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Posts Tagged ‘evolution’

Badgers in a social group- but why?Following my thoughts on the evolutionary significance of delayed implantation, Pablo asked the very good question why (if my theory is correct)  badgers stopped being solitary and started living in clans.  Fortunately, I think I can answer this one.   What follows is not my own thinking, but based on the work of Hans Kruuk, a giant of badgerology upon whose shoulders I gratefully stand.  His The Social Badger is highly recommended reading for anyone interested in the subject.

Kruuk looked at the evolutionary advantages for badgers of living in social groups.  Social living is relatively common in mammals.  The usual benefit it gives is increased vigilance against predators (think of meerkats or rabbits – one or two individuals can keep watch while the others feed).  Social carnivores are more rare.  Lions, wolves, dogs and hyenas gain an advantage from hunting in packs – wolves and hyenas, for instance, can bring down prey much larger than themselves by attacking as a group.

Badgers don’t fit neatly into this scheme.  They are social carnivores but they do not hunt in packs.  In fact, for social animals they are actually not very sociable at all.  Badgers live together in a sett, and they will play and groom and interact with each other outside the sett, but once they leave the immediate area of the sett they forage as individuals.  For most of the time they are above ground they are alone, gaining the advantages of neither mutual vigilance nor pack hunting.  So why do they live in clans?

Kruuk’s theory is based on defending territory.  Badgers, in the UK at least, are omnivores.  They predominantly eat earthworms but will happily feed on anything from wheat and barley to rabbits and dead lambs.  Kruuk observed that badgers take advantage of different sources of food depending on weather conditions and time of year.  Success, for a badger, means making full use of these different ‘food patches’.  In order to have a guaranteed supply of food, the badger must have access to a wide enough range of food patches so that if one is not productive there will be others that are.

In order to have access to these food patches, the badger needs a large territory.  The problem is, a territory large enough to be productive is too large for a single badger to defend.  Hence, so the theory goes, badgers join together so that collectively the clan is able to defend a territory large enough to cover sufficient food patches.  Each badger plays a part in marking and patrolling the boundaries.  This makes perfect sense – many people have found a relationship between the size of badger territories and available food resources.  The territories in my fairly lush Bedfordshire landscape of woods, arable and pasture seem to be quite small, reflecting the good supply of available food.  Those in more sparse areas (such as Scotland, where Kruuk did a lot of his work) are much larger.

The theory accounts for why badgers live in clans today.  If my thinking is right, this clan living is a relatively recent evolutionary adaptation.   This does raise the question of why it should occur in the comparatively food-rich environment in Britain whilst badgers in other, poorer environments are solitary.  One would imagine that the advantage of defending food resources would be more pronounced where the is less food available.  Instead, the opposite seems to be the case.

Sorry Pablo – the answer to your question ended up a bit longer than I thought.  As ever, once I start to think that I understand badgers, I realise that actually I really don’t.

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Badger - one of the mammals that practices delayed implantationAs 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.

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