Sun Bear Cubs: Multiple Personalities

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Our newest little sun bear cubs at the San Diego Zoo are now three weeks old (see previous blog, A Star (or Two!) is Born. Thankfully, things seem to be going well for them in the den, and Marcella is doing a great job of caring for them. We don’t get to see too much of her babies since Marcella tries to keep them covered up and warm most of the time. However, our occasional glimpses are enough to confirm that both cubs have big, round bellies that suggest she is feeding them well.

I am most intrigued by the differences I have noticed in maternal care between this twin litter and Marcella’s two previous singleton litters. With a single cub, Marcella was easily able to cover the baby completely and protect it from cold air. She would tuck her body tightly around the cub and use her body heat and the warmth of her breath to heat the infant. With two cubs, she has more difficulty obtaining a tight curl around them: there is just too much infant body mass, I think. I am able to get more regular glimpses of these youngsters than I did with previous cubs, as one of them usually has a portion of its little body sticking out between her paws or under her limbs.

Another difference I have noticed is that Marcella is less attentive to these cubs than with her previous two. With Bulan or Danum, little whimpers or soft sounds could elicit a comforting response from mom. With these twins, Marcella has made it pretty clear that unless a cub has something to really complain about, she isn’t going to bother to interrupt her rest to comfort the cubs. The youngsters have to protest fairly loudly before she will consider a quick nuzzle or an adjustment in her position. This doesn’t mean she is being a bad mother; rather, it suggests she is making decisions about how to expend her energy more judiciously. This can be an important strategy with twins, who are presumably twice as demanding as singletons.

Or perhaps this “selective mothering” is a result of the multiple personalities residing in the den with her. One of the cubs seems very calm and sedate; it has a softer voice and appears not to get riled up too often or in response to little things. When Marcella leaves to feed, this cub cries for a shorter time period and is apt to fall asleep before mom returns to the den. As easy as this little one seems to be, its sibling is a polar opposite: a fussy, demanding, and very loud little bear who makes a lot of demands on Marcella. This cub will screech continuously in her absence and push itself around in little doughnuts on the floor until mom returns to comfort it. This is the type of cub that may have Marcella deciding that it would be too much for her to respond to every little vocalization…and so, she doesn’t.

I expect that the cubs will remain denned up for the same length of time as Marcella’s previous litters. Danum didn’t spend more than 50 percent of his time out of the den until he was 105 days old. Perhaps twins will become mobile faster, because they will have a sibling to play with? Perhaps they will develop more slowly, since Marcella has to divide her resources between them? The future with the cubs promises to be as informative, and as fun, as the last three weeks have been.

Suzanne Hall is a senior research technician at the San Diego Zoo.