A badger’s homestead is referred to as a sett. Reflecting the dwelling of what is, in parts of its range, a highly social mammal, badger setts are large and spacious enough to accommodate as many as 35 animals (in the largest naturally supported clan ever recorded in Woodchester Park during 1989), although between six and eight individuals is more common. Setts are generally constructed on sloping ground in woodland or on the periphery of farmland, although they have been found in scrub, natural caves, tips, under buildings, in embankments, quarries, hedgerows and sea cliffs Andrew Byrne and colleagues, in their 2014 paper to Ecological Indicators, reported a quadratic relationship between slope and elevation in terms of sett presence. In other words, badgers were most likely to dig setts on gentle slopes (angles less than 15-degrees) and at moderate altitudes (70-170m above sea level).
Byrne and his coworkers also noted that setts were strongly associated with hedgerows, woodland cover and pasture and least likely to be built facing east. In the UK more generally, badgers seem to show a preference for deciduous woodland and copses, with just over half of setts found in this habitat; fewer setts (10-15%) are found in hedgerows and scrub and even fewer (<10%) in open fields.
A large sett may consist of up to 100 m (330 ft.) of tunnels, with as many as 40 entrances/exits, each about 30 cm (12 inches) in diameter. Here in the UK, a study found an established sett in the Cotswolds with twelve entrances and tunnels totalling more than 300 metres (1000 ft); the study estimated that the badgers had moved 25 tonnes (55,000 lbs) of soil to create the network. In his book Badgers, Mammal Society chairman Michael Clark described the internal architecture of a sett. Clark noted that the sleeping quarters are more like extensions off the tunnels than 'rooms' per se and these chambers contain bedding in the form of dried grass and leaves, which provides vital insulation of the chamber during the winter. In one particular experiment, Clark discovered that bedding may stay underground for as long as 14 months, before it is replaced.
Clark also wrote that dung is often found in discrete corners of the sett complex, although not to the extent seen in the dens of – or parts of the sett used by – foxes. The entrances may lead back into random blind tunnels and a maze of interlinking routes between bedding chambers is generally found. Around the edge of the sett's main entrance is a mound of highly compacted earth and, in many instances, discarded bedding material.
Classes of sett
The sett system often consists of a primary and several 'secondary' setts spread around the territory. During her Ph.D. thesis at the University of Exeter, Penny Thornton classified these setts into four groups, based on their frequency of use and topography. Thornton's scheme classifies setts as follows:
|Sett Type||Distinguishing Characteristics|
|Main sett||Many entrances (both used and disused), large spoil heap and well-worn paths. One main sett per clan.|
|Annexe setts||May also have well-used entrances with numerous paths to the main sett, which is typically 50 to 150m (164 - 492 ft.) away.|
|Subsidiary setts||Variable number of entrances. Only some entrances connect to the main sett by obvious paths.|
|Outlier Setts||One or two entrance holes and no well-defined pathways to the main sett. Used sporadically.|
In the main sett, there are usually chambers on several levels, with the main nesting chamber often between five and ten metres (16.5–33 ft.) from the tunnel entrance and about three metres (10 ft.) below the surface. The chambers are lined with dry vegetation that is regularly changed – during late winter and early spring, it is common for badgers to drag the bedding outside to 'air' it out. Clans may dig a new chamber for each successive brood of cubs and setts may be inhabited by several generations of badger. In one particular case, a sett was inhabited by the same family of badgers for more than 200 years. This sett extension by successive generations may partially explain why badgers are often very reluctant to leave well-established setts.
In very cold regions, the main sett is dug below the level at which the ground freezes and all members of the clan will sleep in the same chamber, presumably to share body heat. Additionally, and because of a lack of air circulation within the sett, ventilation holes have been recorded in some setts. These holes are typically about 4cm (1.5 inches) in diameter and extend from the ground's surface to a tunnel directly below. Studies by Tim Roper at Sussex University suggest that in the deepest parts of the sett, it is only movement of the badgers themselves that engenders airflow.
A study by the University of Sussex, published in Behaviour, looked at sett use in European badger populations and found subsidiary and outlier setts were used mainly during the summer. Sett use was not related to sex or body condition, but outliers did tended to be used by younger animals and had a larger number of fleas than the main sett. The biologists also recorded that the main sett was communal; it wasn’t divided into separate interior territories.
A similar study by Jacek Goszczyñski and Iwona Wójtowicz at the Polish Department of Forest Protection and Ecology found that badgers had two peaks of sett use: one in April and another in August and September. The Polish biologists observed that the same setts were visited by foxes and badgers, which tallies with previous observations by Ernest Neal that various species (including woodmice, bank voles, brown rats, rabbits, weasels and cats) sometimes share setts with badgers. Although foxes may share setts with badgers, even raising their cubs there, they typically emerge from different parts, suggesting they're living apart, and some authors suggest that the faeces, urine and food remains frequently found at fox earths can cause the badgers to abandon the sett it they're not breeding.
