The foundation for the historically massive trade in otter skins (see Chapters 2 and 14) lies in the vastly superior quality of otter fur, in its structural design that provides thermo-insulation and makes the otters' watery exploits possible. The queen of furs is that of the sea otter, but the other species are also better protected by their pelage than almost any other mammal.
Seals, which may live in the same places as otters, are shielded against cooling by a subcutaneous layer of blubber, some 7-10 cm thick, which contains up to 40% of their body mass (Irving and Hart 1957; Tarasoff 1974). Otters, however, have no blubber at all and very little fat; for them, blubber would have some serious disadvantages and fur is all important. A thick layer of blubber would interfere with an otter's agility under water, but, in particular, it would make locomotion on land extremely clumsy (as in seals). It clearly would not be suitable for any semi-aquatic mammal such as otters, mink (Mustela vison, M. lutreola), water shrews (Neomys fodiens) and others (Estes 1989). In North America the river otter has an average of 13% of its body mass as subcutaneous fat, especially around the legs close to the body, and also around the ventral side of the body (Tarasoff 1974).
However, in the 'fattest' Eurasian otter dissected in Scotland, only about 3% of its bodyweight consisted of fat as adipose tissue (Pond 1985), and that was mostly mesenteric (inside the body cavity). Most Eurasian otters are as lean as can be, with almost no fat at all—even those living in the colder parts of their geographical range. This compares with, for instance, a seasonal presence of thick layers of subcutaneous fat in another mustelid, the Eurasian badger Meles meles (Kruuk 1989), comprising more than 30% of its body mass. There is some fat under the skin of Eurasian otters in the loins, and especially around the base of the tail, but even there small muscles and vascularization are found in the layer between the fat and the skin, and in general this can hardly contribute much to thermo-insulation. Sea otters have no blubber or subcutaneous fat, but their skin is about twice as thick as that of the river otter (Tarasoff 1974).
In water, fur is less efficient as an insulator than is blubber, yet all otters depend on fur, and their coat shows special adaptations. Under an outer layer of guard hairs, each 2-4 cm long, there is a layer of under-fur, an extremely dense mat of about 1 cm thick, through which the skin is quite invisible even when one tries to penetrate the fur. When examining an otter skin closely, the under-fur gives the impression almost of being the skin itself, so dense is the hair. The river otter has the same arrangement and lengths of hair, but the fur of sea otters is about half as long again as that of Lontra and Lutra (Tarasoff 1974).
One of my students, Addy de Jongh, studied the fur of Eurasian otters by electron microscopy. He counted the hairs in bundles of about 20 or 22 under-hairs around each guard hair. There were 20 to 30 of such bundles per quare millimetre, or about 50,000 hairs per cm2. River otters have about 60,000 hairs per cm2 (Tarasoff 1974). Sea otters go even further, with hair densities of up to 164,000 per cm2 (Williams et al. 1992). Compare this with dogs, which have fewer than 9000 hairs per cm2, cats with up to a respectable 32,000 hairs per cm2, and humans with only about 100,000 hairs on our entire head.
During an otter's dive, air is trapped in the under-fur; this is important for thermo-insulation, as in a scuba-diver's dry suit. For instance, it has been shown in polar bears (Ursus maritimus) that the thermal conductivity of fur is 20 to 50 times greater when wet than when dry (Scholander et al. 1950). It is therefore vital for otters to maintain the air-holding capacity of their fur, even at the cost of considerable effort. It has been demonstrated for sea otters that the air-holding capacity of fur is lost very easily, after even moderate fouling of the pelt, or by contamination with oil (Costa and Kooyman 1982), which increases the thermal conductivity of a pelt 2-4-fold (Williams etal. 1988).
Marine-living Eurasian otters spent quite a lot of time grooming themselves (Fig. 10.5), and especially rolling on grass and seaweed. Bart Nolet, a Dutch student working in our Shetland team, found that three Shetland otters with radio-transmitters spent 6% of their time grooming. The length of each grooming bout was correlated with their previous behaviour in the water (Nolet and Kruuk 1989). The correlation with length of time fishing was not significant, but there was a significant correlation when the depth at which the animal had been
fishing was taken into account (comparing grooming bout length with time in the water multiplied by the depth of dives). This suggested that diving, especially deep diving, stimulated grooming. In later years I have become convinced that the radio-transmitters, which in Shetland we attached to the otters with a small harness or collar, affected our grooming observations, because the attachments stopped otters' access to some small parts of their fur. Anything that interferes with the maintenance of the natural functions of the fur affects the otters' behaviour. Nevertheless, the effect of previous diving on grooming was likely to be real, and not caused solely by the attachment of the transmitter.
