In this chapter I will attempt to tie together some of the strings from previous chapters of this book, summarizing results, spelling out implications, and speculating on their biological significance. The emphasis will be on habitat, foraging and population dynamics, referring to the otters' spatial organization and social behaviour. Inevitably, some points have been made earlier, but perhaps one should not object to that. All aspects of otter biology are so closely interwoven that it is unavoidable that a survey of present knowledge will lead one repeatedly to some of the same road intersections.
In these generalizations there are many aspects of otter biology that illustrate rather general principles in animal ecology or behaviour, such as the energetics of foraging or the function of scent marking. But otter specifics will necessarily come into it. The question will repeatedly arise of how do otters differ from one another, and from other species. These animals obviously have some unusual problems; their numbers are often threatened in unusual ways, hence some answers to questions posed need to be based on comparisons.
Undoubtedly, the main distinction between otters and other similar-sized carnivores is the fact that otters spend much time in water (Fig. 13.1). This relates to their shape, metabolism, locomotion, food and other fundamental aspects of their biology, such as foraging behaviour, social organization, survival and mortality.
Because of this distinction, otters are highly unusual carnivores. They have evolved in habitats that are now threatened more than most. They have a lifestyle that is more risky than that of most other mammals. Some of their important prey species are vanishing, and they are unusually exposed to human-made changes in the environment such as climate change, pollution and over-fishing. Several otter species are subject to fierce exploitation. What
are the aspects that are especially significant and different in their natural history?
Substantial ecological and behavioural knowledge exists for only a few species, mostly for the Eurasian otter, the sea otter and the North American river otter. However, a fair amount of hard data is also available for most others, as well as many casual observations. One of the important conclusions that can be drawn is the striking similarity between all otter species, not just in appearance, but also in their natural history, ecology and behaviour. When researching a river otter in Prince William Sound, a neotropical otter in the Pantanal, a spotted-necked otter in Lake Victoria or a Eurasian otter in Shetland, I am hard pushed to note differences between them. Many of the observations and conclusions made on one of these species will therefore probably also be relevant for the others. At the very least, they will provide a template for our understanding, with working hypotheses. Some species, such as sea and giant otters, are from a somewhat different mould, however, and one has to be more careful with comparisons.
A main aspect of the relations between otters and their environment, often critically affecting populations and behaviour, is feeding ecology, as for most other carnivores (Kruuk 2002). Much of the research that I have described here, for all otter species, is relevant to this in one way or another. However, what is becoming more evident in our knowledge of these animals is the significance of predation, not by otters, but on otters, and, at least in some species and habitats, this has reverberations throughout their ecology.
Evidence from studies of DNA on relatedness and evolution of otters suggests strongly that not only do species all over the world look very similar (except the sea otter), they also are closely related (Koepfli and Wayne 1998; see Chapter 3). There is a concentration of otter species diversity in south-east Asia (starting with Lutra), from where they appear to have moved along the waters' edges into Africa (Aonyx and Lutra), and across the Bering Strait into the New World (Lontra and Enhydra), then south into Latin America (Lontra and Pteronura). They did not make it across large stretches of open water, for instance into eastern parts of Indonesia and towards Australia, or to Madagascar, because of their restriction to the waters' edges.
In the process of this dispersion, the animals have changed—albeit not that much. Some have affected distinct morphological specializations, notably the sea otters (with unusually dense fur, and a short tail adapting to a cold and exclusively all-aquatic existence), the giant otter (a large flat tail enabling fast movement in open water), and the long-fingered clawless and small-clawed species, adapted to foraging for crabs. This list could be much longer, as, naturally, all species show differences, yet the overall similarities between the otters are much more striking. Almost any other family or subfamily of Carnivora shows more diversity of shape and size than do the otters. They are the remarkable result of a trend starting in terrestrial mammals, to utilize the vast resources of the aquatic environment.
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