In Groton, a black bear and her cub search for food in September 2016. (Photo: Tom Murray)

As far as I know, black bears (Ursus americanus) have not been spotted in Cambridge (yet). But they have been spotted in nearby Arlington and Waltham. Black bear populations have been increasing in Massachusetts since the 1970s.

Before 1952, anyone in the state could hunt and kill a black bear, making them scarce in Massachusetts in the 1800s and early 1900s. In 1952, however, new state laws made black-bear killing illegal except during regulated hunting seasons. There are three hunting seasons for black bear today, during which around 250 are killed. In the 1970s, there were fewer than 100 black bears in Massachusetts; in the latest bear census in 2011, there were estimated to be 5,000, and the number grows by about 8 percent yearly. So far, however, experts have seen no breeding females venturing inside Interstate 495.

In November or December, black bears use their long claws to dig out a den in a tree cavity, cave or crevice, under a fallen tree, or in a brush pile. Sometimes a bear will just curl up in a depression on the ground for the winter. During hibernation, a bear’s body temperature drops to 88 degrees from about 100 degrees. This is not a large drop compared with other hibernating species, whose body temperatures may decrease to 40 degrees in the winter. Because of their relatively warm body temperature, black bears are easily aroused in the winter and can respond to danger quickly. A hibernating bear’s heartbeat drops to about 8 beats per minute from 45, though.

For three to six months, hibernating black bears do not eat, drink, pee or poop. Their thick fur and inches of body fat keeps them warm, but they lose about 30 percent of their body weight. When food becomes available again, black bears emerge from hibernation (often in April). Males appear first; females with cubs appear last.

A New England black bear is completely black with a brown muzzle. (Photo: Tom Murray)

Bears are omnivores who will eat just about anything, but plants make up about 85 percent of their diet. When bears first emerge from hibernation, they look for carrion of animals that have died over the winter and young animals such as fawns. Bears have a phenomenal sense of smell, and can sniff out a hiding fawn while foraging. When shoots and buds appear, bears feast on them to rebuild muscle and strengthen their bones. They can often be found in wetlands this time of year. Bears also eat eggs and nestlings. They are good climbers who can reach many tree nests, even the nests of bald eagles.

During summer, black bears eat mostly fruits and berries, grubs, bees and other insects. During the fall, they eat all day long to build up fat reserves. They eat berries and hundreds of acorns or other nuts per day and, if available, food associated with humans – from barbecued ribs to trail mix, sunflower seeds to suet, pet food to corn flakes. Ripening fields of corn and orchards can attract hungry bears. Black bears will even steal game from hunters. Bears have superb memories and can remember the locations of different seasonal food sources for years.

Black bears in the Pacific Northwest fish for salmon at night because during the day, salmon can easily spot their black fur. It may be for this reason that western bears have more color variation than our New England black bears. In the Northeast, black bears are almost always black – it’s why early English settlers called them black bears. But in Western states, more than half of “black bears” are brown, cinnamon or even cream colored. White-furred black bears on islands of western Canada have a 30 percent greater success rate at catching salmon than black-furred bears.

In the spring, black bears, like one in Groton on March 31, 2022, can often be found in wetlands. (Photo: Tom Murray)

Black bears reach maturity at about 3 years of age. In June or July, males leave their mother at 18 months and roam looking for their own territory. Males 3 years and older look for females to mate with, mating with as many as they can find. After a female’s eggs become fertilized, some cell division occurs, but then stops. It is not until December that these cells attach to the uterus wall and grow continuously for eight weeks. No matter when they mated, all female black bears give birth in their winter den to two or three cubs in January or February.

The hairless newborn cubs each weigh less than a pound. They grow fast on a diet of high-fat bear milk – it’s about 20 percent fat, while human and cow milk are only 4 percent fat. Bear cubs open their eyes after a month and start to walk at about five weeks. By April, the cubs weigh 5 to 9 pounds. At this point, they begin eating solid food, although they still nurse throughout the summer. Most yearlings weigh about 100 pounds.

Cubs stay with their mother for two winters. When cubs are about a year and a half old, the mother bear is ready to mate again and shoos the males away to find their own territory. The females may be allowed to remain on the mother’s territory. It is these young males that most often make their way into residential neighborhoods in June and July.

A black bear checking out bird feeders in Groton. (Photo: Tom Murray)

In the spring, black bears, like one in Groton on March 31, 2022, can often be found in wetlands. (Photo: Tom Murray)Female black bears reach about 200 pounds when fully grown, but males continue to grow until they are 10 or 12 years old and can be as heavy as 600 pounds (although 300 is more common). The largest black bear ever taken in Massachusetts is estimated to have weighed about 700 pounds. Because of their size, older adult males can dominate younger males without fighting.

Black bears are entirely capable of killing humans, but they typically avoid confrontations. A black bear has never killed a person in Massachusetts, but there is about one death per year in the United States attributed to a black bear. Normal trail noise in the woods alerts bears to human hikers, and they will disappear before you can spot them. Female black bears are not as protective of their cubs as grizzly bears. If you should spot a mother with cubs, back away and give it space. If you spot a hungry lone bear, yell at it. Raise your arms. Throw rocks. Back away. A bear’s first response to something unusual is to leave, so it will most often go away (but of course every bear, like every person, has a different personality, so there are no guarantees). An agitated bear will snort, paw at the ground and make mock charges to let you know it wants its space. Back away. You should never turn your back on a bear and run or climb a tree. Despite its size, a black bear is better runner and climber than you are.

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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.