A team of researchers at the Woods Hole Oceanographic Institution (WHOI) has discovered a peculiar ability of Weddell seals. In their latest study, the researchers reveal that mother Weddell seals transfer iron from their livers to the milk they feed their pups. The high iron content allows the pups to have the extraordinary diving skills for which Weddell seals are known. 

Weddell seals are large seals that live around Antarctica. A healthy adult Weddell seal can stay underwater for 20 minutes, and some of them even have the power to dive for over 1 hour and 30 minutes. This is why Weddell seals are considered one of the most proficient underwater divers. However, after giving birth, the mother seals have to sacrifice some of their own iron stock so that their offspring can become better divers. This direct transfer of iron results in reduced diving time for the mothers. 

How do Weddell seals transfer iron to their pups?

Before making this interesting discovery, the WHOI researchers studied Weddell seal behavior for seven years, from 2010 to 2017. They noticed that not all Weddell seals chose to breed every year. So during the study, they divided the seals into two groups: skip breeders and currently breeding females. Soon the researchers realized that the lactation period (the time period during which an animal releases milk from its mammary glands to feed its offspring) in breeding Weddell seals was longer than other comparable seals.

Weddell seals are a species of true seals belonging to the Phocidae family. While most true seals have a lactation period ranging from two to five weeks, Weddell seals can lactate for six to seven weeks. The researchers were keen to know the impact of longer lactation periods on breeding Weddell females and their pups. So they decided to analyze the micronutrient composition in the milk and blood of the breeding seals and compare their diving time with skip breeders.   

The analysis revealed that iron mobilization (the amount of iron in a seal’s milk) increased in breeding females after the birth of pups, and iron from their livers was being released into the blood. This iron-rich blood further led to the production of iron-rich milk in the mammary glands of the seals. The researchers claim that a mother Weddell seal has to breastfeed 2.75 to 5.46 liters of milk to her pup daily which also suggests a daily transfer of 309 to 614 milligrams of iron to the pup.

This is eight to 15 times more than what a healthy human (of the same size as a Weddell pup) would require to maintain the optimum micronutrient level in their body. Moreover, the iron content in Weddell seal milk can be 100 times more than what is found in the milk of land mammals. Weddell seals require such high levels of iron proteins (such as hemoglobin) to carry large amounts of oxygen to their blood and tissues so that they can dive longer while searching for food underwater.    

“Because breath hold capacities are so tightly linked with O2-carrying heme proteins in marine mammals, large endogenous O₂ stores dictate how proficiently marine mammals can forage underwater,” said the authors in the paper. 

The mother seals sacrifice a part of their ability 

The transfer of iron from mother seal to pup decreases the iron levels in the former’s body and therefore, the Weddell seals have to reduce their diving time after they become mothers. The researchers noticed that breeding females spent five minutes less underwater on average during lactation compared to skip breeders.

“In Weddell seals, exceptional iron mobilization during lactation hinders the female’s ability to maintain steady-state hemoprotein production. Reduced blood Hb and muscle Mb post-weaning resulted in a significant decline in the female’s TBO₂ (total body oxygen) stores and this decreased aerobic dive capacity was associated with behavioral shifts. Females may be essentially ‘transferring their dive capacities’ to offspring,” the researchers note. 

Although more research is required to further understand the factors affecting the transfer of iron from mother seals to pups, these findings clearly explain how baby Weddell seals acquire the diving talent of their mothers via micronutrients. Apart from iron, the pups also receive hormones, calories, and various other bioactive compounds from their mothers that are necessary for their holistic development. 

The study is published in the journal Nature Communications.