Lutrinae are amphibious members of a group of %. mammals called the order Carnivora, family Mustel-idae. This family also includes the weasels, badgers, and skunks. Technically, otters are defined according to the following suite of characters: (1) upper fourth premolar with a hypocone or hypoconal crest; (2) first lower molar with a strongly basined hypoconal crest; (3) entoconid reduced to a low crest or altogether absent; and (4) numerous aquatic adaptations, which are examined in more detail later.
I. Modern Diversity
Thirteen is currently agreed upon as the number of living species. There is less agreement on what to name these species of otters. Table I lists the 13 extant species recognized by Wozencraft (1989, 1993) and approximately where they are found.
Which genus name is used does not affect the number of species, just how they are hypothesized to be related. The biogeography is important in determining possible relationships and evolutionary history. Fossils, though, have to be considered too.
TABLE I Lutrinae
|
Genus species |
Common name |
Location |
|
Aonyx capensis |
Cape clawless otter |
Central and southern Africa |
|
A. cinerea |
Oriental small-clawed otter |
India to Indonesia, Southeast Asia, south China |
|
A. congica |
Congo otter (Zaire clawless otter) |
Central Africa |
|
Enhydra lutris |
Sea otter |
Coastal North Pacific |
|
Lutra canadensis” |
River otter |
North America |
|
L. felina” |
Marine otter |
West coast of most of South America |
|
L. longicaudis” |
Mexico, Central and northern two-thirds of South America |
|
|
L. lutra |
European otter |
Widespread in Eurasia-Ireland to Japan, South to Sri Lanka |
|
L. maculicollis |
Spotted-neck otter |
Most of sub-Saharan Africa |
|
L. perspicillatab |
Smooth-coated otter |
South Asia-Afghanistan to Southeast Asia |
|
L. provocax” |
Southern river otter |
Patagonian region of South America |
|
L. sumatrana |
Sumatran otter (haiiy-nosed otter) |
Southeast Asia and Indonesia |
|
Pteronura brasilicnsis |
Giant river otter |
Eastern and northern South America |
“Lontra rather than Lutra appeared for these species in Wozencraft (1993). See Species List. bLutwgale rather than Lutra appeared in Wozencraft (1993).
II. Evolutionary History
The first mustelid fossil is from the late Eocene in Europe. Many different lands of mustelids are known from the early Miocene (23 mya) of both Europe and North America. Lutrines (otters) are present by the late Miocene (16-18 mya) in the ho-larctic region. Forms considered to be probable lutrines are known from much earlier in the Miocene. The term “probable lutrines” means that we are not sure if they are otters because of a lack of information. Many early fossil specimens are known only from fragmentary data.
Because otters are placed in the Mustelidae, we can easily use the mink (Mustela vison) as a model of the possible morphology and ecology of the earliest proto-otter. An evolutionary scenario that leads us to the otters would read something like this: The ancestral lutrine probably lived in an area with a considerable amount of shallow water where many fish and invertebrates lived close to shore. It was terrestrial in habits but was able and willing to occasionally make use of the abundant food in the water. It had little specialized anatomy for aquatic locomotion or for food capture. Gradually, over the course of generations, this proto-otter tended to spend more time in the water, and slight alterations of the morphology began to appear. What are these alterations? They are the adaptations we now identify in modern otters. Swimming by the earliest otter-like form would have employed a terrestrial gait much like the mink still uses. Most likely, it started out hunting fish or other vertebrates in the shallows. Fish eating is considered primitive in lutrines (Berta and Morgan, 1986), and feeding on invertebrates evolved later. Piscivory (feeding on fish) involves using the mouth for capture of the prey. The portion of the brain of otters associated with facial sensitivity is larger in most otters compared with other mustelids. This would improve the capture success when using the head to capture prey. A few species of otter that feed on invertebrates such as crayfish show an enlarged area for tactile sensation in the forepaws, which is in line with their increased use in prey capture.
Body shape also changed. The mink shows no special shape adaptations. Gradually the proto-lutrine took on a more fusiform (streamlined) shape. The neck became shorter, the front legs also got shorter, and the hindlimb lengthened mostly through the elongation of the metatarsals and phalanges. These elongate digits on the hindlimb became encased in webbing. These changes went along with changes in the gait. Two types of locomotion developed to produce greater speed in the water. The limbs were used in a joint dirust and recovery stroke (simultaneously, both hind or both front legs would pump and then recover) and the tail was employed to add thrust. In some species, such as the giant river otter, the tail is used for high speed and the limbs just for maneuvering. The adaptations to life in the water reach their extreme in the sea otter. In essence, the sea otter embodies the story of otter evolution. In this species we see the completion of the evolutionary process that turned a land-dwelling mustelid into an animal quite at ease in its marine environment.
