One Life: International Day of the Seal

22 March 2021

One Life

Seals are a widely distributed and diverse semi-aquatic marine mammal. They spend most of their lives in the water, but come ashore to mate, give birth, molt or escape from predators such as sharks and killer whales.

They feed largely on fish and marine invertebrates; but a few, like the leopard seal, feed on large vertebrates, such as penguins and other seals.

When they are on the land, they live in large social colonies which may contain over one thousand seals.

All different shapes and sizes

Seals vary greatly in their size. The biggest is the southern elephant seal, which can weigh up to 4000kg and reach nearly 4 metres in length. The smallest seal species is the Baikal seal which is around 45 kg in weight. 

All seals have fin-shaped feet which make them excellent swimmers, and they can also be used to move around on land.

Perfectly adapted 

Seals have remarkable physiological adaptations which allow them to survive within their aquatic environment. They can dive to depths of over 1000 feet and some species such as the elephant seals, have been recorded holding their breath for up to two hours. 

Seals can do this as they have a higher concentration of haemoglobin in their blood and large amounts of myoglobin in their muscles. Both haemoglobin and myoglobin are oxygen-carrying compounds, therefore, when diving or swimming, they can store oxygen in their blood and muscles and dive for long periods.

Like cetaceans, they also conserve oxygen when diving by restricting blood flow to only vital organs and slowing their heart rates by 50-80%.

Understand abstract concepts

Seals have extraordinary mental capabilities, and can understand the concept of time in the same way humans do, with the ability to detect micro differences in time as small as 420-milliseconds.

In behavioural experiments, seals could tell the difference between a 3-second display, and a 3.42-second display. They may have evolved this unique skill as a result of having to make split-second decisions whilst chasing fish, or from detecting differences in the rate of vocalisations of other seals.

Great communicators

Seals communicate with each other with calls and whistles providing information to others on their intentions and emotional states.

Wild grey seals also clap their flippers underwater during the breeding season, as a show of strength that warns off competitors and advertises to potential mates. The loud high-frequency noise produced by clapping cuts through background noise, sending out a clear signal to any other seals in the area.

Leopard seals also communicate with each by purring like a cat to show off their strength to other males and to attract a female seal.

Male bearded seals make calls as loud as a chainsaw to attract a mate. These elaborate vocalizations are essential for bearded seal reproduction.

Under Antarctica's ice, Weddell seals produce ultrasonic vocalizations. They chirp, whistle and trill under the ice using over 30 social calls at sound frequencies audible to humans and nine ultrasonic vocalizations that are inaudible to us.

Great mums

Seal mums raise and protect their offspring from harm.

Atlantic grey seal mums make a large maternal sacrifice for their pups, going without food for some 20 days whilst nursing their pups. Transferring huge amounts of their own body reserves, including about one-third of their own bodyweight, without feeding themselves, to nourish their rapidly growing pups that need to double their weights in their first few weeks.

Mother grey seals recognise their pup’s calls

Seals live in large colonies, and so finding their pups from amongst many others can be tricky. Some populations of grey seals have developed skills to address this. The pups perform specific and unique calls, that only their mother will respond to. This helps to ensure that their mother returns to feed them, and not another pup. 

Is that you, mum?

For Antarctic fur seals it is easy for a mother and infant to lose one another in the crowd, and mother seals have to go off regularly to forage for food. 

When they return, they need to find their pup from amongst hundreds of calling pups. To help them to achieve this seal pups are able to identify their mothers vocal pitch from a distance, and they use components of her vocal signature to find her. When the mother seal returns, she and her pup both vocalise to locate one another and once they get close enough, the seals use their sense of smell to confirm that one another is who they think they are.

What’s in a seal call?

To find out what exactly seal pups recognise in their mother’s calls, researchers carried out playback experiments on wild pups at different distances. In these experiments, they played both synthetic signals and real recordings to see how the seal pups reacted.

The seal pups were found to use both the sound’s amplitude and frequency modulations to identify their mothers from other seals. The frequency of the call could be reliably recognised at up to 64 metres, whereas differences in amplitude were only useful for up to 8 metres. 

The seal pups initially use the frequency modulation pattern of their mother’s calls when they were further away, and then, when they were closer, they add other components such as amplitude to identify her calls.

A useful skill in a busy crowd

These seal pups rely on being located by their mother, they are helpless until around 4 months when they can go off and forage themselves. A seal colony is a very noisy and crowded place, and so it is imperative that seal mothers can find their infants quickly and efficiently. Having the ability to pinpoint one another’s calls is truly an amazing skill, but it is also essential in such a chaotic environment.

Seals can learn a task and know that they learnt it

Captive seals can be trained to perform various tasks. When seals are given the command to “repeat” they can successfully repeat the task. This means that they know what they have learnt. This is an indication of self-awareness in seals and shows a level of conscious thinking. 

Seals really are amazing animals!

Seal conservation

Seal populations are often severely impacted by ocean pollution and oil spills. Seals are also victims of the commercial seal hunt as their furs are still valued within the fashion industry. Large numbers are also killed due to the competition they are perceived to present to commercial fisheries.

Seals are also threatened by a lack of food, as their natural food sources are overfished by humans. Seals also become entangled in discarded fishing gear, causing them to drown.

Climate change is another major threat to seals, especially those species that rely on sea ice. Some need sea ice for breeding, and others use it for dens. If the sea ice melts too early, then they are at a greater risk of predation, and are unable to come together into groups to breed.

Read more:

Animal aspects: The incredible mind of the octopus
Sea turtles: Animal kingdom’s greatest navigators map the world’s oceans through magnetic fields