As I write this I can be quite confident that in badger setts across the country, female badgers are either giving birth or getting very close to doing so. Up to mid-February is the peak time for badger births.
The reason I can be so confident is that badgers have amazing control over their reproduction via a process called delayed implantation. The badgers can mate at any time of year (spring and late summer seem to be particularly favoured times) and yet give birth in February.
What happens is that the egg gets fertilised in the normal way. Egg and sperm combine and the cells start to divide. But the fertilised egg does not implant itself into the uterus and continue to develop as would be the case in most mammals. Instead, the small ball of cells, called a blastocyst, stops developing and goes dormant, drawing just the small amount of oxygen and nutrition it needs to survive. In late December the blastocyst attaches itself to the wall of the uterus and starts to develop into a full foetus, to be born in February. This is delayed implantation. It means that the badger mating I witnessed in August (see Fieldnotes: 8th August 2009 – Sex) could result in cubs being born now.
Delayed implantation is not unique to badgers. It occurs in a number of other mammals such as stoats, bears, Roe Deer and Grey Seals. The evolutionary advantages of the process are clear – it means that the young are always born at the optimal time to take advantage of the best food resources in spring.
For badgers, there a few interesting implications. Ernest Neal speculates that delayed implantation allows the badgers to mate throughout the year, which may help to strengthen clan relationships. Hans Kruuk makes the point that multiple matings with different males could result in multiple blastocysts, meaning that each cub in a litter could have a different father. Badger families must get complicated sometimes! This is even more interesting when you remember that the dominant female badger may kill the cubs of other females to maintain her position. This control over the genetic make-up of the clan is very much a female thing. It must be related to the fact that you can never be sure who the father is, but there’s never any doubt about the mother.
Now, I’ve been thinking about badgers, and about delayed implantation, and I’ve got my own little theory. This is just my own idea, so if it’s wrong then I take full blame. My line of thinking goes like this: most of the mammals that practice delayed implantation are solitary by nature. This means that the males and females come into contact only irregularly.
Delayed implantation offers an evolutionary advantage to these species because it means that a male and female can meet up at any time of year, mate, and still have the offspring born at the best time. It is a way of compensating for geographical and territorial dispersion.
But badgers are different. Badgers live in social groups where males and females come into contact every day. For clan-living badgers, delayed implantation offers no great advantages. Why don’t female badgers simply come into season in December and have cubs via direct implantation? Neal’s theory about the regular mating strengthening group bonds is one plausible explanation, and there may be a further advantage gained by the genetic diversity of litters sired by different fathers, but I think it tells us something about the evolution of badgers.
I think that delayed implantation is an evolutionary throwback to a time when the Eurasian Badger was a solitary animal. I think it points to a period in the history of the badger when they didn’t live in social groups and therefore gained an advantage from it much as stoats and seals do today. In turn, it suggests that clan living is a relatively recent development for badgers.
It isn’t as wild a theory as it sounds. Other badgers around the world are still solitary – the American Badger, the Honey Badger, the Indonesian Stink Badger, and so on. Even our own Eurasian Badger is solitary across large parts of its range. In Mediterranean regions, where food is scarce, badgers are virtually solitary. Rather than this being an adaptation to the dry conditions, it’s my belief that delayed implantation shows that this is their natural state, with clan living a relatively recent adaptation to the conditions of North West Europe.
Tales from the Wood - The Diary of a Badger Watching Man 
That sounds a very plausible theory. I was never really content with Neal’s own theory for delayed implantation. I never thought that a biological system (the process of delayed implantation) would be governed by a wholly un-biological practice such as clan bonding. I always thought that it had to be something biologically oriented.
Kruuk’s multiple male theory is interesting, but does not really give an answer to why implantation is delayed.
I had always gone along with the idea that delayed implantation ensured favourable birth and rearing conditions. But you are correct, why wouldn’t the female simply become available in December, thus producing a February litter.
As far as I know, your theory comes closest to answering all of the inherent questions in this situation. And it is one of those theories that one has to ask oneself, “Why didn’t I think of that?” Surely a sign it is legitimate.
Thanks Jay – I appreciate the feedback. You’re thinking on the same lines as me. I’ve always accepted the idea of delayed implantation without ever asking ‘why?’
My theory does depend on an ancestral, solitary ‘proto-badgers’, but it does make sense and fit the facts.
All the best
BWM
Certainly plausible but let’s go a little further on your theory and ask why did the badger become non-solitary. Was it merely becasue it was running out od space or is that too simplistic?
Good question Pablo, and one that deserves a full response. Fortunately there is answer. I’ll stick it on a post as it’s a bit long for a comment.
Hi BM. Bit late on this thread I know, but i’m up in Cumbria most of this month. Your theory regarding Badgers is sound for sure. Delayed Impantation is still a mystery even to the most pragmatic biologists. It seems to me that it suits differant species in differant ways. The common factor in most is that the young are born in time with a bountiful season-as you said. Seals apparently originally developed it in order they breed and haul out to calf at the same time to avoid predation in spending too much time out of the water. Indeed Bears, as Badgers ,needed the extra long breeding cycle in order they find a mate over a vast area. However not the case with the Roe Deer,they don’t have to wander far to find a mate,but yes they are more solitary than most deer by nature. They are mono-oeustrus being fertile for just a small ammount of time in their month long rut. Why rut in July/August? One theory is that being slight in stature,the more quality food in summer helps them get back in condition for the leaner months and aid in fertility. This is triggered by photoperiodism. But how so the Badger who spends that time below earth or in darkness. It still has so much mystery to it. It keeps us guessing which is just fine by me.
Thanks Ghillie. Yes, it is still a mystery to me. I suspect you’re spot on when you say that delayed implantation has different uses for different species, although it is interesting that species as evolutionarily diverse as deer, badgers and seals all have it. But as you say, there’s always something more to learn.