European badgers have been confusing scientists with their flexible and geographically variable lifestyle.
In United Kingdom and Ireland badgers appear to form social groups while in the continental Europe badgers mainly seem to lead a solitary lifestyle (similar to most mustelids) or pair up to raise cubs together.
It is not entirely clear, either, what the benefits of the group living are in the species.
Theories have been proposed to explain these differences and lack of clear social profitability.
Mainly it is thought that badgers are only in the initial stages of evolving social group living and that they are only discovering the structures and motivations that will guide their social systems in the future.
Also, it has been proposed that badgers form groups where resources are abundant enough to partner up and to retain kin (offspring) in the same territory.
Resource dispersion and abundance are dependent on climate which is why badger social life can be different from one region to another.
I would like to share some ideas regarding badger sociality.
To begin with, I believe that mustelids find it difficult to evolve social living in general due to their high metabolic needs.
Mustelid body shape is not very energy-efficient and mustelids can lose body heat very rapidly, not they are very tolerant toward hot weather.
Mustelids usually do not accumulate fat (badgers are an exception) which pushes them to feed frequently.
Frequent feeding can place constraints on the use of resources (the resource use might be too high per territory to share the resources with other non-offspring individuals).
Energy is also needed to maintain group relations (bonding activities).
Unless there are no great resources losses due to social group forming or the formation of social groups allows to reduce energy expenditure (through cooperation, sharing of chores etc.), species with high energy demands would be less likely to form social groups.
While badgers can accumulate fat during autumn and their resting metabolic rate in autumn and winter was not higher than predicted for mustelids > 1 kg, metabolic rate in summer was higher than predicted (McClune, D.W. et al., 2015).
There are also seasonal differences observed in badger weight during summer vs. winter (Parish, T. & Kruuk, H., 1982) with badgers demonstrating markedly lower weight values in summer even if the weight data was acquired from both wild badgers and badgers who lived in captivity and were fed as much food as they wished to consume.
Regional differences were also discovered with badgers being heavier in southern England than in Scotland.
The weight fluctuations are thought to be caused by fat reserves that are not accumulated during summer but they are accumulated beginning with autumn.
This means that the onset of fat accumulation might depend on, e.g., photoperiod and not merely on food availability.
Parish & Kruuk explained the regional differences as related to earthworm availability as well as the shorter nights in Scotland (shorter foraging time).
I believe that the ability to accumulate fat is what differentiates badgers from other mustelids and also what determines their potential for social living because it is likely that social group formation is less dependent on high food availability season (summer) and it is more constrained by low food availability season (autumn, winter, early spring).
The ability to grow fat reserves and to enter torpor during winter could have formed the basis for social life potential in badgers.
If there are differences in fat accumulation capacity among regions, these differences might also be significant in determining where badgers would form pairs and groups and where they would not.
Food abundance aside, it could be other factors which initiate fat accumulation and which affect fat accumulation rates that either allow badgers to live socially, or not.
It might be a combination of resource abundance, photoperiod and temperature.
Badgers should have high food availability (determined by local abundance but also by climate) during summer when their metabolic needs are the greatest (also accounting for the energy expenditure invested in social interactions MINUS energy savings due to shared sett maintenance, territory patrolling and partly cooperative raising of cubs which can be observed as babysitting and allonursing) but the food availability should still be high as the period of fat accumulation begins.
If this is determined by photoperiod, it follows that the change in the day length should allow for plentiful foraging while the prey resources have not succumbed to near-winter weather.
If this gap is narrow and fat accumulation starts shortly before the ending of the vegetation growth season (when vegetation and herbivorous prey are available), the badgers might struggle gaining reserves to survive winter and social group formation likelihood would decrease.
However, in some regions autumn might be abundant (warm, rainy weather extending well into November or even December) and badgers would not be constrained as much to build up fat reserves and to lower their metabolic rates.
The duration of night (foraging hours) should also be considered because short nights might not compensate for greater food availability.
On the other hand, if the badgers can manage to accumulate enough fat during a relatively short autumn and the torpor period is long, this could also facilitate social living because during torpor the resource abundance and availability is no longer important and fat-enough badgers can coexist peacefully during these months of inactivity.
Long activity seasons might sometimes prove less supportive of social living unless resources are highly abundant and available during the entire period.
Meanwhile, summer could be the limiting region in hot and arid regions if summer is the season if the highest basal metabolic rates in European badgers.
These factors would determine whether badgers can afford to share their resources and also whether they can afford investing in group interactions.
I think that badger social living began with males moving in with the females (and their cubs) whose territories overlapped with their territories.
This could be the reason why breeding females dominate adult males in badger social groups (Hewitt, S.E. et al., 2009).
I find it possible that badgers used to live like most mustelids with male territory overlapping female territory but badgers are unlike other mustelids.
