Brown bears possess anal sacs and secretions may code for sex

1. What they did

Many carnivores leave scent marks with secretions from glands near the anus, and these secretions can carry information such as sex, age and individual identity. For bears other than the giant panda, almost nothing was known, and earlier anatomists disagreed about whether brown bears even have anal sacs.

As part of a long-term project in south-central Sweden, the researchers darted free-ranging brown bears from a helicopter before the mating season in 2007 and 2008. While each bear was immobilized, they found the anal sacs by sight and by feeling for them, and squeezed out secretion from each sac, wearing latex gloves so their own scent did not contaminate the samples.

They then:

• Compared colour. They matched each secretion against a standard colour chart and also ranked it by eye from 1 (light) to 12 (very dark).
• Analysed the chemistry. They separated and tentatively identified the compounds in each sample with gas chromatography–mass spectrometry (GC–MS), a laboratory method that splits a mixture into its parts.
• Tested for sex differences. They asked whether males and females differed in the overall chemical profile, in the number of compounds, in which compounds were present, and in how much of each compound there was.

Samples came from 17 sexually mature bears (5 males and 12 females, aged 3 to 18 years). Some chemical data were lost, so the compound-by-compound comparisons used 4 males and 7 females.

2. What they found

Anatomy

• Every captured bear had a pair of anal sacs. Their layout is similar to the domestic dog’s, with a duct from each sac opening on either side of the anal canal.
• The secretion was clay-like with an unpleasant smell. The researchers could not smell any difference between males and females.

Colour

• Colour ranged from nearly black to light grey.
• Male secretion was on average darker than female secretion, but the two overlapped.

Chemistry

• They found 90 different compounds, mostly fatty acids, hydrocarbons and steroids. Cholesterol was by far the largest part, about 77% to 81% of the measured compounds on average.
• Of the compounds with a known molecular weight, 68% were heavy (above 300), the kind that evaporates slowly.
• Males had 74 different compounds in total and females 59, but the number per bear did not differ significantly between the sexes. They found no sex-specific compound (one present in every bear of one sex).
• Overall chemical profiles tended to separate males from females.
• The amounts of five shared compounds, all steroids, differed: females had more of four of them, and males had somewhat more of one (a marginal result).
• Grouping bears by which compounds they had did not separate males from females.

Timing

• The amount of secretion that could be squeezed out seemed to decrease as the mating season went on (a personal communication from a project member, not measured in the study).

3. What they concluded

• This is the first study to confirm conclusively that brown bears have anal sacs, and the first to analyse their secretion chemically.
• The secretion likely carries information about sex, through the mix and relative amounts of several compounds rather than any single male or female compound. It probably carries other information that matters to the bears’ social life too.
• Colour overlaps between the sexes, so colour alone should not be used to tell a bear’s sex.
• The many heavy, slow-evaporating compounds suit long-lasting scent marks, useful for a solitary animal with a very large home range.
• Bears may use the secretion more to advertise their presence in the mating season, when they are more likely to meet, though lower production after the mating season is also possible.
• The authors think it likely, but unknown, that brown bears can release the secretion separately from their droppings, as wolves can.
• They mention that bears have been seen to sit down in front of trees after rubbing them, and often sniff the ground before rubbing (a personal communication), which suggests the secretion may be used in marking.
• Future studies should look at differences between age groups.

6. Other technologies, skills or innovations

• Helicopter darting and immobilization of free-ranging bears; age from tooth cementum layers.
• Gas chromatography–mass spectrometry with a library of about 70,000 known compounds; unidentified compounds were added to a new library so they could be recognized across samples.
• Partial least squares regression, a statistical method suited to many closely related measurements from few animals.