that have not yet been studied in detail.
In many inland waters of Africa the clawless otter is in competition, at least to some extent, with the spotted-necked otter and the water mongoose, which both also eat crabs at times. However, there are substantial differences in their diets and habitat preferences. Predators of the otters include crocodiles and seals in the water, and various large carnivores and humans on land.
As is to be expected, these animals are persecuted for their fur, for various parts that are used as medicine, and because of their occasional slaughter of domestic ducks and hens, or their depredations in fish farms, which can be substantial. Along many coasts their favourite habitat is being built over, and disappearance of riverine vegetation inland also has detrimental effects. However, the otters' reliance on crabs, which often associate with somewhat eutrophic waters, also means that this species derives certain benefits from human land use, and their wide distribution throughout Africa suggests a tolerance for environmental conditions that may stand them in good stead.
General references for Cape clawless otters
Arden-Clarke 1986; Rowe-Rowe 1977; Skinner and Smithers 1990; Somers and Nel 2003, 2004.
Congo clawless otter, or swamp otter, Aonyx congicus (Fig. 2.24)
A single large otter is foraging in one of the damp clearings in the Congolese rainforests. It walks a few steps, then pushes one of its front feet deep into the soft mud. It is feeling around, its gaze averted and, after a quarter of a minute, it pulls out a most un-otter-like prey: a very large earthworm. It holds the worm firmly in its fingered paw, then eats, chomping quickly, before moving a little bit forward and trying the same thing again—an amazing but normal foraging bout of the Congo clawless, eating worms at a rate of three per minute.
Very little is known about the Congo clawless otter. It lives in difficult places in the Congo basin of
Africa (Zaire and adjoining areas), looks very similar to the Cape clawless, and the geographical ranges of these two species are more or less exclusive of each other (Fig. 2.25). Also called the swamp otter, it lives in rainforest, in open clearings or muddy banks, or in dense swamps, seen occasionally but studied rarely.
Its teeth are better adapted for dealing with earthworms; the molars are narrower than those of the Cape clawless otter, and they have more pronounced, sharp cusps. The entire skull is massive, but not as broad as that of the Cape species; however, the animal itself is of similar large size and shape, up to 150 cm long and weighing up to 25 kg. Its colour is slightly different, the hair on top of the head, neck and shoulders showing a grey 'frosting', and with more white on the face than the Cape clawless, white edges to the ears, and in particular a strikingly large dark patch in front of each eye.
The animal is active at night and during the early morning, foraging singly or in family parties. Even its food has not yet been studied properly, but is said to include crabs, fish, frogs, insects and various other prey, as well as the many large earthworms.
Congo clawless otters are widely hunted for 'bush meat' and for their fur throughout their range, and their rainforest habitat is diminishing daily. Yet they still appear to be quite common—a fascinating challenge for a research project.
General references for Congo clawless otters
Jacques etal. (in press).
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