Thermal Physiology
The thermal conductivity of water is 23X greater than air, which makes it very effective at cooling a warm body, (think diving into a pool on a hot summer day!). Temperature depending, this can be very refreshing, but that isn’t always the case.
Like most mammals, the Asian small-clawed otter is an endotherm, which means they warm their tissues by the production of metabolic heat. The average body temperature of mammals falls between 38-39℃, and those who spend time in the water need a way to maintain this temperature. If they don’t, their body will quickly lose heat, which can cause permanent
damage or even death.
Most otters will increase their activity on land before diving or swimming, which means they enter the water with an already increased body temperature, allowing them to remain in colder waters for longer. However, A. cinereus relies on its fur for warmth, which is also one of the reasons it is hunted, (see the IUCN page).
As otters have very little blubber to keep them warm, their fur is very important for thermo-insulation. Their coats have special adaptions that enable them to maintain their relative temperature while swimming or diving. The outer “guard” layer has long, shiny hairs that help protect the dense, fluffy underfur beneath it, (L.C. Wright, n.d.). Much like a scuba diver is insulated with a dry suit, the underfur of A. cinereus holds air which works as insulation while underwater. As they dive the air is squeezed out of their fur; the deeper they dive, the faster the air is squeezed out, which is why you often see bubbles behind a diving otter.
Fig. 2. “Asian small-clawed otter grooming,” 2012, December 18. Cute Animal Channel, 0:39. Retrieved November 19, 2021, from https://youtu.be/S9IE6Bq8u4g
Keeping their fur clean is crucial for otters to maintain correct body temperature while swimming and diving, which is why they spend a good deal of their time grooming, (see figure 2). According to Kruuk (2006, 169), if the otter hair becomes oiled or dirtied, the thermo-insulation is breached, and pneumonia can quickly develop. Water type also influences thermo-regulation and grooming habits. In a study of captive Eurasian otters by Kruuk and Balharry (1990, 405-15) it was found that the otters diving in seawater spent almost twice as much time grooming as those in freshwater. They also preferred to rinse their bodies in freshwater after leaving the salty sea, shaking off the excess water, as seen in figure 3. After a few days, the pelts of those who were unable to rinse appeared wet and heavy, unlike the usual fluffy, insulating coat. During the study, otters who were unable to rinse after a sea dip would remain shivering poolside, until freshwater became available. Data supporting the hesitancy of otters to re-enter the sea without a freshwater rinse can be seen in figure 4.
The hesitancy indicates the necessity for clean fur to properly maintain body temperature in the water. The salt crystals left behind from seawater dry on the otter fur. This interferes with the oil secretions that help the hair to repeal water and maintain an insulating layer of air, (Harry Martin, n.d.)
Although this may not affect the thermal insulation capabilities of those otters who live in riverine habitats or freshwater wetlands, it would undoubtedly be of concern for those who spend their time hunting and swimming in the salty mangrove swamps.
Fig. 3. Otter Shaking Water Out of Fur. (Photograph by Bence, M. “European Otter Shaking Off Water Hungary,” Pixels [2019]. Retrieved November 19, 2021 from https://pixels.com/featured/european-otter-shaking-off-water-hungary-bence-mate--natureplcom.html)
