Each winter I regularly don my hard hat, waterproofs, torches etc and deliberately choose to descend into dark, wet, muddy and essentially unpleasant places. Hardly the act of a sane adult, but then nobody's ever accused me of being one of them! Surveying underground bat hibernacula (places used by hibernating bats) provides survey data for the National Bat Monitoring Programme (NBMP), creating useful information about bat population change. I am also undertaking a long-term study of how bats vary their use of underground sites through the hibernation period.
One site I visit regularly is a tunnel in the Dumfries and Galloway hills (See "A Most Peculiar Hibernaculum", 28 February 2009). Built several hundred years ago to channel a water supply for a small village, one end is now blocked and the open entrance is less than one square metre. Inside the tunnel opens up to a sufficient height to walk upright and continues through the solid rock for 350 metres (not quite the 500m I said in 2009, but I've measured it now!).
The entrance to the tunnel - about a metre across.
Four years ago I was carrying out a survey in the tunnel and found a Daubentons Bat (Myotis daubentonii) that was a bit different from the others: it only had one ear. The other appeared to have been torn off. Perhaps the ear had been lost in a predator attack, or maybe the ear had been caught on a thorn in flight. Whatever the cause, most of the ear was missing, though the tragus (the inner part of the ear appeared to be intact).
Even more interesting was the fact that this bat was carrying a ring. Unlike birds, which are ringed wholesale, bats are only rarely ringed. This is because it isn't possible to ring bats round the leg in the same ways as birds: their tail membrane extend all the way to the ankles. Instead, bat rings are actually horseshoe-shaped and fit loosely over the forearm. The are difficult to fit and the chances of the bat being later injured by the ring is greater than for birds. For this reason bats are only rung as part of specific research projects, with clearly-defined aims.
"One-ear" - you can see the ring on her left forearm and her missing right ear.
So One-ear's ring number was an important piece of information, but the number extends around the forearm, making it impossible to read without disturbing the bat. I decided to leave the bat and returned four weeks later, fervently hoping it was still there (I didn't want to return more quickly, to minimise disturbance to other bats). This time I was equipped with scales and measuring calipers, so that I could measure the bat's weight and forearm length and make an informed decision as to whether, having been disturbed, it was likely to have sufficient fat reserves to complete hibernation safely (A bat carer was standing by, in case it wasn't). Happily One-ear turned out to be a female and have plenty of fat reserves to see her through the winter.
Armed with the ring number I was able to discover something of her history. She had been rung at a summer maternity roost the previous August by Sue Swift and Iain Mackie of Aberdeen University, as part of a research programme. The roost was 35km away in a straight line, but given the hilly nature of the area it's more likely she had followed valleys to the tunnel, making the journey closer to 45km.
Since then she has turned up in the tunnel as regularly as clockwork every January. Whilst it's nice to see her (and she's starting to feel like and old friend) she is also a useful source of information. Normally I can't identify individual bats within the tunnel but One-ear can be easily identified, allowing me to track some of her decisions about where to hibernate within the tunnel.
Whilst I have seen bats with all sorts of injuries, One-ear is the only one-eared bat I have encountered...until last year, when another bat turned up in the tunnel with a very similar injury, but without a ring. So when I carried out the January NBMP survey this year I had my fingers crossed and I'm happy to report that not only was "One-ear" there again, so was "Lugless"!
It's interesting to conjecture the effect that having a missing outer ear might have on an echolocating bat. It is easy to assume that, with only one fully functional ear a bat might be unable to gain a three-dimensional image of it's surroundings. The outer part of the ear acts in a similar way to a satellite dish, reflecting sounds into the ear and normally a bat can move the ear to channel sounds from varying directions. However the survival of One-ear and now Lugless shows that they are able to make allowances for this disability and carry on navigating and hunting successfully.
Remember it is a criminal offence in the UK and Europe to disturb roosting bats without an appropriate license. Bats in hibernation are especially vulnerable. Please don't go poking about underground, looking for bats unless you have been properly trained to do so. Not only are you running the risk of prosecution (quite rightly), your actions may be harmful to bats and underground sites can be very dangerous for the uninitiated. If you'd like to get involved join your local bat group and learn how to do it properly (and legally).
