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.