Histological, chemical and behavioural evidence of pedal communication in brown bears

1. What they did

Bears live alone, roam large home ranges and do not defend territories, so they need good ways to leave messages for other bears. Most research has looked at tree rubbing. A study of polar bears found sweat glands in their paws, which suggested bears could leave scent simply by walking. The authors looked for the same thing in brown bears from three angles:

• Skin glands. They took skin samples from the paws (between the toes, the soles behind the pads, and the footpads) of two dead male bears, a captive adult and a wild yearling, and compared them under the microscope with skin from the lip and shoulder.
• Scent chemistry. They rubbed cotton swabs on the paws of six wild bears captured in the northern Carpathians of southern Poland (two adult males, one yearling male and three adult females; 24 swabs in all) and analysed the chemicals with gas chromatography and mass spectrometry.
• Behaviour. In the Cantabrian Mountains of northwestern Spain, a video camera trap hung above a rubbing tree at a spot where bears were known to mark with their feet. It ran from January 2012 to February 2015. They counted “pedal marking” when a bear placed their feet exactly in the existing ground marks (visible as depressions) while twisting each foot vigorously, and “sniffing of pedal marks” when a bear lowered their head and sniffed the tracks.

2. What they found

Glands in the paws

• No glands could be seen with the naked eye.
• Under the microscope, the skin between the toes and on the soles of the adult male had prominent apocrine sweat glands and sebaceous (oil) glands next to hair follicles, more prominent than in the lip and shoulder skin.
• The yearling had the same glands, but they were less developed.
• Both bears had a small number of eccrine sweat glands deep in the footpads.

Scent chemistry

• The swabs held 26 compounds. Twenty were found in every sample. Six were found in all samples from the adult males and in none from the adult females or the yearling male.
• Several were skin-borne carboxylic acids, which are common in mammal body odours. Cyclohexanone, suggested to be a typical scent-marking compound of large mammals, was in all bears.
• The male-only compounds included octanoic (caprylic) acid, thunbergene (a cembrenoid, which can come from animals or plants), two compounds of probable plant origin and two that could not be identified.
• The yearling male’s scent matched the adult females’ profile.
• No urine compounds (such as urea) were found.

Behaviour at the marking site

• Bears were active at the site on 18% of 752 camera trap days and appeared on 232 videos, in every month except January. At least 15 bears could be told apart by the shapes and colours of hair marks on their head and neck.
• Pedal marking appeared in 81 videos (35%). Adult males did 96% of it; sub-adults of unknown sex the other 4%.
• Bears marked with a special gait, stepping carefully and twisting their feet into the depressions left by earlier marking, so that the site carried a worn trail of footprints.
• Pedal marking was seen from March to November and peaked in the mating season (April to June, 54% of marking videos).
• Bears sniffed the pedal marks in 38 videos (16%), from March to December. Adult males did most of the sniffing (58%), but other bears, adult females included, sniffed them too.

3. What they concluded

• Brown bears have the glands needed to leave scent through their feet, and the scent of adults differs between the sexes, which suggests it could tell other bears whether the marker was male or female.
• The field data support this: males pedal-marked more often, especially in the mating season, at a site used for several years by a number of bears.
• The mix of oily sebaceous secretions and sweat could help the scent last in the environment.
• It is hard to say whether pedal scent comes only from the glands or also from urine, but no urine compounds were found.
• Because the marks are deep, worn depressions used for years, pedal marking may also send visual signals, not only scent.
• The yearling’s weaker glands and female-like scent suggest the glands become active with puberty.
• Brown bears are another solitary carnivore that communicates through their feet, and this may be an important way of passing on information.

4. How tracking was or was not used

The study did not use trailing or a tracks and signs survey. The pedal marks themselves are tracks: depressions worn into the ground by bears stepping into the same spots over years. The researchers recognized the marking site from these marks and other field signs seen during field work in the Cantabrian Mountains, and then watched it with a camera. On the videos they recorded when bears stepped into the old marks and when bears sniffed the tracks.

5. Tracker or observer certification

Not reported. The paper does not say who found the marking site or how the field signs were identified.

6. Other technologies, skills or innovations

• A video camera trap (Bushnell Trophy Camera) mounted above a rubbing tree, looking straight down, recording one-minute clips.
• Individual bears recognized from hair marks on the head and neck; age and sex judged from genitals, body shape and size, and accompanying bears.
• Skin biopsies stained with hematoxylin and eosin and examined at 100 times magnification.
• A pilot test of swab types, solvents and extraction times to find the best way to collect paw scent (wooden-core cotton swabs, hexane, two hours).

7. Implications for conservation (from the paper)

The paper does not discuss conservation directly. The authors say the findings add to what is known about how bears communicate and may encourage research on foot scent in other wide-ranging species.