European badger is not the only mustelid species which can breed throughout the year but badger is one of two European mustelids (if I am not mistaken) that can achieve delayed implantation of blastocysts that have been fertilized by different males.
I suppose that in most mustelids that do not live in the southern regions with long growing seasons, mating throughout the year is possible and not viable and probably not sought after by the females.
In badgers, mating is important both during early spring (after parturition) while the female is yet restricted to the sett where she attends to her cubs and also later in summer when the female might attempt to fertilize additional eggs (I cannot recall if there are different success rates in eggs fertilized earlier or later in the season).
This would promote male visitations of females close to sett while they are still nursing their cubs unlike in other species where females might be less admissive even toward the males near the cubs.
However, it is curious that females mate with multiple males and it might indicate – European badgers never really had the typical mustelid spatial organization in which males are territorial to ensure exclusive access to the females within his territory.
Many current badger reproductive strategies (infanticide, promiscuous mating, multiple paternity) do not suggest that males were territorial in the traditional mustelid-sense.
Unless current mating system evolved after sociality, perhaps the badger males did not hold fixed territories which would have meant they were roaming to meet many females and they also did not have permanent setts.
Females might have benefited from hosting certain males or one specific male at least temporarily because that would have limited the nuisance of too many males on their ranges exploiting resources (which the breeding females likely found hard to defend during cub-rearing season when movements were restricted) and making too frequent advances as well as endangering the cubs (fewer and more specific males would have been easier to confound regarding the actual paternity of the cubs).
The male (or few males) thus could have helped with territory maintenance during spring/summer until the female regained her full mobility.
The male (males) visiting the female near her sett might have also helped in sett maintenance.
The female might have evicted the male if she found the resources limited during certain periods.
The female could have determined the duration of the male’s visit and during times of abundant resources, she might have allowed the male to stay longer.
The male might have profited from access to a permanent sett which could have provided more efficient thermoregulation than transient setts (especially if the males, indeed, did not hold their own territories and therefore also changed hibernation setts from year to year).
I sometimes have wondered why European badgers do not always live at least in pairs where climate is harsh because having another badger in the sett during winter torpor might improve thermal qualities of the underground chambers.
My answer was that there could have been energy costs unaccounted for even during this period of very low metabolic activity (not much reliance on food).
Of course, food inaccessibility during winter is an important factor.
But I was also wondering about hormonal production in males.
European badger females have induced estrus and this might mean that males do not respond to estrous females but that the males produce hormones that encourage mating activity irrespective of female responsiveness.
Nowadays, badgers tend to have a ‘quiet’ period regarding hormonal production which coincides with fat accumulation and torpor (Sugianto, N.A. et al., 2019).
I wonder if this has always been the case and whether males in all populations (regardless of their social system) attest to this hormonal cycle.
Perhaps (at least initially) if the males were exposed to females in close proximity (e.g.., in the same sett), they could not cease hormonal production which weakened them due to energy expenditure caused by lack of reproductive quiescence.
I cannot tell if the reproductive quiescence was the result of lower metabolism independent f proximity to females, or if it could only be achieved if the male kept distance from females in autumn and winter period.
I wonder if males sometimes did not begin to inhabit separate chambers in the setts of females during winter because such permanent setts would have been more effective.
The gradual exposure to the female’s proximity might have allowed the male to evolve a stable reproductive quiescence irrespective of female’s presence which would have facilitated moving in to the same chamber later on.
Otherwise, the costs of hormonal production during autumn and winter (as a result of being near the female) might have been too high offsetting any thermoregulatory benefits.
If the males used to reduce their hormonal activity through ‘withdrawal’ (physical separation from females), a new mechanism had to be acquired where pair bond or more extensive social life was developed.
Once the males could join females in the same wintering chambers, there could have been gains regarding thermoregulation during the torpor period.
It is also possible that during periods of either food overabundance or extreme food scarcity male badgers could join females in a more permanent capacity.
Food overabundance would result in lower costs due to continued reproductive activity during autumn while food scarcity would have cancelled out hormonal production due to poor body condition.
While it is less likely that females would accept males in their core territories during periods of scarcity, perhaps the thermoregulation gain (sharing the wintering chamber) might have been an important consideration where fat reserves have not been accumulated efficiently.
On the other hand, if hormonal activity is affected by some climatic variables, for example, photoperiod or food abundance, or temperature, in some regions, badger males would have to avoid females because their hormonal production would not be offset and they would lose body weight due to higher metabolic needs.
I have seen some publications where subadult/adult offspring retention has been regarded the pathway to badger sociality and it is not clear whether this precedes female+male pair bonding or if it the ‘next step’ in social life evolution, or if these social groupings occur independently in different biogeographic regions.
I hope to address offspring retention mechanisms in another post.
Notes by Ieva 