Black bears are on the move during their mating season in May and June. If you encounter a black bear, with or without cubs, be reassured that it is highly unlikely that the adult will act aggressively toward you. (Photo by Mary Holland)

During May and June most black bears have an extensive courtship and mating season following their emergence from hibernation in April. While males are ready for action every year, most females are not. Cubs remain with their mother for roughly 17 months, during which time the mother does not mate. Thus, most mate and give birth every two years.

For the first few weeks after they emerge from their dens, bears have great difficulty finding enough food, so much so that they must continue living off the fat that they accumulated last fall. As green plants begin to mature, the adults seek lush grasses and herbaceous plants, as well as other greenery such as poplar leaves, to eat.

By June, most vegetation has matured and toughened, so black bears now get sustenance from insects such as ant pupae (or bee larvae, as evidenced by the recent raids on beehives in Harvard). Mothers tear apart rotting logs, exposing the pupae, which they and their cubs ingest with relish. These familial days are coming to an end, however, for it is now time for sows to encourage their cubs to go off on their own. The females' biennial mating period is approaching, and the presence of cubs might limit their opportunities. Anywhere from one to six days before they become sexually receptive, mothers drive their year-and-a-half-old cubs off, forcing them to stop traveling with them.

Once they have separated from their cubs, the adult females travel extensively throughout their one- to two-mile diameter territories and beyond. Males also increase their movements during mating season, covering much of their 10- to 15-mile territories, which overlap the territories of several females. As the females travel, they lay down a scent trail, which advertises their availability, and males are quick to follow. Because females that are accompanied by this year's cubs are not available for mating, males usually avoid their territories.

Female black bears continue to forage and are usually able to maintain their weight during the mating season. Males, on the other hand, eat very little at this time of year, and often lose up to 20 percent of their fall weight. Given that they also lose 20 percent of their fall weight during hibernation, they can be quite thin at the end of the two-month mating season. A male black bear weighing 300 pounds in the fall would weigh roughly 180 pounds at the end of July, but will gain back the lost weight by the time it enters hibernation.

Large males chase smaller, younger males away, but are known to fight for dominance and mating rights with other competing mature males. It's not unusual for a male to follow a female and guard her against rivals for more than a week before the female becomes receptive and mating occurs. Copulation usually occurs numerous times, over a period of several days. When done, the male moves on to find another mate; the female is receptive to additional mates as well.

Even though mating takes place in May or June, black bears don't give birth until January or February. This isn't because bears have a gestation of seven or eight months. Rather, the lengthy time between copulation and giving birth is due to a process called delayed implantation. Once the eggs are fertilized, they develop into a small ball of cells called blastocysts. The blastocysts suspend further development until November, when they become implanted in the uterine wall, unless the mother is very undernourished, in which case they are often reabsorbed into her body. After implantation, the blastocysts develop rapidly into the cubs that will be born within two or three months—each measuring about 7½ inches in length, and weighing between half a pound and a pound.

Mary Holland blogs about natural history at www.naturallycuriouswithmaryholland.wordpress.com.