Life in the sea

Grey seals are streamlined for life at sea. Alternating sideways movements of powerful, webbed and fanned rear flippers propel them through the water. Average swim speeds are around 4km an hour. They are capable of incredible acceleration reaching top speeds of 25km an hour and distance of up to 100km in a day. As seals forage underwater, all their available oxygen is prioritised for their diving activity. 

A seal preparing to dive will use its muscles to open its nostrils to inhale and exhale a few times before relaxing to close its nostrils. Recent research shows that 15 seconds before diving, a seal will make a conscious decision to dive and this is reflected in its physiology. Blood is withdrawn from its periphery and pooled into its core (heart, lungs and brain). Its heart rate slows from around 120 to as few as 4 beats a minute. This effectively shuts off their circulation. This prevents decompression sickness, or the bends, as a result of pressure changes when returning to the surface. On average, grey seals dive to around 60m and for up 9 minutes. Dives have been recorded to depths of 200m and for longer if needed. Oxygen is stored in the haemoglobin and myoglobin in blood and muscles. Seals are tolerant to the build-up of lactic acid. The deeper and longer the dive, the more time the seal must spend at the surface or on land to repay its oxygen debt.

Life on land

Seals are thought to shut down more of their brain when resting on land. They may haul out every few days to rest, aid the digestion of their food and replenish normal and emergency oxygen supplies. Seals leave the sea more often and for longer in their annual moulting (Winter/Spring) and pupping seasons (Autumn/Winter.) 

Seals prefer to haul out at higher tides and remain there until they are floated off by the next high tide. Moving or hauling across land, a seal’s fore flippers become the principal driving force. Powerful shoulders simultaneously throw both fore flippers forwards in front of the seal’s body. Strong claws (supported by underling claw bones) grip the sand, rocks or weed. Muscle movements ripple through the seal’s whole body propelling it forwards. 

Adaptations

Seals are well adapted for life on land and in the sea. A blubber layer between 6 to 10cm thick in adults provides insulation and dense fur helps to reduce wind chill on land. Most heat is lost through a seal’s extremities, its face and flippers, which have little blubber. A dry, hauled out seal being splashed by a rising tide feels the cold water most at its extremities, which results in ‘bananaing’. This is a characteristic behavioural reaction, where the seal lifts its head and tail up in an arc at the same time! This keeps a seal’s sensitive bits out of the cold water for as long as possible. 

Given a choice, seals acclimatise themselves before fully re-entering the sea. On a hot day, this might mean briefly dipping their head underwater and raising it back up. This is repeated before committing itself to leave the land. A seal’s flippers use counter-current heat exchange systems of veins wrapped around arteries. This recaptures warmth returning it with blood back to the body. In contrast, seals hauled out in direct summer sunshine may overheat because of their effective insulation. At such times, seals may splay their webbing wide on both rear flippers, exposing them to the air and cold sand to maximise heat loss.

Senses

Like us, seals experience the world using their senses of smell, hearing, touch, taste and sight. They possibly possess a sixth sense enabling them to understand all things sea and weather-related such as tides and waves. Seals greyscale eyesight is amphibious, adapted to seeing through both water and air. They have a strong sense of smell similar to that of a dog. Their hearing is similar in range and pitch to ours. 43 pair of whiskers make their muzzles more sensitive than our hands. They use these for exploring their world and finding food. The density of objects if usually gently tested though touch with their lips and mouths. 

Communication

Close observation of seals reveals a considerable amount of communication. Most of this appears to be nonverbal body language. Verbal communication may be generated with their vocal cords on land. Underwater they use body movements to amplify vibrations through their bodies. 

Society

A seal’s unique fur pattern, like our fingerprints, enables us to identify them for life. Photo identification is the least invasive of all research study methods. Photo ID reveals that there is no such thing as an average seal…like us, they are all doing their own individual thing. It continues to demonstrate how far our seals swim and how much they depend on joined up and protected coastal habitat. Identifying individuals reveals just how complicated seal society really is.