Interview: Bear Boss, Yosemite National Park

March 3, 2009 -- Ryan M. Leahy is the Lead Bear Technician at California's Yosemite National Park. As such, it is his job to manage the balance between a healthy bear population and human safety. Earlier this winter, amid unusually high temperatures in the Yosemite Valley, Discovery News correspondent Eric Bland sat down with Leahy to talk about the wonders of hibernation and the perils of incautious park visitors.

Discovery News: Are the temperatures this winter affecting the bears at all, or are they hibernating comfortably?

Ryan Leahy: With such warm temperatures, a number of bears never went into hibernation. Some of the bears have been active, but we haven't had too many issues.

DN: Why are the bears waking up?

Leahy: Winter dormancy for black bears is dependent on food availability and not much else. Right now the ground is uncovered, and there are still a lot of live oak acorns that are a good food source for the bears, so they can continue to forage here in the Valley.

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The pregnant sows go into dens regardless of food availability because they give birth to pretty helpless young. The den acts like an external womb to keep them warm and safe while they develop.

DN: So when do the sows give birth?

Leahy: The sows typically give birth in the last week of January or the first week in February. Usually a litter has between one and five cubs, with two being the most common. They are about the size of a Pepsi can and weigh less than a pound. They will be blind, unable to hear or smell, won't have any teeth, and have very fine fur. They will be very weak but able to nurse, and that's about all they do for the first eight weeks. They are pretty helpless little units.

DN: With some of the bears active, what kind of management activities are going on in the Valley now?

Leahy: Right now we are periodically trapping to identify bears in some of the housing areas, but other than that, we are mostly focused on office work..

DN: When will things pick up again?

Leahy: It depends on when food becomes available, but usually in late March or April the bears will emerge again and be extremely hungry. Bears are constantly hungry and in search of food, but in the spring and the fall they are desperately hungry.

During the winter a sow can lose as much as one-third of her body weight, so she will be looking for lots of shoots of fresh grass, all kinds of green vegetation. Black bears are omnivorous, and 85 percent of their diet is vegetation, which makes sense, since vegetation can't run away from you.

DN: That raises another issue -- humans bringing food into the park. How big of a problem is that?