Our littlest sun bears, a male and female cub, are now more than four months old and are growing like weeds. (Read Suzanne’s previous blog, Sun Bear Cubs: Snips, Snails, Sugar, and Spice.) They have made incredible strides in motor development. Now they can climb the bars of their bedroom abode, hang from the ceiling with two paws while gnawing on things sticking out of their climbing structures, like the supports for their hammock. They drink from mother Marcella’s water source, eat her kibble and veggies (though slowly), and sleep the days away tucked together in that hammock while mom goes about her business below.

It has been many weeks since I have seen much activity in the den, beyond a simple chase escapade in which one cub evades the other by ducking into the shelter. No nursing, snuggling with mom, or grooming of the cubs seems to take place there any more. The cubs are now officially into their next phase of life.

The hammock has been a favorite resting spot for weeks, and they also like to sleep on burlap sacks on the ground near Marcella. Nursing takes place whenever, wherever, and the twins seem to help each other out to ensure no one misses a meal. Whenever a cub nurses or is about to nurse, it often emits a humming vocalization that sounds like a car that just won’t start. This vocalization is very loud and can be heard by anyone in the area, including the cub that isn’t nursing. Once that cub hears its sibling engaged in getting some milk, it usually comes running, even if it means awakening from a good nap in the hammock. Is this vocalization akin to vocal altruism? Or is the sibling benefiting from something designed for another purpose? I’d really like to know.

Listen to a cub humming [audio:http://www.sandiegozoo.org/animalbytes/sounds/sunbear_cub_hum.mp3]

We have been studying the vocal repertoire of these little sun bears for months. Research staff have been recording their humming and other sounds to see how the vocals change over time and determine if there is a correlation between frequency of humming and body size of the cub, among other things. Inquiring minds want to know if this loud vocalization contributes to the squeaky wheel getting the grease: does mom pay more attention to the louder cub?

Marcella is showing us she is ready to return to the outside habitat. She is growing more impatient with being behind the scenes with each passing day. Once we are sure the cubs are hearty enough to handle the larger habitat, with its tall climbing structures and steep slopes, we will release the three bears to the public viewing area. This should happen within a matter of weeks, so if you are a Zoo regular be sure to stop by Sun Bear Forest often to get a glimpse of the littlest members of our San Diego Zoo bear family!

Suzanne Hall is a senior researcher technician for San Diego Zoo Conservation Research.

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