We do not yet know whether Eurasian otters living in fresh water spend as much time on fur maintenance as those in the sea, as they are more difficult to observe continuously. However, it is likely that they groom and roll less, in the absence of the effects of salt water on the pelt. The influence of sea water, and the Eurasian otters' reactions to it, opened our eyes to many of the problems of thermo-insulation. In fact, as early as 1938, Richard Elmhirst, Director of the Marine Biological Station near Glasgow, described the otters' use of small freshwater pools along the coast for rinsing off the sea water. Having seen this behaviour in Shetland, I realized that these basic observations held important clues for the otters' distribution along coasts and for their behaviour.
The effect of salt water on Eurasian otter fur was analysed by David Balharry, who did experiments in a project with captive otters (Kruuk and Balharry
1990). Two adult females were kept in a large enclosure with a swimming pool. They could drink under a tap if the pool was dry. For an experiment in which we tried to simulate conditions along the sea coast, we gave the animals an additional fibreglass pool in the enclosure, which I shall call the feeding pool. We sometimes filled the feeding pool with sea water, and sometimes with fresh water. Thus, we could have the otters using the feeding pool with sea water, whilst they did or did not have access to the freshwater swimming pool, or we could make the otters use the feeding pool full of fresh water, again with or without access to the freshwater swimming pool. Most of the experiments were done in October and November, so the water was fairly cold, between 1°C and 6°C.
The two otters were fed five times per day, each feeding period lasting for 25 min. During one feeding period, each animal would get five pieces of haddock, which were thrown into the feeding pool at 5-min intervals. We measured several different aspects of the otters' behaviour during and in between these feeding times, and there were striking differences in what the otters did when they were being fed in sea water or fresh water.
Significantly, the otters paid many more visits to the alternative, freshwater swimming pool when they were being fed in sea water. This happened during 35 of 40 feeding periods in salt water, but only once during the 20 times that they were fed in fresh water.
However, the behaviour of both animals changed quite dramatically when they were being fed in sea water without having access to the swimming pool for a refreshing dip. For instance, between feeds during any one feeding period, the animals used to dive in and out of the feeding pool, perhaps to check whether any food was left, or just out of excitement. They were much more reluctant to do this if they had not had their freshwater dip for one or more days (Fig. 10.6). During the times when they had been fed in sea water for 4 days or longer, without having access to fresh water for swimming, the otters would be clearly miserable. They would sit at the edge of the feeding pool, shivering and reluctant to enter the sea water even during actual feeding, even when the air temperature was not particularly low. We then put a stop to the experiment and turned on the tap for
Sea water Sea water Freshwater Freshwater (no fresh (with fresh (no fresh (also fresh wash) wash) wash) wash)
Figure 10.6 The unwashed hesitate before a plunge: experiment with captive Eurasian otters. When being fed in a seawater pool, they are more reluctant to enter the water when no freshwater washing facility is present. There is a similar, but much smaller, effect when being fed in a freshwater pool (P < 0.001, Mann-Whitney U test). (After Kruuk and Balharry 1990.)
the freshwater swimming pool. Immediately the two would dive in, playing and splashing.
Often an otter gives itself a good shake after it leaves the water (Fig. 10.7). In our experiments we saw that this, too, was much affected by sea water: in the absence of fresh water for washing, the two otters shook themselves significantly more often, and it was clearly the sea water that caused the increase (Fig. 10.8). The same pattern emerged for grooming and licking their fur, which the otters did about twice as much when they had sea water in their feeding pool, compared with fresh water.
These are just some basic quantitative effects of sea water on otters, but to anyone simply watching the two animals when there was only sea water in their enclosure, it would have been obvious that something was amiss even without these statistics. An otter emerging from sea water, after several days without a good rinse in fresh water, appeared to be quite soaked through, with the pelt hanging heavily around it—totally different from the smooth but fluffy coat we had come to accept as normal. We quantified this phenomenon by making the animals enter and leave the feeding pool over weighing scales, so that we could weigh the quantity of water clinging to their fur. This was somewhat
Sea water Fresh water
Figure 10.8 Longer grooming after swimming in sea water: experiment with captive Eurasian otters (P < 0.05, Mann-Whitney U test). (After Kruuk and Balharry 1990.)
rough-and-ready, but the scales did show that (a) the longer the animals stayed in the feeding pool, the more water they absorbed in their pelt and (b) during seawater feeding the amount of water absorbed by the pelt was significantly greater (Fig. 10.9).
