Phylogeny and evolution

All otters belong to the largest family of the Carnivora, the Mustelidae (martens). It is divided into subfamilies, the otters being the Lutrinae. Others are the Mustelinae (weasels, martens and minks), Melinae (badgers), Mellivorinae (honey- badger), Taxidiinae (American badger) and Memphitinae (skunks). Of all these, the Mustelinae are the closest relatives of the otters, and are their ancestral branch (Koepfli and Wayne 1998). The elongated body shape of the Mustelinae provided an excellent start from which to evolve an aquatic existence.

The otter lineage of 13 species goes back a relatively long time, compared with that of many other animals. The earliest genus that was recognized as otter (Mionictis) occurred right at the beginning of carnivore evolution, in the early Miocene (Willemsen 1992). The directions in which the various present-day species have evolved from there are important as background to the differences in their behaviour and ecology, as will become evident later.

When describing the thirteen different species as in Chapter 2, the natural inclination was to emphasize the differences between them. The variation is striking, and significant. However, what may be at least as important, if not more so, is their similarity: they are all, without doubt and hesitation, clearly otter. Seeing a giant or a smooth otter for the first time after being acquainted with the river otter, one immediately knows that here is another one, another typical otter. Such similarity between species may be due either to relatedness or to convergence in evolution when different species adapt to a habitat in similar ways. In this case, there is good evidence that all otters are closely related.

There is a considerable literature on otter phy-logeny (Oxford Dictionary: 'racial evolution'), and over the years interpretations have changed markedly with new data. Previous knowledge was based largely on evaluation of morphological differences, and on fossil evidence. There was large variation in interpretation between authors (e.g. Van Zyll de Jong 1987; Willemsen 1992). More recently, however, some of the uncertainty has been taken away by the advent of molecular biology, which has provided a much clearer picture of the relationships between species, and of the course and timing of their evolution.

Analysis of mitochondrial DNA suggested strongly that the divergence of several otter species, from common ancestry, took place some 11-14 million years ago in the middle Miocene (Koepfli and Wayne 1998). The conclusion is based on fairly accurate knowledge of the speed of changes within one particular gene, cytochrome b. The molecular comparisons put the date of the first divergences of otter species (i.e. the beginning of their evolution) somewhat earlier than fossil and other evidence had suggested (Bininda-Emonds et al. 1999; Willemsen 1992), which perhaps is not surprising, as the chance of finding fossils of the earliest animals is small.

The two main branches on the otter tree are Lutra (Eurasian) and Lontra (American), and however similar they may look in the field, Koepfli and Wayne (1998) argued that they are genetically sufficiently dissimilar to warrant these different generic names. These researchers are supported in this by most other recent authors, summarised by Bininda-Emonds etal. (1999). Briefly, as far as we now know, the earliest Lutra species occurred in Europe during the Pliocene (Willemsen 1992), and Lontra appears to be descended from the fossil species Lutra licenti, which occurred in the early Pleistocene in China and migrated from there into North America during the Pleistocene (Van Zyll de Jong 1972; Willemsen 1992). Lontra then colonized the Americas, as recently as about 1.7 million years ago, resulting in the river otter Lontra canadensis, the neotropical otter L. longicaudis, the huillin or southern river otter L. provocax, and the sea-cat or marine otter L. felina. There are also many fossil species, now extinct.

It appears likely that the geographical origin of the Lutra/Lontra species was in southern Asia. It is from that general region that the different species groups are branching out, and where we now find the greatest species diversity.

The main Eurasian/African branch Lutra produced the present-day common otter Lutra lutra, as well as the hairy-nosed otter L. sumatrana, and in Africa the spotted-necked otter L. maculicollis, although the position of this last species is still problematic (Koepfli and Wayne 1998; Van Zyll de Jong 1987; Willemsen 1992). The data are as yet insufficient to give a date for the divergence of these three species. Present-day species of both Lutra in the Old World and Lontra in the New World were preceded in previous ages by several species now extinct.

