Behavior

From the poles to the tropics: First molting of a southern elephant seal on Mauritius Island

Reviewing: Ramah, S., Paul, A. T., Mohit, R. D., Cootapen, S. A., Mooloo, D., Heeramun, Y., & Roomaldawo, N. (2021). Southern elephant seal Mirounga leonina molting at Mauritius Island. Marine Biodiversity51(1), 1-2.

What makes the southern elephant seal stand out?

Mostly located in the Antarctic and sub-Antarctic waters, the southern elephant seal (Figure 1) gets its name from its big size, reaching up to 6 meters in length and ~4000 kg in weight, making it the largest pinniped. Pinnipeds, which in Latin means fin-footed, include seals and sea lions. The southern elephant seal is one of only two species of elephant seals (the other one being the northern elephant seal). Males also develop an elephant, trunk-like nose (Figure 1b) as they become sexually mature. Their trunk-like nose serves two important purposes, especially during the mating season: (i) to roar loudly, (ii) to reabsorb moisture from their exhalations, which helps keep them hydrated when they stay on shore. Being open ocean predators, they spend up to 90% of their lives at sea and only come to shore to reproduce and molt. When at sea, they spend most of their time feeding on large fish and squids, sometimes going as deep as 1000 m to hunt their preys! Thanks to hunting restrictions and rigorous conservation efforts, the once depleted southern elephant seal population has been restored to 740,000!

Figure 1. (a) The southern elephant seal (Mirounga leonina). (b) The males have a trunk-like nose, that allows them to roar loudly and reabsorb moisture from their exhalations during the mating season. Images from (a) Oceanwide expeditions and (b) Liam Quinn accessed through creative commons.

Do southern elephant seals venture into tropical waters?

Figure 2. The Mascarene Islands, where Mauritius is found. The arrow indicates the location in the northern lagoon of Mauritius where the female southern elephant seal was sighted. Image modified from Anthony et al. (2018).

As mentioned earlier, southern elephant seals typically occupy the polar regions, but has been reported to travel up to 33,000 km. On rare occasions the seals have been spotted on the coasts of Ecuador, the Panama, Rio de Janeiro and Oman. There have been nine sightings of southern elephant seals on the Mascarene Islands in the Indian Ocean (Figure 2) between 1996 and 2014. These pinnipeds breed from September to November, and molt from December to March. Molting is the periodic process where southern elephant seals shed their outer layer of hair and skin. This process can take up to a month to complete and requires the animals to find a place on shore to do so. Normally, when seals are in the ocean, their blood flow is directed away from their extremities and towards the core of their bodies to keep their vital organs warm and functioning in the cold ocean temperature. But, because the act of molting requires the seals’ bodies to direct blood flow to the surface of their skin, the seals must be on land during the molt. Otherwise, they (and their vital organs) would become too cold. So that’s why seals must find a warm place on the beach, out of the cold ocean, for the entire molt process.

A funny fact is that we, human, go through molting as well and do not even realize it! Molting means periodic shedding of any outer layer, be it feathers, nails, hair, or skins. For example, when hair remains in the brush when brushing our hair, we are molting! Another interesting and perhaps scarier fact is that on average, humans shed about one million skin cells per day! Okay, back to the southern elephant seal in the next section.

An unusual and unique sighting: A molting elephant seal in Mauritius!

The Albion Fisheries Research Center (AFRC) is the government institute responsible for attending to cases of stranded marine mammals. On 27 January 2020, they were alerted of a stranded seal in the northern lagoon of Mauritius (Fig. 2). The stranded seal was later identified as an adult female southern elephant seal, about 2.3 m in length and weighing 150 kg. At first, researchers from AFRC thought that the female was stranded because she was wounded with what appeared to be circular bites covering her body (Fig. 3a), possibly caused by cookiecutter sharks. However, after taking a closer look, the researchers determined that the female southern elephant seal was going through the process of molting, which was the first sighting of its kind in Mauritius!

Figure 3. (a) Image of a female southern elephant seal molting on a rocky shore in the northern lagoon of Mauritius. (b) Molt skin with attached hair collected on the rock where the molting took place. Images from Ramah et al. (2020).

This was considered unusual in two ways: first, it appeared that the seal was going through a catastrophic molting, which means that their fur and top layer of skin came off in larger patches (Fig. 3a), making them look wounded. Secondly, their natural habitat environment is known to be wet, windy, and cold like in the sub-Antarctic islands, but this time this female southern elephant seal chose a rocky area in the warm, tropical Mauritian sun to go through this process. Researchers from AFRC monitored the molting process daily, and the molt skin with attached hair (Fig. 3b) confirmed the molting process. While the molting process usually lasts about a month, it is unknown when the process started for this female southern elephant seal. On 7 February 2020, still midway through the molting process, the seal left its rocky spot, went back to the sea and was not spotted again. Researchers speculated that the reasons it left was because of high human disturbances and the hot weather prevailing during the summertime in Mauritius. They also believe that the seal ventured to a remote islet, away from these disturbances, to complete its molting process. Hopefully, the southern elephant seal found a nice spot to peacefully complete its molting process and make its way back to its polar home. To read more illuminating facts about these fantastic marine mammals, follow this link.

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