Polar bears may suffer stress from coping with rising temperatures

Environment | Tundras and Poles

By Isabel Rowbotham, Co-Editor in Chief

Published August 6th, 2022

Anthropogenically-induced climate change has increased ambient temperatures worldwide. In the Arctic, cold climate-reliant animals are suffering quantifiably. Polar bears in zoos may be experiencing adrenal cortisol (a stress hormone) responses to the rise in temperatures. What does this mean for polar bears in the wild?

Polar bears (Ursus maritimus) are classified as marine mammals and rely on ice sheets to hunt for seals and other aquatic animals. Historically, polar bears are used to stable habitats but rising global temperatures are putting their survival at risk, and the 2015 IUCN Red List of Threatened Species listed polar bears as a vulnerable species.

Climate change and the global rise in temperatures threaten polar bear survival and possibly cause physiological stress. | Judith P. / Instagram

As climate change is their main threat, understanding the physiological impacts on polar bears is an important aspect of conservation. Warmer temperatures induce thermoregulatory problems for the mammals, which limits their survival capacity.

Research on wild polar bears already shows that even in below 0°C temperatures, they experience considerable body temperature rises through exercise and hunting. However, a new study aims to understand the physiological responses to temperature changes in North American zoos by monitoring captive polar bears.

‘A new study aims to understand the physiological responses to temperature changes in North American zoos by monitoring captive polar bears.’

Researchers investigated 25 polar bears across 11 North American zoos by collecting faecal samples to assess the corticosterone concentrations. This adrenal hormone is associated with stress. Alongside the analysis of samples, daily temperatures were recorded and organised into categories: below 5°C, 6°C to 9°C, 10°C to 14°C, 15°C to 19°C, and over 20°C.

The results showed that a temperature of 20°C can significantly increase stress hormone levels in adult polar bears. Temperatures between 15°C and 20°C were associated with higher concentrations of the stress hormone in juvenile polar bears, although results in this age group were not statistically significant. In contrast, elderly bears were affected by temperatures as low as 10°C to 14 °C.

A study shows that zoo polar bears of a wide range of ages may be susceptible to stress when temperatures rise to 15 to 20°C and beyond. | Paul Gorbould / Flickr

Older polar bears—aged 18 to 31 years—can be affected by lower temperatures due to the more fragile health associated with geriatric animals. As bears in the wild do not normally live this long, comparison with captive animals is difficult. Elderly captive bears often develop cancer and joint diseases that might account for the increase in stress. Additionally, their thermoregulatory capabilities may be dysfunctional at this old age.

It is important to note that researchers attempted to account for other sources of stress, such as the relocation of mature polar bears to other zoos for breeding purposes. This is because mammals experience an increase in testosterone in males and ovarian steroids in females during the breeding season.

This elevation in reproductive hormones could have affected the bears’ adrenal system. Similarly to other mammals such as monkeys and koalas, seasonal reproductive activity is associated with an increase in adrenal cortisol levels.

‘The results showed that a temperature of 20°C can significantly increase stress hormone levels in adult polar bears.’

The rise of this stress hormone is also a coping mechanism to counteract changes when the animal leaves its tolerance zone. Despite the results shown in this study, it is difficult to conclude with certainty that 20°C is the exact temperature threshold—the difference between coping and not coping. It is even more difficult to say whether these results may be translated to bears living in the wild; is this bad news for polar bears in their natural habitat?

Due to climate change, the days in which temperatures in Canada reach 20°C have increased by 30% in the last 30 years. Another important factor for wild bears is the effect of high temperatures on an animal's reproduction or ability to run away from danger. All of these stressors can compromise a population's ability to survive.

But it is not all bad news after all. Another report published in the journal Science found that a genetically distinct subpopulation of polar bears in southeast Greenland may have developed adaptation behaviours to survive in areas where ice sheets have melted. Published in June 2022, the research found their habitat consists of glacial mélange or icy freshwater and, against previous understanding, these bears have adapted to this unique environment.

While captive polar bears have different demands than wild bears, research on their coping mechanisms for high temperatures may prove helpful to all marine mammals. | Hans-Jurgen Mager / Unsplash

Overall, this information also has the potential to help zoos around the world to recognise that polar bears have higher demands to be comfortable and safe while they are housed for conservation. This could change how zoos are designed and how management protocols are adapted to care for polar bears in captivity.

Studies on captive animals can help narrow down the effects of temperature on bears in the wild, since other factors such as malnutrition, predation and conflict with other bears are removed.

The study also highlights how rising temperatures are associated with the decline of the arctic mammal. The effects of an increase in stress hormones have many physiological effects, such as diminishing the bears’ ability to counteract long-term exposure to corticosteroids and causing wear and tear. The results of the study may help researchers understand how temperature changes affect polar bears in the wild.

Featured Image: Charles Koh, Katie Jowett, Patricia Prudente, Joshua Kraus, Priscilla Du Preez| Unsplash

, H., M.C., et al. (2010) Climate change threatens polar bear populations: a stochastic demographic analysis. Ecological Society of America. Volume 91, issue 10, pages 2883-2897.

Laidre K.L., Supple M.A., Born E.W., et al. (2022) Glacial ice supports a distinct and undocumented polar bear subpopulation persisting in late 21st-century sea-ice conditions. Science. Volume 376, issue 6599, pages 1333-1338.

Leishman E.M., Franke M., Marvin J., et al. (2022) The Adrenal Cortisol Response to Increasing Ambient Temperature in Polar Bears (Ursus maritimus). Animals. Volume 12, issue 6, page 672.

Wiig Ø., Amstrup S., Atwood T., et al., (2015) ‘Polar Bear: Ursus maritimus’. The IUCN Red List of Threatened Species. Available at: https://www.iucnredlist.org/species/22823/14871490 [Accessed June 26th, 2022]

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