Mating may occur during any month, although the majority appears to happen between February and September; the bulk of that between February and May. Oestrous (the female’s receptive period) is between four and six days, during which a sow may mate with several boars. During courtship, the male will pursue the female and he’ll bite her nape (back of the neck) during intercourse. There are reports describing a curious behaviour during which the sow runs in circles, first clockwise, then anti-clockwise, prior to mating, although it is unknown how common this is.
In their 1996 edition of Badgers, Ernest Neal and Chris Cheeseman describe some mating events. The biologists write:
“The first sign of interest shown by the boar is often the raising of the tail into a vertical position and the emission of a loud and often continuous deep whinnying purr. He may then approach with a shuffling motion taking short steps with the legs kept rather rigid.”
Other descriptions note the presence of mutual grooming and 'ground pawing' by the boar immediately prior to mating. Many field reports describe vocalisation between badgers during copulations, serious attempts at which may last anywhere from ten minutes to an hour and a half; bouts of two to five minutes seems to indicate the sow isn’t fully receptive. Mate-guarding (i.e. males chasing away competitors from their chosen mate) is known in badgers and, in some respects, badger clans are similar to the pride system observed in lions; sows are able to move into neighbouring territories where they are apparently free to mix with, and frequently mate with, other badgers.
In a study of 548 adult female badgers from Offaly in Ireland between 1989 and 1990, Thomas Hayden and Robert Whelan found that between 80% and 90% of females mated, with 65-70% achieving successful implantation and, of these, only 35-40% lactated. These figures are similar to results from studies on badger populations in southern England, where about one-third of females breed annually. The Irish paper goes on to report that reproductive performance of yearlings was inferior to older badgers and observed some evidence for density-dependant reproduction – i.e. reproductive success was correlated with the number of badgers in the population.
All for one?
The promiscuity of badgers has long been suspected. In a relatively recent paper to the journal Molecular Ecology a team of biologists from the University of Sheffield, University of Leicester and the Central Science Laboratory in York reported on the breeding biology of the badgers in Woodchester Park, Gloucestershire. They found that males from outside of the sow's social group, typically boars from neighbouring social groups, sired about half of the cubs to which they could confidently assign paternity. Moreover, there were very few (~22%) successful matings between members of the same clan. These cases of observed and inferred promiscuity undoubtedly serve to reduce the threat of inbreeding. Indeed, a paper published in the Quarterly Review of Biology suggests that the delayed implantation coupled with superfetation may combine to help increase the reproductive fitness of female badgers.
Superfetation is a condition whereby two foetuses, fertilised at different times, are present in the uterus at the same time. Despite claims that many animals exhibit superfetation, verifiable cases are rare and the condition has only been well documented in the group of freshwater fish called mollies (Poeciliidae). The authors of this study argue that this combination of superfetation and embryonic diapause may make it difficult for dominant boars to identify cubs that are sired by other males, promoting what the biologists refer to as “cryptic polyandry”. Not only does this cryptic polyandry reduce the likelihood of inbreeding, but it may also reduce the probability of infanticide—dominant badgers are known to kill the offspring of subordinates—and increase the breeding sow's window of reproductive opportunity by increasing her scope for conception.
After fertilization, sows undergo a phenomenon known as embryonic diapause or delayed implantation. Diapause is a dramatic reduction in, or a cessation of, mitosis in the zygote at the blastocyst stage. In other words, the cells of the fertilized egg multiply to form a hollow ball (called a blastula) and then stop, remaining suspended in the uterus for between three and 15 months. Currently, reproductive biologists divide those species displaying embryonic diapause into those which are facultative (i.e. delayed implantation is induced by environmental conditions, such as in the rodents and marsupials) and those that are obligate (i.e. that present at every gestation regardless of ambient conditions). Badgers undergo obligate diapause; sows delay implantation until late December or early January, regardless when mating occurs.
