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A sea elephant rarely deceives

12/16/11

It is the story of a large seal, the unwitting witness to the carelessness of human beings. This is because Mirounga angustirostris (the northern elephant seal) is a cruel reminder of the old adage: “after us the deluge”. The pollutants that we dump into the environment find their way into the marine food chain, and they contaminate all levels of this pyramid with the risk that they may find their way onto the plate of a super predator: mankind itself. The northern elephant seal is therefore this “great witness” which is observed by two researchers from Liège, Sarah Habran and Krishna Das, of the Oceanology Laboratory of the University of Liège, in an article published by the international journal Environmental Pollution (1). This article, which is a prelude to the doctoral thesis of Sarah Habran, is in line with an extensive Belgian and international study on the effect of pollutants on marine mammals (2).

As carnivorous marine mammals, the elephant seals are the largest representatives of the seal family phocidae or true seals. Their name, which evokes the land elephants endowed with trunk and defences, has been given to them because of two physical characteristics that differentiate them from other phocidae. Firstly, there is their imposing mass: while an ordinary seal of male sex measures 1m70 for an equivalent weight of 100 kilos, the northern elephant seal can reach three tonnes in the case of an adult male! Secondly, there is this kind of rough trunk (called proboscis) that develops in the males. This curious appendage enables them to amplify the eructations and roars they make when they enter into competition with rivals of the same species.

Elephant mer2

This family counts two separate species which don’t meet each other: the southern elephant seal (Mirounga leonina), which inhabits the Subantarctic region and come to breed on the region’s coasts (Patagonia, The Falkland Islands, The Kerguelen Islands), and the northern elephant seal  (Mirounga angustirostris), which frequents the Northern Pacific Ocean and comes to breed and moult on the American coasts, in California or in Baja California to the north of Mexico. Only this latter species is concerned by the present study, which is being carried out at the same time as a study on the grey seals of the North Atlantic.

There is a great sexual dimorphism in the northern elephant seal: the males, who can measure as much as five metres in length, are much heavier than the females who measure a maximum of three metres in length. These animals have a very polygamous system of reproduction: the larger dominant males preside over large groups of females known as a ‘’harem”. A single dominant male can therefore “seduce” and fertilize more than fifty females in one season!

Ten centimetres of blubber: a blessing and a curse

Like all true seals, elephant seals have atrophied hind limbs (the equivalent of our legs). Each “foot” is quite distinctive, however, and has a palm which is intended for propulsion in water. On the other hand, they have become unsuitable for any kind of movement on land. Movement on the beach is by means of the flippers, these “arms” are used as supports to enable them to propel themselves on land by making a succession of small bounds.  They can move quickly over short distances in this way whether to return to water, catch up with a female or chase away an intruder.

The elephant seals possess a thick layer of blubber under their skin, which insulates them against the cold, improves their buoyancy and constitutes a very precious reserve of energy for periods of fasting on land. This blubber, which can reach a thickness of ten centimetres, was the reason these animals were intensively hunted from the 18th century which almost culminated in the disappearance of the species by the end of the 19th century. The seal ships pursued the elephant seals on land during the reproductive period, slaughtered them in great numbers and melted down their sub-cutaneous blubber in order to transform it into an oil of very high quality, which was suitable for use in the softening of leather and the lubrication of machines at the beginning of the industrial revolution. The hunting never really stopped until new kinds of oil of mineral origin took the place of the melted blubber. Since the beginning of the 20th century, the northern elephant seals have been protected by American and Mexican laws, which have allowed their population to grow again to more than 120,000 individuals, even though the species was on the brink of extinction. But the current population, built up again on the basis of a reduced genetic diversity, is probably less robust and more sensitive to diseases and effects of pollutants.

The elephant seals spend most of their lives under water only resurfacing to replenish their oxygen reserves. Essentially this is stored in the blood of which they contain a much greater volume than humans even in relation to the body mass.  During prolonged underwater solitary dives, sometimes to depths of 1,000 metres, Mirounga angustirostris mainly tracks fish and squid. Its hunting is helped by its hydrodynamic shape which is suitable for rapid swimming. But it is also helped by excellent vision and a pair of vibrissae, those “whiskers” which are sensitive to vibrations in the water, enabling it to easily locate its prey. It has no known predators apart from killer whales and great white sharks. Despite its ease in the marine environment the elephant seal remains among the most ‘’terrestrial’’ of true seals because each year it spends several consecutive weeks on land.

