Feeds:
Posts
Comments

Posts Tagged ‘Kruuk’

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.

Read Full Post »

The weather today has been much warmer than of late.  It was a cold night, but the sun came out and the temperature went up to 8 degrees or so.  It doesn’t sound much, but compared to the last couple of weeks it feels almost tropical!

Rabbit tracks in frost

Rabbit tracks in frost

I went on my usual Sunday morning dawn stroll today.  When I set off it was still very frosty.

Here’s an example of tracks that you won’t find in a tracking book.

The pavement was very frosty, although the road had been gritted.  At some point in the night a pair of rabbits had crossed the road, hopped up onto the pavement and then gone through the railings to the field beyond.

They had picked up the salt from the road on their feet, and this salt had melted the frost where their feet had touched it, leaving this perfect set of tracks in the ice.

I decided to make the most of the day and went for a longer walk than usual.  I let my feet carry me in a big loop around the woods.  The Chinese Water Deer were out again, and the local buzzard seems to have found a friend, as there were two buzzards swooping and calling over the fields.  Either that or he was having a territorial dispute with the neighbour.

I thought it was time I checked in at the sett to see how the badgers were doing.  Of course, there was no chance of them being out at 9.30am, but I wanted to have a look round.  It gave me a good chance to look at the different parts of the sett.  In summer, when I’m actively watching the badgers, I don’t like to get to close to the sett for fear of disturbing them as scent can linger for a long while.  Today though, I thought I’d have a look, since the badgers would not be active until much later in the evening.

Everything seemed to be in order at the sett.  There were two entrances that looked to be in very active use.  Here’s a picture of one of them – note the relatively clean hole, without many fallen leaves or other debris.  You can also see how the sides have been polished by the coming and going of many badgers.  This is obviously well-used at the moment.

Badger sett entrance (1)

Badger sett entrance (1)

Very encouragingly, a couple of entrances showed signs of recent digging and of having been cleared out.  In the picture below you can see a furrow pointing directly to the hole, made by badgers dragging out spoil.  This is another classic sign of an active badger sett.

Badger sett entrance (2)

Badger sett entrance (2)

In the picture below, you can see that the badgers have dug out large amounts of dead leaves from this entrance.  This is a sign that they’re clearing out an old chamber for re-use.

Badger sett entrance showing signs of clearing out

Badger sett entrance showing signs of clearing out

Why is this encouraging?  Well, badgers re-dig parts of the sett at this time of year to make ready for the birth of cubs in February.  The sow prepares a separate ‘maternity suite’ where she can get away from the other badgers and won’t be disturbed.  The signs of activity at the sett all point to there being cubs on the way!

The interesting thing is that there is clear activity at both ends of the sett – the east and west sides.  This implies that badgers are in residence at both ends.  There is re-digging going on at both ends too.  Does this mean that there will be two separate litters of cubs from separate mothers?  Has there been a split in the badgers, so that different groups have taken to living in different parts of the sett?

All the books I’ve read suggest that all the badgers in a sett should be part of one single group, with only the dominant male and female breeding.  This wasn’t the case last year, as there were at least two litters of cubs, and the signs seem to indicate that there will be separate litters again this year.

I’ve also been thinking about the number of badgers in the sett at the moment.  If all the cubs survived (and I have no reason to think that they haven’t) then there will be at least 10 badgers in residence.  Do some of them leave home at some point, or do they stay in the group permanently?  Might this account for the active use of different parts of the sett?  If they leave, what is it that determines who leaves and who stays, and where do the badgers that leave go?  Do they join another sett, or start their own?

You see, this is the great thing about badgers.  We’re only in January and already they’ve got me confused.  I’m going to start the badger watching season as I finished the last one – with more questions than answers!

This is a mystery that needs solving.  Does anyone know where I can get a cheap copy of Hans Kruuk’s The Social Badger?  Even better, if anyone knows anything about the clan structure of badger groups and how they change over time, then please do leave a comment and enlighten me.

Read Full Post »