In 1880, S. Fries proposed that delayed implantation may allow badger cubs to be born earlier in the growing season, thus providing them with more time to develop before they must face their first winter. Some 130 years on, Fries' hypothesis is still considered the most plausible explanation for this phenomenon. Indeed, in their paper to the journal Evolution, Michael Thom, Dominic Johnson and David Macdonald of Oxford University argued that delayed implantation is a plesiomorphic (i.e. primitive) characteristic in mustelids, which evolved several times, probably because there are costs associated with it. The observation that delayed implantation increases in frequency as you move further from the equator, argue the zoologists, strongly supports the idea that it provides the beneficial trait of being able to uncouple mating and parturition. In other words, diapause permits sows the luxury of being able to select mates at premium times (e.g. when males have survived a particularly harsh season, suggesting high quality) while allowing births to coincide with the most favourable seasons.
The biochemical cause of this diapause phenomenon has yet to be established. It has been suggested that hormones in the badger’s blood (e.g. progesterone and oestrogen) and possibly some proteinacious development factors are responsible for triggering resumption of blastocyst development. Indeed, studies in rodents have suggested that the female sex hormone oestradiol and Epidermal Growth Factor Receptor (Egfr) may play a role in triggering blastocyst implantation; certainly, a single injection of the female sex hormone oestrogen has been shown to terminate diapause in some species. Similarly, work by Rene Canivenc at the Universite de Bordeaux II in France has demonstrated a pronounced increase in luteal vascularization and progesterone production prior to implantation in badgers.
In April 1979, a paper published in Nature reported on the experiments of Rene Canivenc and colleague M. Bonnin on captive sows. The biologists exposed six sows to a 10:14 photoperiod (i.e. 10 hours of light, followed by 14 hours of darkness) at 5oC (41oF). Blood samples were taken from the badgers each week and analysed for progesterone, a steroid hormone secreted by the corpus luteum to prepare and maintain the uterus for pregnancy. The scientists found that when the photoperiod was decreased (i.e. more dark, less light) progesterone levels rose to those seen just prior to implantation during January in wild specimens (about 20ng per mL).
Through manipulation of the photoperiod, Canivec and Bonnin could induce “whelping” (i.e. birth) during September, while in the wild the litter would not have been born until the following spring. Ultimately, Canivenc and Bonnin succeeded in demonstrating that delayed implantation in the Eurasian badger appears to be under environmental, rather than genetic, control. Subsequent experiments by the same authors, during which the temperature was controlled, suggested it was light, rather than temperature, that triggered implantation.
These experiments give us a good idea of some of the factors at play here, but they don’t appear to tell the whole story. As Neal and Cheeseman point out, there are several questions that have yet to be answered. Among the problems is the fact that from about October-time onwards, badger emergence is entirely nocturnal and not obviously correlated with sunlight. In addition, we’d expect implantation to be earlier at northern latitudes, but we actually see the opposite. Finally, there are (admittedly rather rare) records of two sows living in the same sett giving birth several weeks apart.
Data from Stephen Ferguson and colleagues at the University of Saskatchewan in Canada suggest that reduced nutrition in the sow can lead to a lengthening of the diapause period. More recent work seems to support this idea and an interesting experiment by a team at the University of Oxford found that when females had a relatively high body condition index (i.e. were healthy and well fed), they implanted early and the cub sex ratio was male biased (i.e. more males were born than females). The important point here is that their finding goes against the reasonably well-established Local Resource Competition Hypothesis (LRCH). The LRCH states that in years when female body condition is poor animals should lower competition for local resources by producing males, which are more likely than females to disperse.
The connection between fat and hormones does seem to be an important one and Ernest Neal speculated that, given the lipophilic tendencies of steroids (i.e. they are fat soluble), hormones may be incorporated into the fat reserves built up during the summer and autumn. When the sow enters winter torpor, she begins to live off her fat reserves and as the fat is metabolised, the hormones are gradually released into the bloodstream; it may be this process that triggers implantation. There are various lines of indirect evidence that seem to suggest this is a plausible theory and, if correct, would make the rather inactive month of December a crucial time for the triggering of blastocyst implantation. Indeed, field observations seem to indicate that disturbed setts produce fewer litters than undisturbed ones.
When we consider the foregoing, in combination with reports from the field suggesting litters are closely tied with food availability (i.e. in years of poor food supply cubs are often later than usual), it becomes clear that the trigger for implantation still begs research. Ultimately, it is probably a combination of light, temperature, current body condition and food supply that interact to trigger implantation.
