An adult male polar bear can lose or gain up to 60 kilograms of body weight within just a few days – an amount equivalent to the body weight of an average adult human. What may initially sound like stress, food scarcity, or health problems is, according to new scientific findings, part of a highly dynamic energy balance. Short-term weight fluctuations are therefore not an exceptional condition but a normal component of the physiological adaptability of polar bears.
These findings are based on a study published in the scientific journalArctic Science by Anthony M. Pagano, Stephen N. Atkinson, and Louise C. Archer. It is the first study to systematically investigate how strongly the body mass and energy balance of free-ranging polar bears can fluctuate over the course of just a few days. The analysis was based on GPS tracking, motion sensors, and repeated measurements of body mass in wild animals. This approach made it possible to examine in detail how closely short-term energetic gains and losses are linked to behavior, activity levels, age, and life stage.
One key finding of the study is that individual animals exhibited weight fluctuations of up to 10-12 percent of their total body mass within a few days. Such rapid changes have rarely been documented before and expand the understanding of what constitutes a normal physiological state in polar bears. The researchers explicitly emphasize that these are not measurement errors, but real energetic processes involving the intake or expenditure of large amounts of energy.
(Photo: Steffen Graupner)
Against the backdrop of shrinking Arctic sea ice considered one of the greatest threats to polar bears—the results provide a more nuanced picture. Less ice ultimately means fewer hunting opportunities, but the relationship between environmental change and body condition is more complex than often assumed. The study shows that short-term changes processes unfolding over days or weeks play a central role and do not automatically indicate deteriorating living conditions.
This is particularly evident in spring, when two energy-intensive phases overlap: the mating season and the accumulation of energy reserves. During the breeding period, many polar bears invest substantial time and energy in mate searching, competitive behavior, and mating. Food intake is often reduced during this phase, despite high energy demands. As a result, short-term weight loss can occur even during periods that otherwise offer favorable hunting conditions. Weight loss is therefore not necessarily a sign of food scarcity but may reflect natural, evolutionarily established behavioral strategies.
The study also highlights pronounced individual differences. Older and more experienced animals were, on average, better able to gain mass, while particularly heavy polar bears more frequently lost weight—likely because their absolute energy requirements are higher. Despite sometimes considerable short-term fluctuations, the average body condition of the studied animals remained stable overall. Short-term losses can often be compensated for and are not automatically an indicator of poor long-term health.
Overall, the findings underscore the remarkable short-term adaptability of polar bears. Within just a few days, they can compensate for substantial energetic gains and losses, provided that hunting success, timing, and environmental conditions allow. At the same time, the researchers point out that this flexibility has clear limits. It describes responses to natural, seasonal fluctuations, but not to persistently deteriorating environmental conditions. The study also exclusively considers natural energetic processes; additional pressures from human activities – such as disturbance, habitat loss, or altered hunting conditions—were not included but could further strain an already fragile balance.
(Photo: Heiner Kubny)
Taken together, these new insights paint a more nuanced picture of polar bear energy balance: a species with high short-term flexibility, whose long-term future nevertheless remains closely tied to stable environmental conditions. How well polar bears will cope with ongoing Arctic warming and the profound changes to their habitat remains an open question that will only become clear in the years ahead.
Marcel Schütz, PolarJournal