Iron is the seals’ gold, because the hemoglobin building block enables their blood to absorb a high level of oxygen and thus allows the animals long dives. In this context, a study now provides an interesting example of maternal willingness to make sacrifices: Weddell seals supply their young with iron so intensively during lactation that their own ability to dive clearly suffers. However, due to the timing with seasonal hunting potential, this is apparently still worthwhile. However, the scientists say that this could possibly change in the course of climate change.
Like us, marine mammals also use their lungs to breathe – but as is well known, they can survive much longer with one breath: In order to be able to stay under water for a long time, whales, seals and co. have optimized their oxygen supply. The adaptations also include an increased absorption capacity of the blood for oxygen. It is based on particularly high amounts of the protein hemoglobin. Its binding activity towards oxygen is largely based on iron. It is also known in humans that a lack of this element can lead to poor oxygen supply. In the case of marine mammals, the opposite of such “anemia” is required.
“The system basically acts like a kind of internal diving tank for these animals, allowing them to dive for extra long periods,” says Michelle Shero of the Woods Hole Oceanographic Institution in the US. Accordingly, young animals must develop their hemoglobin equipment during the breastfeeding period and therefore be supplied with iron intensively. “Females pass on their diving skills to their young when they suckle them, through the provision of iron. So far, however, this has not been examined in more detail,” says the marine biologist.
On the trail of a precious element
To gain insight into the system, Shero and her colleagues conducted studies on Weddell seals (Leptonychotes weddellii) in Antarctica. These animals are known for their extensive dives and a comparatively long suckling period for the young. A favorable aspect for the investigations was also that female Weddell seals do not produce offspring every year. This enabled the researchers to compare females with and without young. As part of their study, they examined the blood and milk of the females and the characteristics of their diving behavior.
As the researchers report, the analyzes of the milk revealed that the animals infuse their young with up to 614 milligrams of iron every day. This is an extremely high transmission rate compared to land mammals: the dose in relation to body mass is up to 15 times higher than that which would lead to iron toxicity in humans. In the case of the seals, however, the young animals can use the element to expand their ability to absorb oxygen. The scientists explain that this allows them to start long dives immediately after the suckling period.
At the expense of maternal diving performance
In order to enrich the milk with the enormous amounts of the precious element, the mothers mobilize it from stores in their livers. However, as the investigations into the diving performance of the Weddell seals revealed, this obviously has its price: Compared to females without offspring, the duration of the dives for the mothers after the lactation phase is significantly reduced. Your oxygen supply seems to be limited. From this the scientists conclude: “Due to the removal of the large amounts of iron, the ability of the female Weddell seals to maintain their own endogenous heme stores is impaired, as a result of which they can only dive for a comparatively short time. The high iron requirement during lactation ultimately impacts diving capacity as a cost to reproduction,” the team writes. In other words, this is a new example of maternal investment in the animal kingdom.
As the scientists continue to report, the reduced diving capacity of the females does not seem to exceed any critical values. “It is striking that the females wean their young at a time when productivity in the sea is high. While the female seals’ diving capacities are limited, they may not have to exert as much effort to catch prey during this time,” says co-author Jennifer Burns of Texas Tech University in Lubbock. Despite the restriction, the female seals can even put on weight in late summer.
However, there are two potentially critical aspects to the system, the researchers say: Precisely timing the reduced diving potential with the seasonal productivity pulse could make the Weddell seals vulnerable to climate shifts that decouple these events. In addition, the iron content of the diet could change: “If the environment changes in such a way that the seals have to start catching low-iron fish species, this will affect their abilities in the long term,” says Shero. “I hope that this study can help to consider iron physiology as an important factor in the life of the seals,” says the marine biologist.
Source: Woods Hole Oceanographic Institution, Article: Nature Communications, doi: 10.1038/s41467-022-31863-7