Why are bear cubs so small? -- request for volunteers - UPDATE January 24, 2010
January 24, 2010 - 7:19 PM CST
Peace is occupying more of the bears’ time as the cub finds the nipples in its new environment. With no competition over nipples, we expect fewer screaming outbursts than we’ve heard in larger litters. Eventually, all litters work out a nipple order, although the mothers get anxious during the squabbles.
Bathroom time also brings cries. Mothers must lick young cubs to stimulate elimination, which she ingests. Some of the movements Lily makes while curled up with her head under her chest near the cub are probably taking care of bathroom duty. Undeveloped young digestive tracts do not absorb all the nutrients, so the mother recycles them.
Bear cubs are smaller, compared to their mothers, than any other placental mammal. One would expect cubs to weigh 5-10 pounds at birth, but they don’t weigh that until they leave the den. Some people think of dens as external wombs.
Why are cubs born so small? Part of the answer has to do with hibernation. The mother’s lowered metabolism means lower blood oxygen levels for the fetuses. Early birth gets them out breathing on their own as soon as possible. It also lets mothers nourish cubs using fatty acids rather than costly glycogen. Bears can’t use fatty acids in utero because the molecules are too big to transfer across the placenta. After the cubs are born, mothers can use fatty acids to make milk, preserving her glycogen. The need for fatty acids to make milk when there is no food intake is another reason only fat bears can produce cubs.
We are hearing a lot of nursing, so Lily and the cub are doing well.
Even though Lily is in a state of reduced metabolism—hibernation—her body temperature is likely in the high 90’s F as we have found in other mothers. Again, only fat bears can maintain that high temperature.
The cub is not hibernating. His or her job is to nurse, sleep, and grow as quickly as possible to follow Lily and climb trees to escape danger in spring.
One of these days someone is going to get a good enough look at the cub to know whether to call it a male or female.
A problem for research is that we can’t watch the screen enough to record everything we should. Keeping up with web crises, thousands of emails a day, and the usual work has us short on sleep. We’re wondering if viewers can help record activities, with each volunteer covering a couple hours around the clock. Hopefully, people are watching in time zones overseas to help cover the wee hours when Americans are sleeping. Observers would note which time zones they are in.
What would we record? First, whether Lily and the cub are active or not. That would mean two lists running down a page—one for Lily and one for the cub—to list the starting time of each period of activity and inactivity. For example, the motor-like hum of the cub would signal the onset of nursing. When the hum stops would signal the beginning of the next activity or inactivity. Notes can cover activities that are not easily categorized. Record the times of cub vocalizations to show when the cub was awake. Also, record the intensity and types of cub vocalizations: (1) pleasant murmurs when the cub is awake but not in distress; (2) the motor-like hum of nursing, (3) low intensity cries, (4) medium intensity cries, and (5) full screams.
Notes can describe Lily’s responses to cries. She may or may not move a little in response to low intensity cries but may move a lot to fix whatever causes high intensity screams. She may vocalize herself, giving sweet grunts that show her concern. You heard a lot of those grunts from Lily immediately after the cub was born.
Lily’s position and visibility will limit some observations—especially observations of the cub. It will be underneath her a lot for a month or so.
If an observer gets bored while the bears sleep, it helps to pass the time counting Lily’s breaths per minute. We can later correlate those with weather records. As an example, although most adults breathe a relaxed 3 or 4 times a minute, a different female sleeping in a den in minus 26 F weather breathed deeply 5/minute as if she required more oxygen (and fat consumption) to maintain body temperature in intense cold.
What would all this give? The elapsed times for each activity would make a daily time budget, which would change as the cub develops. The time budget for this family with one cub can be compared with future time budgets for bigger families. The whole thing would allow comparisons of hibernation activities of a well fed mother and cub with those of other bears. Other bears might be mothers with cubs (it could be Lily and the same cub next winter) or might include less well fed bears that have to drop into deeper hibernation. It could include one of the dominant males of the area. Technology is advancing and more should be possible in coming years. Our observations would provide the basis for comparison in this same area or in a different part of North America or the world. We know this kind of recording can be tedious. We routinely have walked with bears for 24 hours at a time, fasting the whole time to avoid distracting bears with food and corrupting the data. Standardized recording like we can do with Lily can produce a clinically detailed record of the behavior of an undisturbed mother in a den. It has never been done before.
Ideally we would like participants to commit to a 2-hour block of time daily. If you are interested in participating in this project, please email us at
Again, a big thank you to the donors who continue to help reduce our debt and enable us to do more for bears than otherwise would be possible.
—Lynn Rogers and Sue Mansfield, Biologists, North American Bear Center