Zoology: Mother’s iron helps Weddell seal pups dive

Lactating Weddell seals transfer iron from their livers into their milk to promote their offspring’s ability to dive, suggests a study in Nature Communications. This process comes at the cost of their own diving ability, resulting in dives of shallower depth and reduced duration.

Weddell seals are known for their diving ability and have been recorded diving for as long as 96 minutes, an ability supported by the high levels of iron-containing proteins in their blood and muscles. Compared to other phocid seals (one of the three main groups within the seal lineage) they have a relatively long lactation period (6–7 weeks), and during this time females primarily rely on stored energy and nutrients, and may lose between 100–150 kg of body mass. Weddell seal females do not breed every year, which provides the opportunity to compare breeding females to ‘skip-breeders’ across several seasons.

Michelle Shero and colleagues monitored breeding females and skip-breeders between 2010 and 2017 to explore the lactation benefits to offspring and the associated costs to breeding females. They examined the females’ blood and milk contents, as well as resulting changes in diving behaviour. The authors found that indices of iron mobilization were elevated during lactation in females with pups, but not skip-breeders. They found that breeding females transferred iron from their liver into their blood and then into their milk, reducing their own iron stores. This process creates milk with up to 100 times more iron than levels found in terrestrial mammals. The authors suggest the high rate of iron offload led to depleted stores in females after weaning and led to shorter dive durations by approximately 5 minutes on average, when compared to skip-breeders.