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.

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.