I visited with our sun bears this morning (see previous post, Sun Bears: Latest Developments). I am amazed at the size of Palu, our male youngster. He is only 15 months old, but he now weighs in at 88 pounds (40 kilograms), making him slightly heavier than his mother, Marcella. For her part, Marcella is still longer than her cubs, but Palu is a big boy with a broad head belying a solid bone structure. I wonder where he will top out with respect to weight. His father, Dibu, who now resides at the Columbus Zoo, approached 132 pounds (60 kilograms). I think Palu is well on his way to being as large as his daddy.

Palu, Pagi, and Marcella are Bornean sun bears, one of two recognized sun bear subspecies. The Bornean is supposed to be smaller than the other subspecies, the Malayan. However, our Borneans are on the larger side, approaching the weight of some of their Malayan counterparts. This has caused more than one wild sun bear researcher to do a double-take and ask if we are in fact sure that our bears are Bornean!

I have seen some weight data from young, wild-born Bornean bears. Fifteen-month-old bears have been recorded to be between 50 and 64 pounds (23 and 29 kilograms), more than 10 kilos lighter than our boy Palu. Our female cub, Pagi, is also a little bigger than these wild counterparts. What accounts for the size difference we are seeing?

The stresses and strains of living in the wild no doubt contribute to some of the lower weights recorded from wild Bornean bears. As emphasis, all of the weight values I have seen are from orphaned bears, which at various times in their young lives were living without any maternal support before being brought into a captive environment. Without mother’s milk and having experienced some period of famine, their growth was likely to have been impacted to some degree. Fortunately, this type of stress is unknown to Palu and Pagi.

Another factor in the difference in weights is likely to be environment. The Bornean sun bear inhabits a tropical evergreen home range in which food is typically abundant year round. Feeding on fruits and insects, the Bornean bears make adjustments for which trees are fruiting in their areas, but typically don’t have to worry about dramatic seasonal changes to food abundance, as a polar or brown bear might. Despite this fairly stable food source, wild bears simply have to work harder for their meals than do bears in zoos. Pagi and Palu are still nursing a few times each day, and they get additional food supplementation from the keepers. Marcella, too, benefits from regular food rations, allowing her to endure the energy drain of lactation with less stress to her body than a wild female, who must always be looking for her next meal.

What’s more, unlike their wild counterparts, our Borneans don’t have to worry about occasional variations in fruit abundance. In Borneo, the El Nino/Southern Oscillation may be responsible for large, simultaneous fruiting events in which food becomes very abundant. During this time the bears eat well. However, the mass fruitings are often followed by a period of low food abundance, when bears need to work harder to eat. Another factor impacting wild bears is destruction of habitat, which makes finding your next meal that much more difficult. Loss of habitat is a critical threat to the bears of Borneo, as the forests are logged and cleared for the development of palm oil plantations.

Finally, there may be a genetic factor at play here, too; Marcella and Dibu may simply be on the large size for Bornean sun bears. Certainly, I can’t imagine Marcella weighing 10 kilos less than she does currently; she would be skin and bones. She’s a very healthy girl around her current size.

Whatever the reason, our bears are happy and healthy, and the cubs are growing into robust and capable individuals. Which is good news, whatever their size!

Suzanne Hall is a senior research technician for the San Diego Zoo’s Institute for Conservation Research.

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