Polar Bear Mothers: An Energetic Balancing Act

All animals face the challenge of deciding how to spend their limited energetic resources. For polar bears—long-lived mammals that can reproduce several times over their lifetimes—females must balance their own energy needs for survival with the needs of their cubs. 

Seasonal changes in Arctic sea ice coverage mean that polar bears often face long periods without access to important habitat for hunting seals, making these energy allocation decisions particularly challenging for reproductive females. During times of reduced food availability, polar bear mothers must rely on their body fat reserves to support themselves, while also producing milk to support their cubs. Previous research shows that polar bear milk is generally extremely high in energy (up to ~33% fat), and cubs will typically stay with their mother for around 2.5 years, with females lactating for much of this time. 

​​As a researcher in polar bear energetics, I have been curious about the impact of changing sea-ice dynamics and longer fasting periods on the ability of polar bear moms to nurse their cubs. This topic is growing all the more relevant as sea ice loss from climate warming is rapidly altering foraging opportunities for polar bears across the Arctic.

Some valuable data

A serendipitous meeting with wildlife scientist and veterinarian Dr. Stephen Atkinson provided a rare opportunity to tackle this question. Sea ice disappears entirely from Hudson Bay in the warmer summer months, forcing the polar bears here onshore. While on land, bears are fueled solely by their energy reserves until temperatures cool and the sea ice returns in the fall. As part of his graduate research on Western Hudson Bay polar bears in the 1990s, Stephen collected milk samples from females with cubs while the bears were fasting on shore during the ice-free season. Along with measuring the composition of the mother’s milk, Stephen and colleagues recorded other important metrics including the body mass, length, age, and fat reserves of each bear. 

In a new study published in Marine Ecology Progress Series, Drs. Peter Molnar and Stephanie Penk of University Toronto Scarborough, Dr. Anthony Pagano of the U.S. Geological Survey, Stephen, and I recently revisited this data with some new modeling tools and new questions. For every polar bear family, we calculated how long they had been fasting on land based on that year’s sea-ice breakup date. We then estimated the energy content of the mother’s milk and identified important factors that affected milk energy, aiming to understand whether  the bears moderated their lactation investment—and if so, why. 

We found the energy content of polar bear milk decreased the longer bears had been off the sea ice and fasting on land. Female body condition also influenced the energy in the milk: Not surprisingly, females with lower energy reserves produced milk that was the lowest in energy. Some of the females had already stopped producing milk entirely when they were sampled, even though they were still accompanied by cubs. Bears were more likely to have ceased lactation the longer they had been off the sea ice, and when they were in relatively poor body condition. This indicated that as females spent longer fasting and burned through their own fat reserves, they were increasingly forced to prioritize their own energetic needs, with less energy available to allocate to their cubs. 

However, the age of the cubs also affected milk production: Females with cubs-of-the-year (cubs born that winter, termed COYs) produced more energy-rich milk than females with yearlings (cubs from the previous year). Since COYs require lots of energy to support their fast growth rates but have yet to accumulate sufficient fat reserves to sustain themselves, they are more dependent on their mother’s milk than yearlings. Our study suggested that mothers accompanied by vulnerable COYs continued lactating and produced more energy-rich milk (even when in lower body condition) than mothers of yearlings.

Consequences for moms and cubs

For a smaller subgroup of bears, Stephen had obtained milk samples twice within the same fasting season, allowing us to test whether bears that reduced their investment in lactation saved more energy overall. We discovered that for every 0.3 calorie drop in energy per gram of milk, moms burned through 1 kilogram less of their own body reserves between recaptures. Although producing milk with less energy benefited polar bear moms, their cubs didn’t do as well: Cub growth decreased as milk energy declined, with some cubs not growing at all in the capture interval. 

Consequences for polar bear populations

Dusting off these historical data provided valuable insight on how female polar bears balance their own energetic needs with those of their cubs. Almost three decades have passed since these samples were collected and a lot has changed for the polar bears in Western Hudson Bay. Earlier sea-ice melt and later refreeze dates have extended the ice-free period by approximately nine days per decade from 1979 to 2011 (roughly 27 days, total), with corresponding declines in polar bear body condition. Such changes are likely challenging the ability of moms to continue to provide the high-energy milk required by their offspring. Declining lactation performance may have already played a role in increased cub mortality that has accompanied longer ice-free seasons, and the overall reduction in the size of the Western Hudson Bay population, which has dropped by about half since the early 1980s. If human-caused warming is not constrained and sea ice loss continues, female polars bears are likely to increasingly struggle to provide for both themselves and their cubs. 

Dr Louise Archer is a Mitacs Postdoctoral Fellow at the University of Toronto Scarborough, supported by Polar Bears International.