Diving Physiology
Fig. 8. “Milwaukee County Zoo Otters Swimming.” 2020, April 21. Milwaukee County Zoo, 3:57. Retrieved November 12, 2021 from https://youtu.be/mXjE6-Fb53Q
Aonyx cinereus is a semi-aquatic species, and as such their habitat generally consists of lakes, rivers, coastlines, and rice paddies (Selwo Aventura, n.d.) Although these otters do not rely solely on swimming and diving to catch their prey, (see Nutrition), they are still excellent swimmers. As the A. cinereus evolved, their bodies adapted to a lifestyle that included time on both land and in the water. Examples of this include their webbed feet, highly adapted eyesight, and dense fur (Balliet and Schustermann, 1971, 305).
When they are swimming or floating, A. cinereus will use all four feet to “doggy paddle.” However, when they dive, (they can
stay underwater up to 8 minutes), or swim at high speed, their entire bodies become involved. Their bodies move in an up and down, wave-like motion while using their hind feet to steer (Taipan 2018).
Adapting to a semi-aquatic life is not without its consequences, however, and comes at a cost of higher energy requirements for locomotion in addition to thermal constraints due to their small body size. To better understand their energy requirements, Borgwardt and Culik examined six A. cinereus while at rest and while swimming underwater. Figure 9 shows the resting metabolic rate of the otters on land, while figure 10 shows the power and transportation costs of them swimming at various speeds.
Fig. 9. Resting metabolic rate (RMR, Wkg-1) of clawless otters on land, (f female, m male). Duration(s) of resting phases during 4 h experiments was variable. (Image by Borgwardt, N., and Culik, M.B. “Asian Small-Clawed Otters (Amblonyx cinerea): Resting and Swimming Metabolic Rates,” Journal of Comparative Physiology. B, [1999]: Vol. 169.2, 101.)
Fig. 10. Power (MR, Wkg-1) and transport costs (COT, dimensionless) in clawless otters swimming at various speeds. (Image by Borgwardt, N., and Culik, M.B. “Asian Small-Clawed Otters (Amblonyx cinerea): Resting and Swimming Metabolic Rates,” Journal of Comparative Physiology. B, [1999]: Vol. 169.2, 102.)
Linking the cost of locomotion to that of thermoregulation, Hind and Gurney state, “the initial rise in metabolic rate occurs because forced convection causes a rapid rise in body surface cooling with increasing velocity.” (1997, 538) When they are swimming at slower speeds, there is less heat generated and they must increase their metabolic rate to maintain core temperature.
It was found that A. cinereus consume 82% more energy resting in water, than on land. In addition, after periods of prolonged activity, the otters require a minimum of 8 minutes of resting for metabolism to reach a stable minimum (Borgwardt and Culik, 1999, 102).