A study in Poland's Bialowieza Primeval Forest, conducted by a team of biologists at the Polish Academy of Sciences' Mammal Research Unit and led by Rafal Kowalczyk, found that in addition to various setts throughout the territory, these badgers used a number of shelters. In this low-density population (about two individuals per 10 sq-km), the badgers used several setts and daily shelters, particularly tree hollows, to save energy while moving about their large territories. An interesting finding of this study was that setts may play a role in marking the sites and while the number of setts being used by the badgers at any given time increased with increasing territory size, sett density (the number of setts per square kilometre) did not. Subsequently, the zoologists speculate that sett utilization in high- and low-density badger populations are regulated by different factors.
In high-density populations, social factors such as large group size, intraclan aggression, parasites, etc. force badgers to use multiple setts, while the increased food availability that permits clan formation in the first place allows more time and energy to be spent digging new and modifying existing setts. Conversely, in low-density populations, food is at a premium and more time and energy must be spent looking for it, so less can be spent modifying the sett. Making use of natural shelters allows the badgers to cover a larger area more efficiently when foraging.
In their book Badgers, Ernest Neal and Chris Cheeseman note that for the setts studied in the UK, the relative humidity within the main sett was always 100%, while the temperature tended to vary from 6C to 19C (43 to 66F) even though the external temperature can vary from -4C to 33C (25 to 91F). In other words, the setts offer a relatively stable environment, where temperature fluctuates by only about 13C (23F), even when the external temperature varies by as much as 37C (66F). Work by Tim Roper and Jude Moore suggests that the primary factor influencing these maximum and minimum temperatures is the cover surrounding the entrances; setts with woody cover surrounding the entrances exhibited the most stable temperature. (see Central Heating, below.)
In Badgers' Year, Howard Lancum observed that badgers at the sett he was observing in Devon during the early 1950s would use only certain entrance/exit holes during any given period. Lancum's direct observations, coupled with “cottoning” of sett entrances, revealed that this clan only used a few holes of the nine available at a time as he explained:
“For example, for two or three weeks the badgers would use, say, holes Nos. 2, 4 and 5 withour change, but then, for no obvious reason, they would switch over to Nos. 3, 6 and 7 for a period.”
Lancum does not speculate on why this should be, but it may correspond to changes in feeding grounds or reflect a variation in the use of sleeping chambers within the sett, which may be rotated to help control parasite numbers.
The impact of setts
Some interesting work has been done looking at whether the distribution of badger setts can be associated with specific biological features. One paper published in the Journal of Natural History back in 1997 by zoologists at the University College Cork in Ireland, reports on an interesting association between badger sett location and the common woodland fungus Phallus impudicus (Stinkhorn fungus). Paddy Sleeman and colleagues found that Phallus fruiting bodies were clumped in an area 24m to 39m (79–128 ft.) from the entrances of the four setts at their study site.
A linked study, published in the Entomologist's Gazette more recently, recorded 12 families and 22 species of fly (Diptera) at a badger sett in County Cork during June, July and November of 1999. Overall, the biologists suggest that badger setts attract flies (particularly blow-flies) which are primarily responsible for the spread of stinkhorn fruiting bodies. The laxative effect of the fruit’s mucus would then explain the clumped distribution of stinkhorn around the entrances to the setts. Off-hand, the data presented in these papers seem rather circumstantial, which perhaps explains why the association has been neither widely recorded or accepted.
In addition to stinkhorn fungi, several studies have found an association between badger setts and the local floral community. In their study of seven badger setts from the Rogow District of Central Poland, Artur Obdzinski and Robert Glogowski from Warsaw Agricultural University recorded a decline in both number and coverage of acidophilous (acid-loving), oligotrophic (low nutrient-loving) and skiophilous (shade-loving) plants. This decline included species from all three main bud-height categories of flora: geophytes (buds underground); chamaephytes (buds close to the ground); and phanerophytes (buds more than 25cm/10in from the ground). Along with this decline, the botanists found an increase in basiphilous (alkaline-loving), eutrophic (nutrient-loving) and heliophilous (sun-loving) species. Included among the promoted flora were byrophytes (e.g. mosses and liverworts), therophytes (plants that over-winter as seeds) and hemicryptophytes (plants that have their over-wintering buds at ground level).
It seems that during the digging and maintenance of their setts, badgers alter the soil properties and vegetation structure, which in turn promotes the nutrient-, alklaline- and sun-loving species at the expense of those which are acid-, low nutrient- and shade-loving. Furthermore, there is a well-documented association between badger setts, specifically their latrines, and two common species. The latrine areas around badger setts provide favourable habitat for elder bushes (Sambucus nigra) and nettles (Urtica dioica) because the badger dung releases nitrogen into the soil as it decomposes; elder and nettles prefer nitrogen-rich soil.