The reproductive season begins in December with the arrival of large males on the beaches, the strongest acquire a ‘’harem’’ which can count as many as several dozen females. Seal elephant maleThose wishing to challenge them for possession of the females must challenge the dominant male in a fight preceded by powerful eructations unless they succeed in ‘’picking up” a female on the periphery. The females fertilized during their stay “on the beach” the previous year, return to it between the months of December and February to give birth to a single pup weighing between 30 and 40 kilos at birth. Suckling of the new-born lasts between 24 to 28 days during which the mother, deprived of access to food, loses a considerable amount of weight equivalent to around a third of her body mass. On the other hand her offspring can triple its weight during the same period and reach or even exceed 100 kilos at weaning.

The adult female can be fertilized as soon as she has finished suckling her pup and will give birth to the next one around eleven months later. But the development of the embryo does not commence immediately. The gestation period only lasts seven and a half months in fact. This is known as “embryonic diapause”, which is common in many species of true seals. Once weaned, the young seals gather into groups and progressively enter the water to learn about life in the sea. It is also during this period on land that the young elephant seals lose their natal fur, called lanugo. Some of them can remain on land for three months without eating.

‘’Sentinels of the environment’’

Marine mammals in general and the elephant seal in particular are considered as ‘’sentinels’’ of pollution in the marine environment. The word ‘’sentinel’’ can almost be understood here in its proper sense: a sentinel keeps watch for others; and is the guardian, the look-out who warns others of the presence of danger. Why do scientists consider elephant seals to be sentinels?  Because these animals situated at the top of the trophic networks, are precious bioindicators of the health of the seas. What does this mean? ‘’Perched’’ on top of the food chain proper to their marine environment, they eat mostly fish and squid which have themselves previously eaten smaller fish and crustaceans while the latter have ingested the animalcules of zooplankton a carnivore which is itself a predator of herbivorous zooplankton after the latter has absorbed phytoplankton, itself a predator of the herbivorous zooplankton after absorption of the latter by phytoplankton, the ‘’vegetable soup’’ situated at the bottom of the food chain in the marine environment.

Trophic network

The elephant seals are therefore likely to accumulate significant quantities of contaminants in their food such as trace elements and persistent organic pollutants such as PCBs and certain types of pesticides, etc.  They are therefore excellent ‘’bioindicators’’ of the level of pollution of the ocean by various contaminants.

How can this be verified? Thanks to the fact that the elephant seals have the great advantage of spending a significant part of their annual life cycle on land. Therefore, they are easily accessible to scientists in their natural environment, where they can be approached, given an anaesthetic and samples taken from them for analysis. During periods of reproduction and moulting, these animals gather in colonies along the coasts and fast completely, because they avoid going into the sea where their food is. Therefore they are totally dependent on their thick layer of blubber to maintain their metabolism and supply the milk required to help the newborn to grow during suckling. In the northern elephant seal, which beaches on the Californian coasts, the suckling lasts 24 to 28 days, in January and February mainly. After this period of lactation, the pups are weaned abruptly and enter their post-weaning fasting phase, which lasts two and a half months. During this period, they can only rely on their reserves of fat accumulated during suckling before their mother returned to the ocean to feed. They will spend the next four to five months at sea before coming back to rest on the land in the autumn.

Mom pup

The voluntary periods of fasting associated with moulting, lactation and weaning involve not only an important mobilization of energy resources, but also of the contaminants that are associated with them. During the suckling period, the mother’s milk transmits essential trace elements to the pup (such as zinc, iron and selenium), which play an important role in the growth of the newborn. However, the milk also passes on non-essential elements like lead, cadmium and above all, mercury which proves to be very toxic at this key stage of development.

