European Badger Behaviour - Scent-marking

Whatever the underlying cause(s) of group formation in badgers, scent plays a pivotal role in group and territory maintenance. Valuable tools in the act of scent-marking are the subcaudal gland (SCG), which is close to the anus, and the paired scent glands located just inside the anus – anecdotal observations suggest that scent glands between the toes (i.e. interdigital glands) may also be used when marking objects, such as trees, near the sett. Arguably the most important scent-marking tool is the subcaudal gland, which is used to mark objects in the territory as well as other members of the clan, in a process known as allomarking.

The SCG consists of a pouch, the subcaudal pouch (SCP), that’s divided into two sections by a membrane. The SCP is lined with several layers of sebaceous glands that secrete an oily lubricant onto the skin and hair, and apocrine gland cells. It opens to a horizontal slit, two to eight centimetres (about 1-3 in.) wide, situated between the base of the tail and the anus. The gland secretion is predominantly a composition of unsaturated fatty acids and water, with the consistency of a margarine-like paste. The secretion’s odour is partly generated by the bacteria in the pouch. Additionally, each fatty acid has its own characteristic smell. Chemical analysis of the SCP has shown that the bacterial component and fatty acid composition varies from badger to badger, suggesting each individual has its own unique scent.

A badger sniffing the air. Scent is highly important to badgers and a pivotal part of their social system. - Credit: Marc Baldwin

A series of recent experiments carried out by a team of scientists, fronted by Christina Buesching at the University of Oxford, has provided some interesting information on the composition and variability of SCG scent. In one set of tests, Buesching and her colleagues used gas chromatography to determine the composition of the SCG secretion. Gas Chromatography is a method of separating a chemical mixture into its constituent parts; effectively, it’s a ‘mechanical nose’. The study identified 110 different components, 21 (19%) of which were present in every sample. Of particular interest was the discovery that the SCG secretion shows distinct seasonal and clan-specific variation. Buesching's experiments found that clan member secretion chemistry was more similar to members of their own group than to those from outside the clan; the composition varied according to season, sex, age, body condition and reproductive status. These results suggest that not only does the SCG’s secretion convey information about clan membership, but it may also provide information about the health and fitness of the individual.

A second paper by the team reported that not only did sex have important implications for SCG properties (males had larger glands, containing more secretion than females), but so did reproductive condition and season. Although the paper reported no observable effect of pregnancy and lactation on secretion colour or volume, there was a significant negative correlation between the levels of progesterone in the blood and the volume of SCG secretion during the spring and summer (i.e. the more progesterone, the less SCG secretion). For males, those in breeding condition had substantially more secretion in their SCG than non-breeding males, although this finding couldn't be linked to the levels of testosterone in the blood. Reproducing boars also had a significantly whiter secretion than their non-breeding counterparts, although, overall, sows had consistently darker SCG secretions than males. Secretions from both sexes were darkest during spring.

Allomarking is frequently observed in badgers and according to a paper by Hans Kruuk, Martyn Gorman and Alan Leitch published in the journal Animal Behaviour in 1984, can be split into two categories: mutual or sequential. Mutual allomarking involves two badgers pressing their SCPs together simultaneously, while sequential allomarking is characterised by one badger marking the body of another. In a 2003 paper to the journal Behaviour, Christina Buesching, David Macdonald and Pavel Stopka reported their observations on the allomarking behaviour of badgers in Wytham Woods between November 1994 and April 1996.

A juvenile badger allo-marking an adult (right) in the clan. - Credit: Claire Dulanty

The zoologists found that sequential allomarking was significantly more common than mutual marking, occurring in 2,866 (95%) of 3,021 instances while mutual allomarking was only observed 155 times (5%). Both forms of allomarking were considerably more common during the mating and cub-rearing seasons (winter and spring, respectively) than at other times in the year. Females showed no preference for the marking of one sex, while boars preferred to mark sows, although neither sex marked a specific individual preferentially. Boars marked more prolifically than females with yearlings and juveniles seen to mark most frequently (marking adults of the same sex). Non-breeding sows marked breeding sows more commonly than vice versa.

