With climate change, Arctic winters are becoming shorter and shorter, and with it the length of time during which the sea is covered with ice and polar bears can hunt seals. The predators therefore increasingly have to look for food on land during the ice-free summer months – which is more difficult and costs them more energy. Despite adaptation strategies for hunting and nutrition, polar bears are at risk of starvation during this time, as researchers have determined. Even a kind of “summer sleep” does not protect the animals from weight loss.
In spring and early summer, polar bears usually feed primarily on fatty seals, which give birth to their offspring on the sea ice at this time and are relatively easy prey. In the following summer and fall months, as the sea ice disappears, the bears are forced to look for other food on land instead. They then draw on the previously absorbed energy. But this period is getting longer and longer because with climate change, the Arctic sea ice is disappearing earlier every year. In Canada’s Hudson Bay, for example, the ice-free period has already increased by three weeks between 1979 and 2015. In the past decade, polar bears had to stay on land for around 130 days a year. As a result, the polar bear population in this area has decreased by around 30 percent since 1987.
How do polar bears survive in summer?
A research team led by Anthony Pagano from the United States Geological Survey (USGS) has now examined in more detail how much this development threatens polar bears and how the animals are adapting to it. To do this, the researchers equipped a total of 20 polar bears with GPS trackers between 2019 and 2022 and tracked their movements in the Arctic Hudson Bay for three weeks during the ice-free months of August and September. Using video cameras attached to the animals’ necks, the biologists observed what the bears ate and how they behaved. They also weighed the polar bears before and after this observation period and analyzed their blood values.
The evaluations showed that most bears more or less actively hunted birds, caribou and other animals on land in the summer, while others fed on berries, seaweed and other plants. In order to save energy in the food-poor summer months, some polar bears, mainly large male polar bears, also adapted their behavior, fasting and hardly moving at all. They fell into a hibernation-like state. Three bears also swam far out to sea in search of food, but were unable to eat the carcasses they found during the strenuous swim.
Which of these strategies the individual animals chose was independent of their age, gender or body weight at the start of the observation, although these factors were associated with different energy requirements and consumption. Pregnant polar bears also used these strategies to survive. Nevertheless, 19 of the 20 bears lost a significant amount of weight: between eight and 36 kilograms within the three weeks, an average of one kilogram per day, as Pagano and his team report. Only one polar bear happened to find a seal or whale carcass on land and therefore gained weight instead of losing weight.
Without help, the polar bears will starve
The researchers conclude that all of these survival strategies are not enough to noticeably delay the time until starvation. “Neither strategy will allow polar bears to exist on land beyond a certain period of time. Even the bears that foraged for food lost body weight at the same rate as those that simply lay down,” says co-author Charles Robbins of Washington State University. “The terrestrial food provided them with some energetic benefit, but ultimately the bears had to expend more energy to access these resources,” adds Pagano.
Since the sea ice will decline even further and earlier in the year due to climate change, targeted protective measures are necessary to protect the polar bears from starvation during the summer months, according to the team. “As land use increases, starvation is expected to increase, particularly among adolescents and females with young,” Pagano said.
Source: Anthony Pagano (USGS) et al., Nature Communications, doi: 10.1038/s41467-023-44682-1