Brugia Malayi - Clark Science Center - Smith College

The Function of the Tail in Aquatic and Semiaquatic Mustelids
Paula Noonan
Otters and mink, members of the Mustelidae family, occupy aquatic and semiaquatic niches. The pertinent question is: “Why did
some mammals become aquatic in the first place?” (Reidenberg 2007). The answer is the key source of energy: food available
in the aquatic habitat. The adaptations for swimming for these mustelids differ in terms of thrust (amount of webbing of the
feet for paddling) and propulsion (amount of undulation of the body and tail). At the same time, mobility on land is somewhat
hindered by morphological adaptations for aquatic life, such as webbed feet (Estes 1989). But is the length of the tail significant in
an aquatic environment as these otters and mink search for prey? Does the use of the tail by the truly aquatic sea otter differ from
that of the freshwater otters or the semiaquatic mink? Specifically, would a longer tail in relation to the body length be a help or a
hindrance in an aquatic environment?
One way to look at the function of the tail is to determine what its proportion is to the length of an animal’s body. This type
of morphological information exists on the skin tags of specimens in natural history museums and provided the data needed for
the sea otter, Enhydra lutris; the freshwater otters, Aonyx capensis, Aonyx cinerea, Hydrictis maculicollis, Lontra canadensis, Lontra felina,
Lontra longicaudis, Lontra provocax, Lutra lutra, Lutra samatrana, Lutrogale perspicillata, and Pteronura brasiliensis; and two mink, Mustela
lutreola and Neovison vison. Specimen data came from the American Museum of Natural History in New York, the Field Museum in
Chicago, the Smithsonian National Museum of Natural History in Washington, D.C., the ARCTOS and MCZBase databases, and
the primary literature. Male (n=617) and female (n= 455) data were separately calculated to control for sexual dimorphism.
Most otters and mink have tails that are 38-76% of the length of their bodies, with the two mink species on the lower end
of this range (see Figure 1). However, the sea otter, the only mustelid that spends nearly its entire lifetime in the water, has a tail
length of 23% of its body length.
A possible explanation for varying lengths of tails for these species may result from environmental adaptations. Entirely
aquatic, sea otters have the most modified feet for swimming, making them awkward on land. When the sea otter is on its back,
it may use its tail to maneuver slowly, but in normal swimming position and when swimming submerged, the tail and forepaws
are not used in propulsion. Intermediate examples might include the small-clawed otter (Aonyx cinerea), the river otter (Lontra
canadensis), and especially the giant otter (Pteronura brasiliensis), which have webbed feet but undulate their tails for propulsion
(Borgwardt and Culik 1999; Fish 1994, 2000). Mink have minimal webbing, but maintain a streamlined body similar to the otters
(Williams 1989).
If thrust is generated by the tail for some of these species, why would the sea otter have a much shorter tail? The sea
otter’s adaptation for a fully aquatic lifestyle appears to show a trade-off between the efficiency of large, webbed feet against the
potential undulatory thrust of a longer tail. If a shorter tail works best for sea otters, is there a trend for the freshwater otters to
be intermediate in length and the mink (as the most land-based) to have the longest tails? Since mink are not truly reliant on an
aquatic habitat, unlike the otters, one might conclude that their morphological adaptations show less modification. However, as
the results in Figure 1 show, since the mink have shorter tails than the freshwater otters, the question becomes whether a short tail
may be the original trait and a longer tail the derived trait. Further investigation of the relative tail lengths of other mustelids may
provide the answer. (Supported by the Elizabeth B. Horner Fund in the Biological Sciences)
Advisor: Virginia Hayssen
2012
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