The observation that males marked more than females corresponds nicely to the previous work showing that males hold substantially more secretion in their SCPs than females and that both sexes produce more secretion between January and May, corresponding to the peak allomarking period. Another interesting finding of this study was that, while mutual marking varied according to sex and season, sequential marking was dependent upon factors such as age and reproductive status; what we call fitness-related parameters. This implies that the two forms of marking may have, at least partially, different functions. Buesching and her colleagues suggest that mutual marking may serve to maintain a common 'clan scent', while sequential marking may be more involved in advertising individual-specific information, using your clan members as billboards that advertise your status, with perhaps the added function of distributing the clan scent.

Overall, these specific group odours are thought important for intra- and interclan (i.e. within and between groups) communication. Indeed, badgers are well known to be able to distinguish between the subcaudal secretions of clan members, neighbours and unfamiliars. I’m not aware of any studies in badgers, but work on peccaries (wild pigs of the Tayassuidae family) and mice have shown that group odour helps to reduce aggression.

Approximate location of the anus and subcaudal gland (SCG) of the badger. Modified from Neal and Cheeseman (1996). - Credit: Marc Baldwin

The frequency with which a badger deposits its scent within the territory varies according to a host of factors, including reproductive status, season, sex and location on the territory. A paper to the journal Acta Theriologica in 2004 by Christina Buesching and David Macdonald reports on how scent-marking by badgers in Wytham Woods changes according to these factors. The act of SCG scent-marking takes about a second and involves the badger squatting, bending the knees and raising the tail, and pressing the semi-circular shaped opening of the SCG against the substrate, frequently a prominent object such as a tuft of grass or rock.

Between April 1996 and June 1997, 442 incidences of object-marking were observed, the frequency of which varied with the season as well as the sex, age and reproductive status of the badger; breeding badgers scent-marked more frequently than non-breeding ones; adults scent-marked more frequently than juveniles; and, during the cub-rearing season, females scent-marked more frequently than males. Females were seen to over-mark (i.e. re-apply the scent to the exact same area) more frequently and more consistently than males, who only over-marked during the mating season. Overall, roughly one-third of scent marks were over-marked within 24 hours. Buesching and Macdonald concluded that, based on these data, not only do scent-marks play a role in signposting territory boundaries, they also serve as advertisement signals directed at other group members.

Complementary to the scent laid down by the SCG are scents from urination and faeces. In their 2004 paper, Buesching and Macdonald observed badgers urinating on top of their sett, either in specially dug pits, on the grooming area or, most frequently, on the spoil heap. Urination typically involves squatting, although boars have been observed to perform raised-leg urination (i.e. cocking of the leg). Various studies have demonstrated that urine scent marks may be used to over-mark the scent left by foxes, while scenting in general is carried out along the main foraging path before the badgers begin to forage more widely. The badgers then find their way back to this path, which shows them the route back to their sett.

Underside of a captive badger, illustrating the location and cursory appearance of the subcaudal gland. - Credit: Anonymous at Photographer's Request

It has been suggested that countless generations marking the same feeding route may explain why, even when a field is ploughed, the badgers can re-establish the exact same path and how badgers collecting bedding unerringly find their way back to their sett without looking where they are going (bedding is brought back to and carried down into the sett backwards). It may also help explain the results of a fascinating study conducted by a team of biologists in the Croix-aux-Bois forest of northeastern France in 2003. This particular study found that if you take a badger away from its home sett and release it somewhere else on its territory it homes (i.e. finds its way back to the main sett) quickly. If you release it in the home range of one of its neighbours', it will also find its way back after wandering randomly for a while, presumably while it gets its bearings. If you release this badger outside the home range of its neighbours, however, regardless of the distance, it fails to find its way back home. These results are interesting because they suggest that badgers may recognise neighbouring (contiguous) territories as being close to their own.

Faeces is often considered to be a more robust marker than urine. Although the faeces itself may not smell particularly pungent, the smell of the anal sac secretion emptied onto it either during or immediately after defecation can be rather overpowering. Badgers generally deposit faeces into specially dug, strategically placed latrines. Latrines are located close to the sett and along well-used foraging paths.

Observations on captive badgers suggest that scenting behaviour starts when they’re about nine-weeks old, although more recent biochemical observations suggest that cubs don't begin producing SCG secretion until they are about four months old. As Ernest Neal and Chris Cheeseman point out in their book Badgers, the captive observations correspond to roughly the age at which wild cubs begin to leave the sett. They also comment on how living in an area permeated by its own smell seems to bring “assurance and relaxation” to the individual concerned.