Biology of Pinnipeds

Pinnipeds often have to deal with cold conditions. To cope with this they show several adaptations for thermoregulation:

  • A thick layer of blubber, which also has several applications outside of thermoregulation :
    • Buoyancy,
    • Energy storage, allowing for energy reservoir in periods of fasting,
    • It allows for a hydrodynamically efficient body shape. The compact shape and lack of longer limbs also allows for reduced heat loss.
  • Pinnipeds are comparatively large if you consider the size of other land based carnivores. The overall large size allows for conservation of heat.
  • Newborn pups, whilst having very little blubber, have a thick layer of fur and high rate of metabolism to reduce heat loss.
  • Newborn Atlantic Fur Seal pup.

    These adaptations do, however, pose a problem when warm weather is taken into account. Pinnipeds (along with many other animals) have a system which allows them to maintain a state of homeostasis. The system is called 'countercurrent heat exchange' or CCHE. Veins and arteries are situated near each other so that heat from the warmer vessel can be transferred to the other vessel. This acts to keep the internal temperature of the animal constant.

    Countercurrent heat exchange mechanism.
    Countercurrent heat exchange
    Pinnipeds are also subject to tremendous pressure when they dive, so they have mechanisms to protect themselves. They have flexible rib bones which allow for their alveoli to contract in their lungs. Their trachea can also be collapsed to account for pressure.

    Pinnipeds are able to lessen the need for oxygen by forwarding it only to essential organs whilst diving. They can slow their metabolism down tenfold, this state is called bradycardia. This low oxygen state causes the build up of carbon dioxide in the muscles. Pinnipeds remedy this by utilising a transport protein called myoglobin instead of haemoglobin.

    Pinnipeds primarily use aerobic respiration in their diving even though their oxygen supply is finite. Anaerobic respiration does occur in longer diving periods. Pinniped muscles are very tolerant of lactic acid, which allows them to continue heavy exercise in its presence.

    As well as other marine mammals, pinnipeds have heavily lobulated kidneys which are extremely efficient at concentrating urine. The excess salt which is excreted in their urine is often even more concentrated than in salty sea water which as a result leads to a net gain in pure water after drinking. In comparison, humans lose pure water after drinking sea water.