Gestation, birth & development
Once implantation of the blastocyst has occurred, gestation is usually between six and eight weeks, with cubs born anywhere from mid-January to mid-March with the bulk occurring in early February. Births have, however, been recorded anytime from mid-December to April. A female may produce as many as six cubs, although two appears most common. Studies from Ireland have recorded in utero litter sizes of between one and four young. Typically, any given female will only produce one litter in a single year, although under plentiful conditions more than one female in the clan may reproduce. In common with all mammals, the young are altricial (i.e. born blind and helpless). Neonatal (newborn) badgers are thin with pale grey fur and measure about 12 cm (5.5 in.). The cubs have their first teeth at about four weeks old and their eyes open after about five weeks.
The majority of cubs are born underground in a specially modified chamber close to the sett entrance, with good ventilation and a mass of bedding that is moved in prior to the birthing. Based on captive badger births, it has been suggested that the temperature inside the natal chamber is 18 to 20C (64–68F). In rare instances, a sow may give birth above ground on large mounds of straw/hay/grass/reeds. This appears to be more likely where the expectant mother is a subordinate sow looking for somewhere to give birth away from the attention of the alpha sow.
Young badgers emerge from the nursery chamber at about eight weeks old (late April or early May) and weaning begins at about 12 weeks. The permanent dentition is complete by 16 weeks (ca. 4 months) and the cubs are weaned by five to six months old, around end of June or beginning of July. Cubs may be seen foraging with the sow by summer and by June all the juveniles will be familiar with the clan's territory. By the time the cubs are fully weaned, if conditions are good the juvenile badger may weigh 6 kg (13 lbs). By the end of the year it will usually weigh between eight and ten kilograms (19–22 lbs) and measure 70 to 80cm (2.5 ft.).
On average both males and females mature at 12 to 15 months old, although males may mature as late as two years and females may begin ovulating earlier or later than the average. Dispersal is most common in badgers of two years old and seems intrinsically related to clan stability (the more stable the habitat and social dynamics of the clan, the less likely dispersal becomes) and males have a greater tendency to disperse than females.
In a paper in the journal Ecology David Macdonald analysed 17 years of data from a marked population of badgers in Oxford and reported that 35% of individuals never dispersed from their natal territory and in those that did, dispersal distances only rarely exceeded two or three home range diameters. Overall, these and other data show that although excursions into neighbouring territories for the purposes of mating are common, as a species, badgers have low dispersal rates and at a local scale are genetically depauperate (i.e. local mixing of genes is severely diminished). Where dispersal does occur, sows may leave the clan in 'coalitions' of two or three animals.
Generally, about half the cubs will die within their first year through causes other than infanticide (i.e. the weather, disease, on the roads etc.) and there is an average annual adult mortality of about 30%. One study from Ireland published in 1993 observed that more than 60% of cubs died during their first year, 35% to 40% of which died before they were weaned. Males were also more likely to die early in the year, with females dying later. Presumably this trend reflects the tendency for males to be bolder and thus explore further from the relative safety of the sett than females.
Disease and road accidents can have a significant effect on some populations, but weather is perhaps the most significant mortality factor. In a paper to the Journal of Zoology in 2002, a team from Oxford University's WildCRU lab report on the population dynamics of the badgers living in Wytham Woods between 1987 and 1996. During this time, the population expanded considerably, almost quadrupling from 65 adults to 228, as did badger density (15 adults per sq-km in 1987 to 38 adults per sq-km in 1996).
Despite the substantial population increase, cub survival was inextricably linked to rainfall. In 1990, a summer of unusually low rainfall resulted in 15 (65%) of the year's 23 cubs dying, although rainfall seemingly had little impact on the number of adults surviving. Presumably, during prolonged periods of dry weather, earthworms are scarce and difficult to dig for, leading to a decline in overall body condition and an increased chance of cub mortality. Fortunately for the badgers of Wytham, their population appears to have grown in response to an overall improvement in climatic conditions as well as the social rearrangement of territories observed during the study.