The animals are analysed without being hurt

Up to now no study had focused on the mobility of essential trace elements (selenium, iron, zinc) and non-essential trace elements (cadmium, lead, mercury) during the fasting periods of the seals. The objective of the liège-based study has been therefore to understand the manner of transfer or mobilization of trace elements during the key processes of the life cycle involving a voluntary fasting period (lactation, moulting and post-weaning fasting) in the northern elephant seal. In other words: the scientists wanted to understand how, during these periods, the substances begin to move and travel through the body, but also, from one organism to another, in this case from the mother to her offspring.


It was possible to carry out this work on living animals that are in good health representing the wild population. This involved samples of very accessible ‘’materials’’ such as blood, milk, blubber and hair. These samples have the advantage of being ‘’practically non-invasive’’, in the sense that they don’t require any injury to organs and involves little or no breaking of the skin.

Seal elephant biopsySarah Habran, research fellow at the FRS-FNRS in the Oceanology Laboratory of ULg, and the international team of researchers who work with her visited the elephant seal colony at the Año Nuevo State Park, on the Californian pacific coast several times. With the help of park rangers, the scientists ‘’captured’’ some twenty mothers and their pup at regular intervals during lactation, as well as some twenty pups weaned (called weaners) during the post-weaning fast. Each individual was marked with the aid of hair dye, in order to better recognize the animals that were not among their samples. The concentrations of trace elements in the blood, blubber, hair and milk (during suckling) were analysed.

In addition to being interested in the levels of contamination by trace elements in the animals analysed, this study should provide an understanding of the mechanisms involved in the toxicokinetics of these substances. The scientists also wanted to know what happened to these “chemical products” after their absorption, how they passed from the mother to her pup, as well the “redistribution” of these elements in the tissues during the key periods involving a voluntary fast. 


And they succeeded! The research that was patiently carried out on the northern elephant seal showed the existence of transmammary transfer of mercury and selenium, two substances which transit from the mother to the pup through the milk. This study also suggests that the maternal transfer of selenium is high during suckling, while the transfer of mercury takes place mainly during the gestation period. Likewise, lactation and fasting affect the total levels of mercury and selenium in the blood and milk of the mother seals. The milk consumption and tissue deposition (that is to say growth and development) influence the dynamic (movements) of contaminating substances in the body of the pups. This underlines the importance of examining these processes carefully during the interpretation of the levels of trace elements in the context of biomonitoring. Furthermore, to the best of the knowledge of the researchers from ULg, the levels of concentration of mercury and selenium had not been previously determined in the northern elephant seals.


Additional toxicology studies will be necessary to understand the repercussions of these chemical transfers on the health of the elephant seals. But what is certain already is that the progress in the area of ecotoxicology is of paramount importance. This is particularly so because the levels of polluting substances present in the environment continue to grow in spite of measurements taken in matters of protection of ecosystems and the relative regulation of pollution over the last few decades.

(1) Sarah Habran, Cathy Debier, Daniel E. Crocker, Dorian S. Houser, Krishna Das, Blood dynamics of mercury and selenium in northern elephant seals during the lactation period, in Environmental Pollution 159 (2011) 2523-2529, éd. Elsevier
(2) Sarah Habran’s thesis is in line with a larger framework which is interested in the levels and effects of organic and inorganic pollutants in marine mammals. This research convention is financed by the Fundamental collective Research Fund (Funds associated with the F.R.S. – FNRS) and finances two teams at the University of Louvain-la-Neuve (Prof. Cathy Debier) and the University of Liège (Dr Krishna Das and Prof. Jean-Marie Bouquegneau). Other collaborators both Belgian and foreign brought their help and expertise to the table for the acquisition of samples, veterinary monitoring and the analysis of the different organic pollutants: Sonoma State University (California, United States), Sea Mammal Research Unit (University of St Andrews (Scotland, United Kingdom), Dr Ursula Siebert (University of Veterinary Medicine Hannover, Foundation, Institute of Terrestrial and Aquatic Wildlife Research –ITAW–, Germany), Prof. Jean-Pierre Thomé (CART, ULg), Prof. Adrian Covaci (Toxicological Center, Université d’Anvers).


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