Photos: Kali and Luna, polar bears at Buffalo Zoo

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Alaska transplant Kali and fellow young polar bear Luna are the subject of a behavioral study by graduate student Jackie Heatwole at the Buffalo Zoo in Buffalo, N.Y.

On a Tuesday morning in June, warm but overcast, school groups huddle around the Buffalo Zoo's polar bear exhibit. The children come and go in great, chattering waves of 20 or 30, oohing and aahing and balancing on tiptoe to get a better view.

They are here to see the zoo's fluffy white cubs: Luna, a female born at the zoo, and Kali, an orphan who arrived recently from Alaska.

One moment, the bears are romping around their habitat, splashing in a shallow pool and tackling colorful toys. Then, suddenly, Kali bolts to one end of the enclosure. He clenches his right paw in his mouth and starts to suck on it, making a humming noise that sounds like a series of fast-paced clicks.

For two months, Jackie Heatwole has been observing the cubs: Luna, a female born at the zoo, and Kali, a male who came to Buffalo in May after a hunter shot his mom in the Alaskan wilderness. Both are about 6 months old.

Heatwole, a master's student in biological sciences at the University of Buffalo, has been tracking their behavior to better understand polar bear development, as well as how the two bears differ from one another, given their divergent histories.

READ MORE: Studying Kali's behavior offers clues to polar bear development