Seals molt every year to keep their fur healthy

When we think about molting, the most familiar example we can think of is the snake “shedding its skin” and replacing it with a new one. However, did you know that also mammals shed something? Mammals shed their fur. Pinnipeds, such as seals, otariids, and walruses, belong to this category of fur shedding mammals. Once a year, they shed their old fur which is replaced by a shiny, brand new coat. For my honors project, I had the chance to observe the populations of seals inhabiting the Moray Firth, the largest firth in Scotland, and study the timing and process of their annual molt. In this post, I want to share with you some interesting facts about the molting process in these fascinating animals.

Credit: Jariceiii

Why do seals have fur?

The development of hair structures in mammals has been of critical importance for their success on land and the colonization of a broad range of habitats. One of the main roles of the pelage is to keep the animal warm by trapping the heat between the skin and the hair.

In the more marine mammals such as seals, however, the thermal function of the pelage has been almost completely lost. The main function of the pelage in seals is to protect the skin from mechanical damage caused by fights and UV radiation. Still, hair must remain short allowing them to swim fast in the water. In order to maintain these properties, the old pelage needs to be periodically shed and replaced with a new one. This process is known as molting. While other terrestrial mammals shed their fur gradually throughout the year according to the season (just think about cats’ fur and its change in density throughout the year), molting in seals is much more abrupt and happens once a year.

Molting on land or at sea

Most seal species need to spend the majority of their time ashore while molting. This is because the blood needs to flow closer to the surface of the skin to promote hair growth. Spending too much time inside the water would incur severe heat loss as the blood loses heat much faster in water than in air. By increasing their time spent hauled out, they optimize their surface temperature by keeping it around 37°C. Species that adopt this strategy molt faster, because the blood supplies more nutrients to the skin which is needed for the production of new fur. Harbour and grey seals are examples of species adopting this strategy.

However, some species have evolved differently. For example, the California sea lion (an otariid!) regularly enters the water during its molt to forage. In order to avoid too much heat dispersion, it greatly reduces the blood supply to the skin. This results in a longer molt duration which can last up to a few months.  

Catastrophic molt

Did you know that some seals not only shed their fur but also… their skin? This extreme molting strategy is adopted by the elephant seals and the Hawaiian monk seals. Their skin peels off and new skin emerges. This strategy requires a lot of energy, so they spend this period on land in order to increase the blood supply to the skin. During this time they fast and rely on their energy reserves.

Molting in seal pups

Some seal pups ‘first molt’ happens after birth…

At the beginning of the pupping season, most of the females come to land to give birth to their pups. Harp seal pups are born with “lanugo”. The lanugo is a long, fine, white fur that is present in the newborns of most species at the time of birth. Generally, the lanugo is shed after a couple of days/weeks after birth. The loss of the lanugo fur is what we can consider as a “first molt”.  

Some seals molt even before they are born

Harbour seal pups (together with the hooded seals) are unique in the sense that they shed their lanugo prior to birth, while still in utero. The presence of the lanugo in these species is unnecessary, as they are born with a thicker layer of fat to keep them warm. For this reason, unlike other seal pups, they can enter the water within a few days from birth. This enables them to live on the unstable ice (for the hooded seal) or on sand bars that are frequently hit by storms or floods (for the harbour seal).

Molting moms

The hormone’s dilemma

After the nursing period, females start to mate with the dominant male. It is the one that defended their harem from other males attacks, during the nursing period. Mating is both aggressive and loud. It leaves the females with scars from the male’s bites. A few weeks after the breeding period, seals enter the molting season. But sex hormones like estrogen (which is the female’s primary sex hormone that regulates the reproductive system) is thought to inhibit molting and hair growth. In females, estrogen levels that are low during the nursing period start to increase after mating, once the embryo has started to develop.

How can molting still take place after the mating season if the oestrogen levels are so high?

Females pause the embryo’s development

Female seals have evolved an astounding strategy that helps them overcome this problem. It is called embryonic diapause: the fertilized egg remains in a state of dormancy, preventing the embryo to develop. The development will resume within 2 or 4 months depending on the species. This strategy allows females to extend the period of pregnancy so that pups are born when environmental conditions are most favorable. It also allows for the molting process to take place since the sex hormone levels go down.  

Will climate change affect molting?

Environmental fluctuations caused by climate change will undoubtedly affect seals populations and their molting ecology. Seals close to the poles will be the most affected since they rely on sea ice to haul-out when molting.

Sea ice cover has been declining significantly over the past few years, both in the Arctic and Antarctica. Predictions are not optimistic regarding the trends in sea ice cover. The loss of sea ice might force seals to spend more time in the water resulting in a longer molt duration (as the water temperature is too cold for hair regeneration). It might also force them to change haul-out sites for the molting season and choose a colder spot. In addition, climatic changes will likely affect prey availability. For this reason, seals may be unable to regain enough fat after the mating season which can delay the start of their molt. So what are the consequences? Seals molting later in the season have a lower probability to carry the pregnancy to term.  

Understanding the physiological requirements and the factors affecting molt in seals will help us understand how they might respond to these future challenges.

Baby harp seal with its mom – This picture was taken by my dear friend and colleague Kristle Kranenburg during our expedition in West Ice.

The best time to see huge amounts of seals hauled out

The time over which molt takes place differs between seal species, but it always occurs right after the pupping season. In Scotland, for example, grey seals come to land to start their molt between mid-January and mid-February. Harbour seals molt in the summer between the end of July and mid-August with pups being born in June.

Further reading on molting seals:

Chiara paoli

Chiara obtained her Honours Bsc in Marine Biology from the University of Aberdeen. She had the chance to volunteer in Galicia at the BDRI, where she helped with dolphins Photo-ID and boat surveys. She also spent one year in Tromsø as an Erasmus student. She was lucky enough to participate in an expedition in West Ice to study harp and hooded seals.
She enjoys science communication and has a passion for seals. Her honours project focused on the timing of moult in harbour and grey seals, in relation to sex and reproductive status.

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