The Northern fur seal population in the Eastern Pacific/Pribilof Islands (Alaska) is designated by the Marine Mammal Protection Act (MMPA) of 1972 as a “depleted” stock, meaning this population of animals should not be allowed to fall below the optimum sustainable population level. A management plan (see the URL below) was issued in 2007, focusing primarily on the effects human activities have on these marine mammals and their habitat.
In the accompanying photo, you see two Northern fur seals floating on the surface of one of the MMCC’s pools, in what appears to be an odd, yoga-like position. This behavior, called jughandling, is actually quite common; it is a behavioral adaptation to the environment in which these animals live.
Northern fur seals are endotherms (warm-blooded) and spend 80% of their lives at sea, coming ashore to mate, give birth and occasionally to rest, and as a result, are well-adapted to life in a cold ocean. They have a dense, double-coat of fur more like a sea otter than a sea lion. In order for their fur coat to function as nature intended, they have to constantly groom the coat so that it will trap air bubbles that help provide a warm layer of insulation. Grooming is just one example of an activity that generates body heat which is lost in water much faster than in the air. Animals can increase their metabolic activity which will generate heat, but this can be expensive in terms of the energy spent in the process. The need to conserve energy, as well as body heat, is important to the survival of these animals. Behavioral adaptations allow them to do just that.
Sea lions are often seen floating on the ocean surface with flippers raised in the air. This behavior is called thermoregulation and is a strategy for regulating internal body temperature. The sun will warm cooler blood just below the surface of the skin and carry heat back to the body core, warming the animal as the blood circulates. If the animal is hot, water on a wet flipper will evaporate, cooling the circulating blood.
Jughandling is considered an example of thermal substitution (as is grooming) rather than thermoregulation. In thermal substitution, the loss of metabolic heat is offset by a behavior that aids in the retention or recycling of body heat in order to maintain a relatively constant core body temperature. Since jughandling is done while the animal is floating on the surface, little energy is spent during this activity.
As you can see in the photograph, one front flipper is held between the two rear flippers out of the water, reducing the flipper surface area exposed to the air and water, thereby reducing the amount of lost body heat. The more endotherms can do to prevent the loss of body heat, the less energy needed to generate additional heat as the body’s core temperature cools. This is a survival strategy that allows the Northern fur seal to successfully adapt to life in the frigid waters of the Eastern Pacific Ocean.
- Liwanag, H.E.M. Energetic Costs and Thermoregulation in Northern Fur Seal (Callorhinus ursinus) Pups:
The Importance of Behavioral Strategies for Thermal Balance in Furred Marine Mammals
Physiological and Biochemical Zoology: Ecological and Evolutionary Approaches
Vol. 83, No. 6 (November/December 2010), pp. 898-910
Written by Chris Huff