Finally, in a 2016 paper to the journal Forest Ecology and Management, Polish Academy of Sciences botanists Pzemyslaw Kurek and Beata Cykowska-Marzencka presented their study of the bryophytes found around badger setts in the Kampinos Forest, one of the largest and best-preserved lowland forest complexes in central Poland. Bryophytes are the mosses, liverworts and hornworts that we often associate with woodland and drystone walls. They’ve been around for the last 400 million years, making them the oldest land plant group on Earth, and they play a vital role in the ecosystem, being among the first to colonise bare and disturbed ground, allowing other plants to become established. The botanists identified 55 bryophyte species during their survey, 49 of which were found in and around badger setts. Of these 49 species, just over half (26) were found only on badger setts. Badger setts were also found to be home to a greater diversity of bryophytes than any of the reference (badger-free) plots. Epiphytic species (i.e. those that grow on other plant species, moss growing on the bark of a tree for example) were considerably abundant around setts, with 14 species found growing around setts and only three species found growing away from them.
For these epiphytic bryophytes to be found around the badger setts, the species of fleshy-fruited trees and shrubs on which these epiphytic bryophytes grow, particularly pear and dogwood in this study area, must also be more abundant around badger setts than away from them. Indeed, Kurek and Cykowska-Marzencka conclude that topsoil disturbance by the badgers digging creates niches suitable for the establishment of a variety of vascular and bryophyte plant species, making badgers an important force in increasing diversity in managed forests.
Maintenance and expansion
Sett maintenance seems to be something that all clan members play a role in, although how significant their contribution appears to vary. A study by WildCRU biologists Paul Stewart, Laura Bonesi and David Macdonald found that some 20% of clan members (adults and yearlings) were responsible for 60% to 90% of the digging and bedding collection that they observed during eight months of systematic monitoring at four setts in Oxford’s Wytham Woods between October 1994 and May 1996. The zoologists also recorded, in contrast to similar studies in Ireland, males digging more than females; boars of high social status were also more likely to dig than those of low status. Both sexes collected bedding at roughly equal rates and breeding sows took a more active role in den maintenance than non-breeding females.
Stewart and his colleagues suggested that by extending the sett, large frequently-copulating boars may increase their breeding success by creating a harem resource for several females. Similarly, they considered that highly resident breeding females may have more to gain from extending the sett than non-breeding sows, because they could then reduce reproductive competition. Unpublished observations by the same authors suggest that some breeding females partially restricted entry to certain parts of the sett during breeding, while previous studies by renowned carnivore ecologist Hans Kruuk in the late 1980s demonstrated that, when sett expansion is restricted, there is an increase in repeated aggression and litter infanticide between females.
At his study sett in west Yorkshire, Adrian Middleton observed that both males and females, sometimes also the cubs (although they were more likely to abandon attempts or were take the bedding the wrong way!) collected bedding. Reading his notes, however, one gets the impression the sows played a more active role in sett maintenance. Indeed, when the sow died the boar remained at the site, but Middleton, writing in Badgers of Yorkshire and Humberside, described the sett as looking “ill-kept and rather inactive”.
Given that badgers are well insulated with a double-layered coat and, in many parts of the UK, form clans that can contain ten or more animals, warmth in the sett has never really been a consideration for most naturalists. Certainly, on a cold winter's day a badger sett can sometimes visibly steam. An intriguing paper by German researchers, however, provided tantalising, frankly almost unbelievable, evidence that badgers may actively heat their sleeping quarters.
Helmuth Wölfel and Eberhard Schneider at the University of Göttingen in central Germany kept two sows and a boar in an enclosure with a concrete nesting chamber during the winters of 1984/85 and 1985/86. The badgers were offer meadow hay, cereal (wheat/barley) straw and beech leaves as bedding material, much preferring straw. The badgers “chopped” (they don't say how) the material up into pieces 5-10cm (2-4 inch) long and constructed a multi-layered nest-like structure into which researchers placed temperature probes. As the vegetation began to decompose it generated heat. When only the leaves were offered, the temperature within the pile reached 18C (64F), but when straw was incorporated a staggering 38C (100F) was recorded around the probe.
Intriguingly, there was no evidence that the badgers slept on this pile (i.e. the vegetation wasn't crushed and there was no trace of urine, faeces of food); the badgers opting instead to make normal sleeping nests in the same chamber. In their paper to Zeitschrift für Jagdwissenschaft in 1988, Wölfel and Schneider describe how the badgers moved their sleeping nests closer to, or further from (i.e. into an adjoining concrete tunnel), the structure as the temperature outside dropped and rose, respectively. Based on this behaviour, which, according to Tim Roper in his book Badger, happened during both winters (the paper only gives data for the first winter), the researchers considered this structure to be a “bio-oven” that the badgers constructed to help warm their sleeping chamber. As truly fascinating as these data are, we don't know whether badgers do this in the wild.