Then there are the three clawless and small-clawed otters: in Africa the Cape clawless Aonyx capensis and the very similar Congo clawless Aonyx congicus, in south-east Asia the small-clawed otter Aonyx (or Amblonyx) cinereus. The work of Koepfli and Wayne (1998) strongly suggested that these three are descended from the Lutra branch, some 5-8 million years ago. These authors also found that the three between them are so closely related that they should all be put into one genus, Aonyx, which is the name I shall also use for the small-clawed otter. It is likely that their common ancestor evolved the finger adaptation and dental structure needed to exploit the rich resource of crustaceans in the Asian and African rivers.

This leaves the phylogeny of three species. All three of them are odd ones out: the sea otter Enhydra lutris, the giant otter Pteronura brasiliensis and the smooth-coated, here called smooth otter, Lutrogale perspicil-lata. All three have flat tails; they are large and gregarious. The sea otter is the most aberrant of all. It is the species that looks least like any of the other otters, and it has a very different lifestyle. It is not surprising, then, that its lineage probably diverged at a very early date from the Lutra/Lontra species (Koepfli and Wayne 1998). Earlier, there were also several other sea otter-related species (Enhydritherium, Enhydriodon; Riedman and Estes 1990), which are now fossil. The actual species Enhydra lutra evolved quite recently, in the early Pleistocene; another close relative was Enhydra macrodonta, also now extinct (Kilmer 1972).

In addition, the giant otter split from the others at a relatively early stage (Koepfli and Wayne 1998). It is related only distantly to the American Lontra, and the evidence of Willemsen (1992) suggests that its story began not somewhere around Brazil, but with the now extinct otter genus Satherium, emigrating from Asia into North America and spreading south. All members of that genus became extinct, but it did give rise to Pteronura, in South America.

Many open questions remain over the evolution of the various species. At present the most enigmatic is the smooth otter Lutrogaleperspicillata in south-east Asia. I found no published records based on its DNA, but there have been suggestions that its closest living relative is the giant otter, because of similarities in the morphology of their skulls (Willemsen 1992) and in their behaviour (Duplaix 1980). Further research will tell.

In general, the pattern of evolution of the Lutrinae, as suggested by DNA analysis as well as fossils, appears to be a very rapid divergence into several different branches at an early time in the history of the Carnivores, in the Asian Miocene. This rapid diversification was then followed by long-persisting genera, which between them covered all terrestrial parts of the globe, except Australia and Antarctica. There are no marsupial otters, which, given the aquatic lifestyle of these animals, is perhaps not surprising.

We are left with a scenario of two main otter lineages, the three Eurasian Lutra and the four American Lontra, the three Aonyx as a more recent offshoot of Lutra, and the three 'odd ones out' (Enhydra, Pteronura and Lutragale). In addition, the position of Lutra maculicollis is still uncertain (Fig. 3.1).

Within each species there often are characteristic genetic differences between animals from populations in different parts of their range. Such geographical variability can sometimes be quite obvious, for instance when we compare the otters in Shetland with those from mainland Britain. The Shetland animals almost always have clear, individually shaped, white patches on their throat (Fig. 3.2), a trait that occurs only rarely in otters from the

Figure 3.1 Family tree of otters. Species and genera of the subfamily Lutrinae, with approximate ages of separation in millions of years. Based on data from Koepfli and Wayne (1998) and Bininda-Emonds etal. (1999).
Figure 3.2 The individual yellowish-white throat patch of a Eurasian otter in Shetland.

British mainland, although Irish otters also have it. Shetland otters are also smaller, but that may be a phenomenon controlled by differences in their environment (e.g. food), and not necessarily genetically fixed. Intraspecific variation has been studied rigorously with molecular methods, and the results have important implications for conservation management, such as decisions on transplantations (Blundell et al. 2002). It will be discussed in more detail in Chapters 11 and 14.

Was this article helpful?

0 0

Post a comment