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Eurasian badger (Meles meles) ecology: REPRODUCTION AND POPULATION DYNAMICS


Reproduction and Development

Mating and fertilisation in the Eurasian badger can take place during any month of the year although the main period of sexual activity, as determined from field observations and anatomical investigations, is during spring, with a second smaller peak in the autumn. During oestrous, which generally lasts from 4 to 6 days the sow may mate with more than one boar. Copulation usually lasts from 10 to 90 minutes although many brief encounters of less than two minutes occur when the sow is not fully receptive. However, births are synchronised to occur at the same time of year regardless of the time of mating. This is achieved through a phenomenon known as delayed implantation which was first identified in badgers by Fischer (1931).

In delayed implantation the fertilised eggs start cell division as they travel down the fallopian tubes and enter the uterus as tiny blastocysts (hollow spheres of cells). In most mammals these blastocysts would then implant themselves in the wall of the uterus. However, in some mammals, including badgers, they remain free in the uterine cavity for 2-10 months depending on the time of mating.

The onset of implantation is undoubtedly related to the concentration of the hormones progesterone and oestrogen in the blood stream. It has been proposed that variations in daylength, and seasonal changes in body temperature are also key factors. However, the amount of fat reserves accumulated by sows during autumn is likely to determine whether blastocysts implant and result in successful litter production. Sows that are heavier in the autumn are more likely to successfully produce litters in the following spring.
Badgers mating ©Eunice Overend

Delayed implantation ensures that cubs are born at the most appropriate time for survival. The length of gestation after implantation is about 7 weeks. The majority of births are known to take place between mid-January and early March although births as early as December and after April do occur, albeit rarely. Although the sow gives birth in wintry conditions the cubs are well protected in their underground chamber.

The number of cubs born in a litter ranges from one to five although the usual number is two or three. In low density areas it is believed that only one litter is produced per social group. In larger groups and when conditions are favourable, a maximum of four litters can be produced in the main sett and surrounding annexe setts.

Cubs are born blind and with a light covering of white silky fur with the characteristic dark eye stripes becoming more conspicuous a few days after birth. At about 6-7 weeks old their eyes begin to open and they start to explore underground tunnels and chambers. Cubs emerge from the sett for the first time after 8 weeks, usually around April in southern England, but stay close to the entrance and the sow while they familiarise themselves with their immediate surroundings.

Lactation, during typical weather conditions, lasts for a minimum of 12 weeks. This is followed by a period of weaning when supplementary suckling takes place. Due to the long lactation period the cubs are already large by the time they are weaned and emerge at a time when food is most abundant. Sows are capable of maintaining their milk supply into the autumn so that the cubs do not die of starvation during periods of extreme drought.

By 12 weeks cubs are much more boisterous with play and exploration becoming more purposeful and often resulting in a characteristic flattening of the vegetation around the sett. Cubs born around February in the wild can be as big as small adult sows by the end of autumn. Growth rates vary with food availability, which is in turn affected by the prevailing weather conditions. Very slow growth rates are found during exceptionally dry summers, which is probably due to a lack of available earthworms.

Although the cubs are still dependent on their mothers milk they supplement their diet by foraging near the sett and later following the sow further afield and feeding nearby. On average weaning starts in the first half of May. A few weeks after weaning, the sow and her cubs may move from the breeding sett or if space is available they may occupy another part of a large main sett.

Badger body-weight increases most sharply between June and November for the first two years of life with males putting on more weight than the females. In the south west of England, sows ovulate for the first time when 13-14 months old, although this may occur earlier if food is abundant. Boars mature slightly earlier, at about 11 months. Eventually some of the cubs will leave the parental sett either in the autumn or the following spring whilst others may remain as residents. Generally females stay whilst yearling males may emigrate on nearing maturity, although much depends on the composition and size of the social group

 Eurasian Badger Life cycle 

 

Population Dynamics

The badger population in Britain is thought to have increased considerably in recent years.  The last National Survey (1994-1997) indicated that the number of badger social groups had increased by around 24% since the previous survey (1985-1988).  The badger population in Britain is estimated to have increased by approximately 70% during this period.

At Woodchester Park, the badger population increased from 1978 until 1993.  Much of this increase occurred in the period 1980 to 1988, when the population doubled.  However, the population has not reached a higher level since then, suggesting that the carrying capacity may have been reached.  Most of this increase occurred via growth in the average size of the social groups, although a small number of new groups have appeared over the period of study.
© John Davis

One of the explanations cited for the increase in badger numbers nationally, is the strengthening of legislation that started with the Badgers Act in 1973, and concluded with the Protection of Badgers Act 1992.  Increases in survival rates due to protection from forms of persecution such as digging and shooting may have lead to the population growth we have seen.  However, without information on levels of persecution, this is impossible to confirm.

Any changes in population density must be caused by factors influencing births, immigration, death and/or dispersal.  The birth rate is dependent on the sex ratio, age at maturity, proportion of breeding females, average litter size (see Reproduction page) and life expectancy.  At Woodchester, adult males have a higher mortality rate than females, resulting in an adult sex ratio biased towards females.  The average litter size for the population is estimated as 2.7 cubs and approximately 50-70% of cubs die in the first year of life.  Although badgers have reached 14 years old at Woodchester, 90% of individuals die before the age of 7. 

Environmental conditions are also likely to influence fluctuations in population size.  Hot dry weather throughout the summer and autumn leads to hard ground, low numbers of available earthworms, and therefore difficult foraging conditions.  It is important that badgers put on as much fat as possible before the winter, so they can survive the cold months when food is in short supply.  Also, it is possible that blastocyst implantation is less likely when badgers are in poorer condition, leading to lower fertility and poor recruitment. 

Dispersal of young badgers in high-density areas was thought to be limited.  However recent research at Woodchester Park illustrated that movement between social groups by individuals was relatively common, although many of these moves were temporary.  Badgers of all ages permanently dispersed away from their natal groups, with a tendency to move to smaller groups.  The amount of emigration out of Woodchester Park into the surrounding countryside is unknown.  Each year, only a small number of adults immigrate into the study area.  It is likely that Woodchester Park is a source rather than a sink population, as historically the surrounding area has been subject to badger control and is likely to be at a lower density.

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