Mating in European Badgers
A Badger Trust guest article by Paul Saunders
European badgers are fascinating creatures and the more I learn about them, the more interesting they become. One of their most remarkable features is how they mate and the methods they use to try and maximise the number of offspring that they produce.
Badgers live in groups of typically around four to seven individuals. This group is known as a ‘clan’ or a ‘cete’. They are social creatures and will bond with each other through actions such as grooming each other or pressing their bottoms onto another badger in order to leave a scent mark on them.
Another frequent behaviour they display is during mating, when the males bite at the necks of the females. This usually takes place within the badger's clan, but males will travel to neighbouring setts to try and mate with a receptive female if one is present.
Badger mating, gestation and birth
Badgers reach maturity at around a year old, but most sows will not have their first cubs until the winter of their second or third year. Mating can happen all year round, but it mainly occurs during two periods – one in January to May and another in July to August. Research suggests that this is when the fertility of the females is highest and therefore mating during these periods is most likely to produce a pregnancy. Badgers don't just mate once though, and females can mate several times with different males over many months.
Cubs can be born from mid-December, but more usually between January and March. Technically their gestation period is only six to eight weeks, yet the female badger may have mated as much as eleven months before giving birth. This long time period between mating and birth is possible due to something called ‘delayed implantation’.
What is delayed implantation
Delayed implantation, also known as embryonic diapause, is where an egg is fertilized soon after mating, but the cub will not be born until several months later. Shortly after mating, the fertilised egg develops into a very early foetus known as a blastocyst; development then slows down almost to a standstill.
The blastocyst floats around in the female's womb until December or early January, when the short day length triggers hormonal signals that cause changes in the womb and cause the blastocyst to implant in the wall lining. Once implanted, the early embryo resumes its normal development and completes its transformation into becoming a fully-formed cub.
What is superfetation?
There is another phenomenon that is used to increase the number of offspring that badgers produce and that is something called ‘superfetation’. After a badger has successfully mated, it is possible for there to be a blastocyst in the womb and for more eggs to be released so that the female can mate again and become pregnant with more than one embryo.
This method of having an existing embryo and fertilising more eggs at the same time is used by many mammals. However, most mammals that use superfetation have a very developed foetus that is almost ready to be born and fertilise an egg that will develop after the birth so that there will be two pregnancies.
The European badger is one of only six mammals that displays superfetation, where all of the offspring are born at the same time with only one pregnancy. It is a very clever way of ensuring that the maximum number of cubs are born each year, as badgers usually only release one egg per cycle. By mating over multiple cycles, this allows more than one cub to be born in each litter.
Why use delayed implantation and superfetation?
There are two main reasons that are thought to be behind delayed implantation and superfetation in badgers.
One is to maximise the reproductive success of the females. Not only does it increase the number of cubs born in a litter, but by mating with several males at once there is a higher likelihood that one of them will be able to fertilise an egg.
Also, as the blastocysts develop very slowly during the period that they are floating in the womb, those that are fertilised earliest are less likely to produce viable cubs. Being able to fertilise more eggs throughout the year means that blastocysts of different ages and sizes are present and the chances of a successful pregnancy increase.
Another advantage it has is that it potentially reduces the risk of the males killing the cubs, something that is seen in many mammal species. Male infanticide is thought to be less common in badgers because, if all of the cubs could potentially belong to one male, he is less likely to kill any of them as he is fooled into thinking they are all his.
There are also other advantages that have been suggested, such as keeping more males in the clan as more of them mate, obtaining material benefits from the males such as bedding, avoiding male harassment and increasing the genetic diversity of the population.
Think of those badgers about to be born in the next month or two
As the winter nights draw in and the weather gets colder, spare a thought for those tiny bundles of fur that are lying underground, blissfully unaware of the environment waiting for them when they emerge in a few months' time. They couldn't possibly imagine the complicated and highly evolved methods their mums used to allow them to be born.
Hopefully in just a few months you may be lucky to spot a young cub as it emerges from its sett, sniffing the outside world for the first time. If you do see one, just think that it could already be well over a year old, despite having only been born a couple of months previously.
About Paul Saunders
Paul is a vet and zoologist and is passionate about the natural world and conservation. In his spare time he enjoys observing wildlife of all species and is a keen wildlife photographer. The pictures in this article are his and if you would like to see more of his work then head to his Instagram @campaulcam.