Saturday, 28 February 2009

A Most Peculiar Hibernaculum

Bats are very good at doing what they shouldn't do: ignoring the books and turning up in the wrong habitat or behaving in an unexpected way. That's part of the charm of working with them. There's a constant chess game in which we try to stay one step ahead.

Usually these unexpected things turn up one at a time and mercifully they are outnumbered by "correct" behaviour. Recently however, I have done some hibernation surveys at a site where the bats seem to specialise in intriguing behaviour.

The site is a tunnel about 500m long, accessed via a small hole, high in the hills of southern Scotland. The tunnel was hewn by hand from the solid rock, in order to carry water from one valley to the next, for industrial use. On end has long since collapsed, but the other is still accessible to those who know where it is. Despite it being far from habitat likely to be used by foraging bats, it seems to be well-known to bats and is regularly used by Daubenton's Bats (Myotis daubentonii) and Natterer's Bats (Myotis nattereri).

The tunnel is small, mostly just high enough for me to stand up in (I'm just over 6 feet tall) and between one and two metres wide. As there are few crevices, the bats tend to tuck themselves into angles in the rock, meaning that it possible to be fairly confident that the majority of bats will be seen during a methodical survey. This is a rare luxury: in many hibernation sites it can usually be assumed that, for every bat seen, there are likely to be more out of sight.

Because of this, I thought it would be interesting to place some temperature dataloggers within the tunnel, to measure the temperature variation at different depths. The dataloggers were in place for four weeks, during which there was a warm spell, followed by several days of very cold weather, accompanied by snowfalls. Despite this, a datalogger just 7m from the entrance (C on the chart) showed a temperature variation of less than one degree either side of 4.5 degrees. A second logger 100m from the entrance (B on the chart) showed a steady temperature of 6 degrees, never varying more than a tenth of a degree. A third logger another 100m in (A on the chart) showed similar consistency around 7 degrees. Hardly surprising then that the tunnel suits the bats well.

Water runs along the floor of tunnel throughout, but never more than a few centimetres deep and I can be quite confident that it rarely rises above that height. Why? because last week I saw a Daubenton's bat tucked into an angle in the rock less than 30cm from the floor. I have rarely seen bats low on the walls of other hibernacula, but here they have been seen doing this several times.

In January this year I walked through the tunnel, noting the various Myotid bats. almost 250m from the entrance was an especially small bat. I had to look at it for a few moments before the evidence of my own eyes registered: it was a Pipistrelle. Why on earth a Pipistrelle had chosen to hibernate so deep in an upland underground site is beyond me! Normally Pipistrelles hibernate in conditions with far less consistent temperature and humidity. I would love to have known which Pipistrelle species it was, but without handling it, it was impossible to tell.

Undoubtedly my favourite oddity at this site was a Natterer's, which had found itself a small crevice. Perhaps the bat would prefer it if the site had more crevices for them to crawl into, as at other hibernacula, because this bat seems to have decided it wanted to be in the crevice come hell or high water. It had managed to get it's head in and no more and there it had settled down to hibernate. It had it's backside stuck in the air and it's wings akimbo, looking for all the world as though someone had hammered it in with a mallet...

Regular reader of this blog will have heard this many times by now, but please be aware that it is usually a criminal offence to enter a bat hibernaculum in the UK without a licence issued by one of the statutory nature conservation organisations (NE, SNH, CCW etc). It is also extremely dangerous to venture underground without training. Blundering about below ground in winter is a great way to harm both yourself and hibernating bats. Sorry, but my sympathies are with the latter! If you want to take part in these activities, join your local bat group. What are you waiting for?

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