Iron in breast milk helps Weddle seal pups dive

A Weddell’s seal on Antarctic sea ice is caring for its cubs Image courtesy of Michelle Shero

A female Weddell’s seal with her child in front of Mount Eriebus Image from Michelle Shero

Weddell’s seals in an ice cave, ready to dive In the image by Michelle Shero

The research team studied a Weddell seal at a site in Hutton Cliff in Antarctica, and the weather was sunny and windy. The smoke in the distance comes from Mount Eribers, the southernmost active volcano in the world. Image by Michelle Shero

One study showed that Weddell seals transfer iron from their livers into their milk to improve their offspring’s ability to dive. This process sacrifices the mother’s own diving ability, resulting in a reduction in their depth and duration. The study was published August 2 in Nature Communications.

Weddell seals are known for their diving ability, which has been recorded for a maximum of 96 minutes, an ability that is largely present in their blood and muscles. Compared to other members of the seal family (one of the three main species of the seal lineage), they have a longer lactation period (6-7 weeks), during which the female seal relies mainly on the energy and nutrients previously stored in the body, and may lose 100 to 150 kg of body weight. Female Weddell’s seals don’t breed every year, which allows researchers to compare the differences between breeding female seals and “non-breeding female seals” in different seasons.

Michelle Shero and colleagues at the Woods Hole Institute of Oceanography in Massachusetts monitored breeding females and non-breeding female seals between 2010 and 2017 to study the benefits of breastfeeding for offspring and the associated losses to nursing females. They analyzed the blood and milk composition of these female seals, as well as corresponding changes in diving behavior. It was found that females with cubs had an increased iron transfer index during lactation, while female seals rather than reproductive did not.

They found that breeding female seals transfer iron from their livers to their bloodstream and into milk, which reduces their own iron reserves. The milk produced by this process can contain up to 100 times more iron than terrestrial mammals. The authors argue that this iron transfer not only reduced the iron reserves of female seals after weaning compared to non-breeding female seals, but also shortened their average diving time by about 5 minutes. (Source: China Science Daily Feng Weiwei)

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