Giant otters live in family groups, but every individual usually fishes for itself, independently. There are occasional reports of cooperative action, such as groups driving schools of fish into shallow backwaters, or a family going eight abreast driving fish into shallows between sandbanks (Duplaix 1980; see also below). The animals are active in daytime only, and their most common method of fishing consists of shallow dives, lasting fewer than 10 seconds, along banks or into overhanging vegetation, or in flooded woodlands. The members of a foraging group communicate continually, with slight growling sounds (Staib 2002), but there is no evidence of any concerted action.
Because the giant otters often operate in close proximity to one another, observation of individuals is confusing and quite difficult, and there are few good data on actual diving times. When they are fishing in deeper waters, often near floating vegetation, dives may take up to 20 seconds. In about one-third of observations, Elke Staib noticed that in deep open water the animals were fishing with a behaviour that she called 'jump-diving'. The otters were diving repeatedly within a small patch, surfacing almost dolphin-like whilst taking a deep breath and immediately going down again, sometimes with the entire body out of the water. This jump-diving had a strikingly high success rate.
In an interesting difference with Lutra and Lontra species, giant otters are frequently seen fast-chasing fish, over distances of up to 50 metres, especially in shallow water or close to the surface in deeper areas. They cause a great commotion in the water whilst doing so (Duplaix 1980; Staib 2002). This suggests that vision is highly important during fishing, and the large, bulging eyes of this species lend support to this.
Once prey has been caught, it is eaten on the surface, head first, whilst the giant otter is treading water and feeds the fish into its mouth with the forepaws (Fig. 9.4). Only larger prey are landed, and if giant otters happen on a really large catfish or turtle they may pull it out together and share it with other family members (Staib 2002).
Of the other South American otters, details of foraging behaviour are known only for the marine otter. Until more studies are available, we may have to assume that both the neotropical otter and the southern river otter catch their prey in a manner similar to that of the river otter. In my own casual observations of neotropical otters, their fishing and diving appeared similar to that of river otters and Eurasian otters. The marine otter, a largely diurnal species, was studied by Richard Ostfeld and his team
(Ostfeld etal. 1989) and by Gonzalez Medina (1995). It also fishes by diving from the surface, like the river otter in the sea, and similarly its mean dive time was 28 (±9) seconds, with the longest dive lasting 64 s. Although these dives were of similar length to those of river and Eurasian otters, marine otters are smaller (3.2-5.8 kg) (Lariviere 1998), and one would have expected mean dive times of around only 16 s (see Fishing behaviour of Eurasian otters above). Marine otters appear to be more than averagely efficient divers. Catching crabs involved dives that were 4-7 s longer than those for fish. When marine otters emerged with a fish, the prey was eaten on the surface if small, but medium-sized or large fish or crabs were taken ashore; if other marine otters were present, they occasionally stole from one another.
The foraging behaviour of otters in Asia has been little studied. The smooth otters that I observed fishing in the Huay Kha Khaeng river in Thailand were in shallow, running water, and in one observation two individuals repeatedly swam alongside each other from one side of the river to the other, rapidly driving fish in front of them into some reeds, where they could easily be captured. Similar behaviour had also been observed by others (references in Kruuk et al. 1994a). Foraging behaviour of the hairy-nosed otter is quite unknown, and may be similar to that of the Eurasian otter. The small-clawed otter, however, does things differently.
With their curiously long fingers and only rudimentary claws, these small otters poke around under boulders and logs, and into nooks and crannies, debris and vegetation both under water and on the banks where their mostly invertebrate prey is hiding. Crabs, insects and molluscs, as well as frogs and fish, are their quarry; perhaps they can dive and fish in deeper water like the other otters, but I have not seen it and it has not been described by other observers (see review by Sivasothi and Nor 1994). An interesting snippet is the observation of small-clawed otters digging up molluscs and leaving them in the hot sun, so they soon opened up (Timmis 1971).
In Africa, foraging has been researched more thoroughly in a much larger Aonyx, the Cape clawless otter. Its forefeet have similarly long, sensitive fingers, also used for catching crabs, by feeling under stones and in dense vegetation, in the sea or in fresh water. This is often in shallows, where the otter walks or swims, often foraging with its head above water (Fig. 9.5). Alternatively, an animal may dive in deeper water, in the same fashion as described for the Eurasian otter, its tail showing before going down at an angle of some 60° (Somers 2000a).
The mean dive time of Cape clawless otters foraging in the sea was 21 seconds in both Arden-Clarke's and Somers' studies. This is somewhat shorter than the 26 s predicted for an otter with a mean body mass of 13 kg (Somers 2000a), and in freshwater habitats it was shorter still, at 17 s (Rowe-Rowe 1977). These values probably reflect the shallower depths at which the Cape clawless operates, compared with other otters.
Cape clawless eat small prey at the surface, often holding it in the front paws both when transporting and whilst taking bites from it, and, just like other otters, they take large prey ashore. When carrying it on land, prey may be held with one of the front paws (Rowe-Rowe 1977).
Only a few comments have been published about the foraging of the very similar Congo clawless otter, specifically about their remarkable habit of feeding on earthworms in swamps of the West African rainforest. The description of this foraging suggests a
very un-ottery behaviour (quoted from Jacques et al. in press):
Worms are located by pushing the forepaws deep into soft mud banks, and clawing through the mud with the fingers, gaze averted. After several seconds the forepaws are withdrawn and the worm transferred from the digits to the mouth. An average of 3 worms per minute are obtained in this manner. Parent otters have been observed feeding immatures, with worms passed from mouth to mouth.
The frequency with which worms are caught is rather similar to that of another earthworm specialist, the Eurasian badger (Kruuk 1989).
Somewhat more is known of fishing behaviour of the third otter in southern Africa, the spotted-necked otter. Where we watched them on Rubondo island, in Lake Victoria (Kruuk and Goudswaard 1990, p 324),
. . . the groups mostly moved from site to site as a pack with individuals swimming close together, but during actual foraging they spread out, each one fishing on its own. Almost all fishing was done within 10 metres of the shore . . . and mostly within 2 metres. The otters hunted between rocks and stones and especially between branches of shrubs . . . moving through the water much faster and more silently, without splashing, than does the Eurasian otter, with shorter periods on the surface between dives. Fishes smaller than about 10 cm were eaten in the water, larger ones were landed.
Along the shores of Lake Malawi, spotted-necked otters were similarly fishing between large rocks and branches, not in open water. However, John Procter (1963) saw them fishing for prolonged periods in waters of up 1.5 m deep, with dives lasting about 15 s 'remarkably constant in duration', and an exceptional dive of 21 s. Procter (1963, p 96) also commented on the unusual agility of this species: for their near-vertical dives, their bodies arched almost clear of the water, and the animals
. . . seem able to turn faster than their prey. Once a small fish which was being chased at full speed near the surface flicked out of the water just in front of the otter's nose and landed where the latter's tail had been, but the otter had already turned and caught the fish as it re-entered the water.
In Lake Muhazi, Rwanda, Anne Lejeune frequently saw spotted-necked otters dive in open water of unknown depth, with mean lengths of dive of 22 s (Lejeune 1989). This was rather longer than the calculated expected dive length for an otter of 4 kg, of 16 s.
In general, therefore, all otters show fairly similar behaviour mechanisms to acquire their prey, with relatively small species-specific variations on the central themes. The sea otter is the most aberrant, and the three Aonyx species are different, with the use of their long fingers. There is species-specific fine-tuning of the general foraging behaviour, but, to me, the similarities in their behaviour are more remarkable than the differences.
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