The results indicated that sea water interferes with the capacity of the fur to hold air under water. The fur soaks up water instead, thereby jeopardizing the insulating function of the pelt. Microscopic examination showed that when sea water dries in the pelt,
700-1 600500- JL
Sea water Sea water Fresh water
(no fresh available) (fresh available)
Figure 10.9 Sea water interferes with insulation: experiment with captive Eurasian otters. When fed in sea water in the absence of a freshwater washing facility, more water is retained in the pelt after emerging on land. (P < 0.001, Mann-Whitney U test). (After Kruuk and Balharry 1990.)
salt crystals are formed along the length of both guard hairs and under-fur, with many hairs sticking together in small bundles. Perhaps the salt interferes with the lipid secretions from the skin glands on the hairs, instrumental in retaining the air layer (Tarasoff 1974). Possibly, the crystals also interfere with the surface structure of the individual hairs, the scales.
All of this will not come as a great surprise to anyone who has ever swum in sea water without showering afterwards: hair gets sticky and ropy. However, to confirm these results in the laboratory, we did some experiments with pieces of otter skin, measuring temperature at several points inside the fur with small thermo-couples. The pelt was stretched over a copper plate at 35°C (just below body temperature) with cold (6°C) water flowing over the outside. The temperature near the skin, inside the fur, was 29.1 ± 1.1°C, which showed that heat permeated the skin from the inside, and not much was lost through the fur. After the pelt had been rinsed twice in sea water and dried again, there was no significant change in this, but after rinsing five times in sea water the temperature in the same sites inside the fur dropped to 23.0 ± 1.1°C, a significant decrease. There was no significant change in a set of control pieces of otter skin, which had been washed in fresh water. This demonstrated the havoc played by sea water.
Apart from their fur, otters have a few more adaptations against the cold. All but one of the species have a long tail, which functions in propulsion, during diving and with underwater steering, but which is a liability in terms of thermo-regulation, because of its large surface-to-volume ratio. The species that is the most temperature challenged, the sea otter, has a tail that is much reduced in length—a compromise between different environmental requirements. At the other extreme, the giant otter, in tropical waters, is the species with a long and flattened, broad tail. The smooth otter, another warm-water species, also has a tail that is flat at the base. Apparently, these last two can afford a tail that is more effective for locomotion. Most other species are similar in shape to the Eurasian, and some, such as the river otter, live in environments that are at least as cold, or more so (Fig. 10.10).
Semi-aquatic mammals of the same size as otters, or smaller, have a problem with heat loss that is more serious than for larger animals, such as seals. Small bodies have a smaller thermal capacity (i.e. they can hold less heat), but the relative surface area is also greater in a small animal. A small animal in cold water cools much more rapidly than a large one with the same thermo-insulation, so size helps.
Keeping warm is therefore slightly easier for sea otters, as the most aquatic of all carnivores, as they are larger (two to four times the weight of Lutra or Lontra). Their metabolic rate is two or three times higher than that of similarly sized mammals (Costa and Kooyman 1982; Morrison et al. 1974), higher also than that of other otter species, and this means they have to eat more. Most of their heat flux goes through the large rear flippers, which are sparsely furred and heavily vascularized (Costa and Kooyman 1982; Estes 1989; Iverson and Krog 1973; Morrison etal. 1974). These flippers may be used as 'solar panels' to absorb heat, and prevent aquatic cooling, with the animals floating on their back, flippers out of the water (Tarasoff 1974) (Fig. 10.11).
As sea otters leave the water far less frequently than other otters do, if at all, the sea water does not dry out in the pelt and there is less salt encrustation in the fur. Nevertheless, they have to keep their fur scrupulously clean and salt-free by grooming a great deal every day. If the hair gets only slightly oiled, or if the animals get dirty under conditions of captivity, their
Figure 10.10 North American river otter on ice, in Yellowstone Park. © C. Reynolds and J. Stahl.
thermo-insulation is breached and they develop pneumonia very quickly. This has been shown, initially more or less accidentally, when animals died in captivity (Kenyon 1969), and later by other scientists in deliberate experiments to study the effects of oil pollution at sea (Costa and Kooyman 1982; Siniff etal. 1982).
Especially exciting is the grooming behaviour of sea otters. Each day Enhydra spends 1-2 hours grooming—longer than any other otter, and whilst afloat on the surface. Generally, after a feeding bout and in a highly stereotyped sequence, the animal first goes through energetic somersaulting and rolling, with vigorous rubbing of the entire body, and actually blowing and whipping the air into the fur (Kenyon 1969). It squeezes and rubs its fur with its forelegs. Then, floating on its back, it rubs its face, neck and paws, and the hind flippers rapidly together. The tail and hind flippers get a good lick, finally the entire body is licked, and the animal wraps itself in kelp, several times (Loughlin 1977, in Riedman and Estes 1990).
In contrast, Lutra and Lontra never groom in the water but on land, rubbing on grass and seaweeds, drying the fur in the air and relying on this and the use of fresh water to rid the pelt of salt, and to allow air back into it.
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