Whilst walking down near the river Ouse recently, in an area which can occasionally flood in very wet conditions, I found this huge molehill which was about 100 wide x 60cms high! According to our knowledgeable members this is a mole fortress, so I decided to search on the internet to find out more about this interesting phenomena which I had not heard about before and I thought I would share my findings with you.
Moles sometimes build huge molehills weighing over 500 kg. Usually they contain a network of tunnels, a nest chamber lined with hay and caches of earthworms. The worms have their heads bitten off and remain immobile in a tangled ball. They act a store of food for when the mole cannot obtain food from its tunnel system, for example during periods of flooding or freezing temperatures.
The vast majority of molehills are relatively small and without internal structure but on occasion moles construct these large and structured mounds called fortresses which are commonly found in areas with a high water table which are liable to flooding. When the waters rise the mole can retreat from the waterlogged tunnels and take refuge within the fortress. There it can remain, dry in its nest and sustained by the stores of worms, until the waters recede.
Fortresses also feature in shallow soils lying on a hard substrate. Moles prefer to make their nests deep in the soil where temperatures are relatively stable and for most of the year rather higher than at the surface. Moles living in thin soils cannot dig deep nests and a fortress may offer a degree of insulation to the mole asleep in its nest.
Fortresses are built with the soil excavated from tunnels that would have been dug anyway but there is a considerable extra cost in moving this large quantity of soil to one central point.
Word and picture by Julie Lane