Loss of sea ice negatively impacts breastfeeding of cubs | Polarjournal
The images of a polar bear accompanied by her cubs are always among the most touching. The task of caring for their offspring, however, is not an easy one for these animals whose habitat is undergoing drastic change. Image: KT Miller / Polar Bears International

Due to the increased time spent on land fasting, female polar bears have difficulty producing milk for their cubs. These are the findings of a recently published study that links sea ice loss and breastfeeding among polar bears.

A study published last week in the Marine Ecology Progress Series demonstrates a potential link between disappearing sea ice and polar bears lactation. Forced to stay on land longer and to fast, the females would produce less milk and milk less rich in fat.

To come to these conclusions, the researchers used data from milk samples from polar bears captured in western Hudson Bay between 1989 and 1994.

Hudson Bay, particularly the town of Churchill, is famous for the polar bears that spend the summer season there. Females give birth in dens. In autumn, the bears gather while waiting for the pack ice to form, allowing them to reach their preferred hunting grounds. However, due to global warming, the sea ice is forming later, leading to conflicts between the inhabitants of the region and hungry bears.

Polar bears are finding it increasingly difficult to nurse their cubs. With global warming, hunting for seals is delayed, leaving mothers with fewer reserves to ensure sufficiently rich milk for their cubs. Image: Daniel. J. Cox / Polar Bears International

A significant decline in the size of the Hudson Bay bear population had previously been documented. However, the recently published study demonstrates that the reduction in sea ice also has a negative impact on the lactation performance of females and therefore on the survival probabilities of cubs. Females who themselves have difficulty feeding thus produce much less rich milk for their offspring who therefore gain less weight. A survival mechanism for these mammals: “Polar bears Ursus maritimus face considerable allocation challenges when seasonal sea-ice melt precludes access to prey for several months, and females rely solely on energy stores to cover their own energetic needs and provision offspring“, the authors mention.

So, as the females fasted longer and burned their own fat reserves, they were increasingly forced to prioritise their own energy needs, with less energy available to provide for their cubs. The results of the study on milk samples showed that after around three months on land, a female with cubs born during the year had a 53% probability of breastfeeding, compared with 35% for a female with cubs from the previous year. 

The energy content of milk decreased by half after three months on land. For females with two young, the energy content of their milk even decreased by more than 75%. A real problem for the survival of the young, especially since the forecasts relating to the loss of sea ice are not encouraging. Indeed, this loss is set to continue as the planet warms up. “ To quantify the exact consequences for demographics, the fate of offspring once a female reduces or terminates lactation effort is an area for further exploration, particularly given that rapid Arctic warming will increasingly force individuals to undertake longer periods without access to primary prey “, the authors plead in their conclusion.

A female polar bear gives birth to one or two cubs, sometimes three. At birth, the cubs are blind and weigh only 600 grams. The female will take care of her young for several months, forgoing food herself. She will spend the first months of her youngs’ life feeding them fat-rich milk. By the time they leave the den in spring, the cubs already weigh around a dozen kilos. The female will have lost half her body fat.

The cubs are then brought up and stay with their mother until they are two years old, before becoming independent. The females nurse their offspring for 18 to 30 months with extremely rich milk. Like other marine mammals, polar bear mother’s milk contains 33% fat, which is essential for the cubs’ survival in the cold Arctic.

Link to the study : Archer LC, Atkinson SN, Pagano AM, Penk SR, Molnár PK (2023) Lactation performance in polar bears is associated with fasting time and energetic state. Mar Ecol Prog Ser 720:175-189. https://doi.org/10.3354/meps14382 

Mirjana Binggeli, PolarJournal

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