The tunnel has been converted as a bat hibernaculum with a stable temperature and humidity environment for several species of bats (Natterer’s, Daubenton’s, Brown Long-eared, and Barbastelle) to use during freezing weather during the past 21 years.  Accelerated global warming during that period has resulted in more extreme weather although the frequency of very cold winters has decreased, so the bats’ use of this winter hibernaculum varies year-on-year and week-by-week, but it appears that generations of bats remember where it is when they need it, so its design has focussed on providing a very wide range of hibernation habitats inside the tunnel, so that different species of bats can find the locally specific environment needed for their species and for their specific lengths of stay in the tunnel, which now vary every year.

The initial design, following Bat Conservation Trust advice, has involved restricting the through-flow of air due to local weather by building ferro-concrete walls across each end, to close off half of the tunnel entrance at the Littleworth end, while leaving a grilled access aperture of just one square metre, set 3 metres above ground level, at the slightly lower Horspath end of the tunnel. This design has not only reduced the airflow in the central part of the tunnel to a minimal value, allowing the temperature of the tunnel walls there to stabilise at about +8 degrees Celsius, but has also facilitated easy measurement of the through-flow in terms of cubic metres per second at the 1 metre square grill. Thousands of infra-red measurements indicate that an air temperature and wall temperature gradient exists along the length of the tunnel, with the greatest variability near its ends, and that a vertical gradient extends up the walls, to which bats are very sensitive in selecting their spots for hibernation. To provide a variety of options for bats, the initial 120 ceramic bat bricks, each designed for 6 bats, were recessed at intervals into the side walls at 2-3 metres above the tunnel floor, and in addition, another 100 thick wooden bat boards were fixed at 10-metres intervals along the side walls of the tunnel to provide an alternative range of bat accommodation which is easier for Licensed Bat Handlers to monitor. To further increase the range of choice of bat hibernation environments in the tunnel, a large number of bat bricks have been fitted inside the alcoves built into the tunnel walls for railway personnel 160 years ago. These bat bricks have proved to be very popular with Natterer’s bats, and they have been installed externally on steel brackets in a ‘sandwich’ as shown in the photo, with a grooved brick for bats to land on below them, and with a high density brick above them to increase the thermal stability of this sandwich. This hibernaculum therefore offers hundreds of choices of habitat to bats according to their changing hibernation requirements as we experience more extreme weather.

Hibernating bats require a humid environment, and the internal source of this humidity has been provided by controlling the water level in the tunnel at the Horspath end to produce a 40-metres-long internal lake where the water temperature, typically of around +6 degrees Celsius promotes evaporation, and this is also enjoyed by a noisy chorus of male frogs who enter through the overflow channels when coming to the frog-breeding pond outside to wait for females in early spring. Flooding part of the tunnel to a shallow depth to raise the relative humidity to around 90% has not interfered with human access because a very secure steel platform with steel steps has been installed inside the Horspath entrance, leading to a wide elevated board walk beside the flooded section, to facilitate any monitoring of the bats by the Oxfordshire Bat Group and for educational visits by interested groups. As the owner of the tunnel, Oxfordshire County Council has prohibited any further entry into the tunnel by people for whatever purpose, so monitoring has ceased, but with a steel platform inside, it is hoped that the installation of automatic bat activity recorders may be permitted at some time in the future. Fortunately this hibernaculum was designed to mitigate the effects of future climate change, so the bats should still be very happy in there.