Human Food May Alter Hibernation, Aging in Black Bears

Written by Dr. Karen Shaw Becker

bear hibernation

Story at-a-glance -

  • Bears who consumed more human foods had shorter hibernation periods, by up to 50 days, compared to bears who ate mostly wild foods
  • The shorter hibernation periods were also linked to shorter telomeres in the bears; telomeres are caps on the end of chromosomes that are sometimes used as markers of cellular aging
  • One of the benefits of hibernation appears to be reduced cellular aging, as bears who hibernated longer had reduced rates of telomere shortening
  • Over time, it’s thought that bears who eat more human foods may lose some of the advantages associated with hibernating, including slower cellular aging

Human activities influence bear habits in many ways, some of them obvious, such as habitat encroachment and destruction, and some of them far more subtle. It’s widely known that human food sources, including garbage, crops and livestock, can alter bear activity. The bears may be drawn into urban areas and are even at risk of being killed as a result, should they be deemed a nuisance or enter roadways.

However, access to a handy buffet of human food is also altering bears’ typical hibernation and foraging habits, according to a recent study in Scientific Reports.1 In addition to shortening their hibernation, the access to human food may also be accelerating aging.

Human Food Linked to Shorter Hibernation in Bears

Black bears may hibernate for up to 7.5 months a year in order to conserve energy during times of food shortage. While hibernating, they do not eat, drink, urinate or defecate.2 Their breathing and heart rate slows and they “sleep” through the winter (technically, bears are awake during hibernation but in a reduced metabolic state).3

Researchers followed 30 black bears in Colorado from 2011 to 2015, analyzing their eating and hibernation habits. Those who consumed more human foods had shorter hibernation periods, by up to 50 days, compared to bears who ate mostly wild foods.

The shorter hibernation periods were also linked to shorter telomeres in the bears. Telomeres are caps on the end of chromosomes that are sometimes used as markers of cellular aging. One of the benefits of hibernation appears to be reduced cellular aging, according to the study, as bears who hibernated longer had reduced rates of telomere shortening. The researchers explained:4

“Our study demonstrates that … food subsidies are also associated with cellular aging indirectly via altering hibernation length. Black bears with a greater reliance on human food subsidies were associated with having shorter hibernation lengths, and these shortened hibernation periods were associated with greater telomeric attrition. Consequently, bears that use more food subsidies hibernate less and thereby appear to experience greater cellular aging.”

Over time, it’s thought that bears who eat more human foods may lose some of the advantages associated with hibernating, including slower cellular aging. Ultimately, this altered behavior among bears may have far-reaching consequences for the species as a whole.

Rising Temperatures Also Shorten Bear Hibernation

Bears’ hibernation is incredibly sensitive to the environment, such that a bear may come out of hibernation six days sooner for every 1 degree C rise in minimum winter temperature.5

In the 2017 study, while the availability of both natural and human foods were again linked to shorter duration of hibernation, warmer temperatures were even more influential on hibernation, leading to delayed hibernation in the fall, sooner emergence from hibernation in the spring and reduced duration of hibernation overall. What this means for bears in the long-run is largely unknown:6

“As changes in land use and climate dramatically alter landscapes for wildlife, there is a critical need for managers and conservation practitioners to understand how animals are adapting. While the initial response of animals to a new environmental stressor is typically a shift in behavior, little is known about the effects of human-induced environmental change on hibernation, an important life-history strategy for many species.”

A number of remarkable processes occur while a bear hibernates, including nitrogen recycling. Waste products are produced while bears hibernate (and they live off of their fat reserves), but it’s not disposed of.

Instead, the urea produced as a waste product is broken down into nitrogen, which the bear uses to build protein and maintain their muscle mass and organ tissues. Through this nitrogen recycling process, bears can actually lose fat and increase lean body mass during hibernation.7

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Hunting Also Alters Bear Behavior

Researchers, who have been following more than 500 Scandinavian brown bears closely, some since 1984, have detected a not-so-subtle shift in the length of time some mama bears are caring for their cubs.8 It used to be a rare occurrence for a mama bear to stay with her cubs for 2.5 years, but it appears to be getting increasingly common.

In Sweden, it’s illegal to hunt family groups, so a single female bear is four times more likely to be shot than a bear with a cub; in short, hunters are shooting more of the females that only keep their cubs for a year. Both mother bears and their cubs gain an increased survival advantage with longer care periods, which compensates for the reduced reproductive rates that occur as a result.

In a study of Scandinavian brown bears, mothers who ventured closer to humans were also more likely to have surviving litters during mating season, compared to those who did not (and were more likely to suffer from a complete litter loss).9 It’s not known if this strategy is widespread, but it’s one more intriguing insight into how human activities influence the lives of animals around them, for better or worse.