What is a bear’s “back rub”? Ecology captured on film

Do you know the method of surveying the population of brown bears? In mid-May, we accompanied Professor Sato Yoshikazu of Rakuno Gakuen University on a survey of the population of brown bears in Sapporo City, which his research laboratory (wildlife ecology) has been conducting for many years, and found that the bears’ habits are skillfully exploited.
Professor Sato analyzed that there are many relatively young male bears, and female bears with cubs near urban areas. At the site of the survey, barbed wire had been wrapped round a 1.5-m-high, 15-cm-diameter stake next to a tree, to create a ‘hair trap’ that collects the fur of bears. But do the bears really approach implements just wrapped with barbed wire?

“Bears communicate with each other in the forest using scents, and they have a habit of rubbing their backs against trees to mark them with their scent,” explains Professor Sato. It’s called “back rubbing” and is done not only by adult bears but also by cubs. It is believed to have various meanings, including searching for mating partners during the breeding season.

A bear and its cub walking during daylight hours, taken from video footage (courtesy of Prof. Sato)
A hair trap wrapped with barbed wire

■For “back rubbing”

The main targets for marking are trees that have the scent of other bears on them. They also like to back rub trees that have unusual odors, such as oil from chainsaws or color-sprayed markings for tree felling.
These habits are taken advantage of by the hair traps, which are said to be pre-soaked with an odorous preserving liquid. Barbed wire is used as the fur easily becomes entangled in it. The bears’ skin must be strong as they rub their backs with apparent pleasure rather than pain. The barbed wire is not only used on the hair traps, but is also placed in the vicinity at a height of about 50 cm above the ground, to catch the fur when a bear steps over it or a cub goes under it.

Research laboratory member Okubo uses tweezers to remove fur caught in the hair trap

■DNA analysis of the hairs
Professor Sato’s laboratory identifies individual bears by DNA analysis of the hairs collected, and estimates the number of bears living in the area. Since blood relations can also be determined, they are working on research to clarify a family tree of brown bears living in the city. In addition, an automatic camera that reacts to movement has been installed on a tree near the hair trap, to capture video footage of the bears. From these videos, the size, sex and physical characteristics of the bears can be determined, as well as the number of cubs they have with them. The laboratory is also studying the reproductive and nutritional status of the bears in combination with individual identification using DNA data.