Several studies have demonstrated that, across Britain, the average number of breeding females per clan decreases with increasing latitude; so Scottish clans, for example, typically contain only a single breeding sow, while several sows may reproduce in clans from southern England. Indeed, in high-density populations such as those observed in south-west Britain, group size may be considerable – the record currently stands at 23 individuals, three of which were lactating sows. In these groups, where several reproducing females may co-exist, it appears that although sows compete directly for breeding status (the number breeding being directly related to the quality of the clan's territory), there is little competition between them thereafter.
Furthermore, data from these groups suggest that, in high-density populations, reproductive suppression is mediated through female-female competition for resources, rather than through a need for co-operative care of the young. In other words, reproductive suppression is a mechanism for adjusting the group size to fit the availability of local resources, instead of being a way of coercing clan members into babysitting. This idea is given further weight by studies from Sweden, Spain and Italy, which have found that when food is very scarce, badgers abandon the concept of sociality altogether and live alone or in pairs.
Fair weather friends?
There is still debate as to the underlying cause of group formation in badgers. Classically, an ecological principle called the Resource Dispersal Hypothesis (RDH for short) has been used to explain clan sociality. The RDH states that the size and configuration of areas defended by an animal (or group of animals) is decided on the basis of food availability. In social primates, for example, work during the late 1970s demonstrated that those which feed primarily on foliage often have smaller territories than those feeding on fruit, which is believed to reflect the sparser distribution of fruit compared to foliage; fruit is less abundant than foliage and so the primates have to move over a wider area in order find sufficient to satisfy their hunger.
In the case of badgers, the spatial aspect of the RDH (referred to as RDHS) is frequently applied. In essence, where an important food resource is sporadically distributed, a pair of animals will decide upon the minimum area that can be shared with their conspecifics before competition for the food becomes likely. Within the context of this theory, badgers are referred to as “contractors”, because they should maintain the smallest economically defensible territory with sufficient resources to permit reproduction (i.e. the smallest patch needed to support them throughout the year, without expanding).
Recent data from the Doñana region of south-western Spain, however, suggest that food availability along can’t account for the group living in this population of badgers. The study found that in this population, female territoriality was driven by food availability, while male territoriality was driven by female availability. Additionally, the ranging behaviour of males suggested that they were 'expansionists', rather than 'contractors' and overall territory size was related to its richness, rather than the patchiness of its resources.
While these data certainly don't disprove the RDH for badgers as a species, the biologists suggest that there is a better hypothesis to explain the sociality of badgers in this area of the Mediterranean. Rather than a single factor (i.e. food distribution) governing the formation of groups, they suggest that a myriad of factors including the availability of key resources, the impact of the dispersing individuals' mortality on the population demography and other behavioural constraints that may favour philopatry over dispersal, may integrate to influence whether an animal stays on its parents' territory or moves away to find its own place.
Aggression within the clan
Within clans there may be “tiers” of affiliation. Work in Oxford has demonstrated that not only do sows sometimes show a preference for sharing sleeping chambers with other females, but sub-adult boars have been observed to form “loose associations”, spending more time with each other than other clan members.
Despite a lack of evidence to suggest a stable social hierarchy in badger groups, fighting between clan members is well known and typically escalates from jaw-to-jaw contests to neck and rump biting. Where direct aggression does occur, the resulting wounds can be serious. Work by Glen Cousquer, Veterinary Officer at the RSPCA Wildlife Hospital in Taunton, shows that fight injuries can vary from incidental puncture wounds to large suppurative (i.e. pus-discharging) wounds. In an article to the World Wide Wounds website, Cousquer writes:
“Bite wounds often penetrate deep into the dermis, introducing bacteria into the subcutaneous tissues and setting up foci of infection. In some badgers these foci of infection burst out and coalesce, resulting in large open wounds that may subsequently become flyblown [maggot-infested]. Many badgers cope well with their wounds and it is not uncommon for badgers to be seen at the hospital with incidental wounds, requiring little or no treatment.”
Cousquer goes on to say that upon successful healing of the bite wound, the badger is often left with an area of tough scar tissue, which may provide some protection from further aggressive interactions. A more recent paper to the journal Animal Behaviour by the mammal team at Oxford University's WildCRU reports that not only did males, and especially heavier males, receive more bite wounds than females, but wounding rates (again particularly in boars) showed a density-dependent increase. In other words, as the population increased, so did the frequency of males were observed with bite wounds. The rate of bite wounding in males was also seen to increase as the number of badgers living in adjoining territories increased. This, coupled with the observations that males were injured at about twice the frequency of females and that older individuals were more likely to sport bite wounds, suggests that the defence of territory may be a significant cause of these injuries.
The biologists noted that severe wounds were fairly infrequent in individuals less than three years old, which implies that, assuming this rate wasn't masked by individuals moving away and dying, agonistic encounters are typically restricted to badgers of breeding age; these fights are thus a result of social tensions. Interestingly, David Macdonald and his colleagues failed to find any correlation between bite wound frequency and season, which is in contrast to both Cousquer (who observed more bite-wounded casualties during February and March) and David Dixon (who documented higher levels of intraspecific aggression on nights when the moon was in its new phase - see: Activity).
Typically, fights between badgers are one on one, but there are accounts of so-called “cooperative aggression”. In their contribution to the 2000 compendium Research Techniques in Animal Ecology: Controversies and Consequences, David Macdonald and colleagues refer to a coordinated attack during which an individual was bitten on the rump by one badger while a second grasped its head.