Natural Connections: Bear Babies

One of the student researchers from UW Stevens Point looks and listens for signs of cubs in an active bear den.

Natural Connections: Bear Babies

With bright sunshine to provide at least the appearance of warmth, single digit temperatures didn’t phase us much. Hand and foot warmers took the edge off, too, as did the brisk pace set by college students from University of Wisconsin Stevens Point’s (UWSP) Wisconsin Black Bear Project.

The students weren’t following a designated trail, instead they were following the faint beeps of a radio collar worn by a female black bear in her den.

For over 30 years, students and professors from UWSP have conducted research on the seasonal movements, habitat selection, and reproduction of Wisconsin’s black bear population. During that period, bear numbers have risen from 9,000 to 24,000. The researchers’ hard work has contributed significantly to the body of knowledge about black bears in Wisconsin, and informs bear management, too.

On this late January morning, a small group of Wisconsin Master Naturalist volunteers were lucky enough to join a reconnaissance mission. The sow bear we headed toward had been caught in a barrel trap last fall and fitted with a collar. Although her age was unknown, she weighed more than 150 pounds at the time of capture, (she topped out the scale) which means she was big enough to have cubs. Students monitored her movements using radio telemetry equipment, and noted when and where she entered her den. Now, the question was, did she have cubs or yearlings?

Black bears have a unique reproductive cycle. Mating occurs in late May and June, and eggs are fertilized then. But after each one develops into a tiny ball of cells called a blastocyst, they hang out on pause for five months. If the sow puts on enough weight during her fall period of hyperphagia, then the blastocysts implant into the uterine wall in November and develop rapidly. If the sow has a lean fall, then her body won’t put her through the rigors of motherhood.

Cubs are born in January, weighing only a pound. According to the North American Bear Center in Ely, MN, “newborn cubs are smaller, relative to their mother’s size, than the young of any other placental mammal.” While it’s rumored that mama bears stay asleep while they give birth and discover a cute surprise in the spring, this is just folklore. Mother bears wake up to give birth and care for their young, although they do their best to conserve precious energy and fat stores.

At a respectful distance from the den, we paused to let the college students go ahead. Assistant Professor and Bear Project leader Cady Sartini answered our questions, and prepared us for what we’d see.

Like many mamas, this bear had excavated a den in sandy soil under roots and brush in a spruce bog. Some bears have been known to just lie down and let the snow cover them, but this expectant mother was a little more picky. Only females are radio collared and ear tagged, Cady told us, since the project is mostly interested in bear reproduction. Males range more widely, and don’t stay with their offspring. Cubs spend their second winter with their mother, too, and, ideally, female cubs who survive to be yearlings get a collar before their mom kicks them out.

A student returned to our group with exciting news—they could see the mama bear’s nose, and hear noises from at least two cubs! Most likely, the young cubs were hidden behind mama, away from the drafty entrance. From previous experience, researchers know that it’s important to observe a den for 20 minutes or so to make sure that they’ve given the cubs a chance to be heard.

Confirming the presence of cubs or yearlings essential because the research team plans to return in March. They will anesthetize the adult. The cubs will be counted, sexed, weighed, and aged by measuring the length of the hair between their ears. If there are yearlings in a den, the process becomes more complicated. They would be anesthetized along with the adult, and any female yearlings would be fitted with collars. Knowing what the researchers will encounter, and being able to plan for it, is key to making the process go smoothly.

Carefully and quietly, a few at a time, Master Naturalists were allowed to approach the den. Cady had explained that the bear would know we were there, but she would be trying her best to stay asleep. Disturbing hibernating bears is something that should be done as little as possible, and with valid research goals in mind. Just by nursing young, a female will lose around half a pound a day, and waking up wastes valuable energy.

Under the snow, the den hole seemed sandy, dark, and damp. The mother bear lay with her face buried and a paw over her ear, just as you would if you were trying to sleep in. From behind her came the pulsating grunts of warm, comfortable, nursing cubs. This sound tells mom that everything’s fine and she can stay right where she is. (You can listen to these sounds in the Museum’s current exhibit, Growing Up Wild.)

Later, one of the Master Naturalists reflected that “it is one thing to read that a sow bear gives birth to cubs while she hibernates and quite another to peer into the small opening of a bear’s den mid-winter, see her snout and a paw, and hear her mewling cubs from the darkness of the confines beyond. This was a gift I believe we all received with great gratitude.”

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is now available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. The Museum is now open with our exciting Growing Up WILD exhibit. Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.

Last Update: Feb 08, 2023 11:46 am CST

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