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Monday, 22 February 2016

Just hanging around

For 3-4 years now we've been checking the Catacombs for hibernating bats.
Bat team, winter 2015-2016
Gloomy, cold, dank, a little spooky, and just about perfect temperature and humidity conditions to keep bats in steady hibernation. There is high risk of serious disturbance at this crucial time, not just torchlight and noise, but also inadvertently raising the temperature a few degrees by our movement around inside. Hence our monitoring group is kept small, and monitors have a special bat licence for carrying out the checks. This being an urban park in the middle of the city, the risk assessment includes looking out for broken glass and discarded needles around the gates and grilles. 
The Batacombs: spot the bat ... (c) Greg Slack
The Catacombs have been here since the late 1830s, comprising a number of arched sub-chambers, very few of which have been filled, and in one spot a further walled enclosed alcove. On our first ever visit, this alcove was where we found some 16 horseshoes bats, mostly lesser horseshoe, but also a couple of rarer greater horseshoe bats, and, although numbers greatly fluctuate, this feature has been one of the more regularly tenanted locations ever since.

Fitting with the dripping, cave-like ambience, our torch beams pick out overwintering peacock butterflies, convincingly camouflaged as dead leaves, herald moths, snails bleached to a limestone white, and massed clusters of flared-skirted southern pill woodlouse. One fissure in a wall contained a huddle of Eristalis drone flies. Cave spiders guard their ping-pong egg sacs high in the corners. Among the darting flattened-plane, two dimensional shadows thrown about by the torch sweeps, suddenly there is a small, darker, more solid shape.

Just as they are meant to, the horseshoe bats hang upside down from walls and ceilings, though it is still somehow eerie to see them suspended impassively there, like strange dormant leathery fruits, or tiny thumb-sized folded-up umbrellas (our other UK, non-horseshoe, vesper bats do not generally do this, but tuck themselves tightly into nooks and crevices). This year curiously the horseshoes have been clinging to the open walls quite low down - something to do with unseasonal temperatures this winter? At the alcove, one almost had to climb in to the stone coffin to see an individual bat clinging to the underside of the lid.   
Strange umbrella fruit: lesser horseshoe bat (c) G. Slack
Cave spider, probably Meta menardi, guarding egg sac
Noting the popularity of this feature, last August we set up a little habitat enhancement, in the form of a free-cycled wardrobe, modified by part-cladding with roofing felt and roughened sarking board, to simulate a few extra secluded nooks and crannies with varying micro-climates. Archaeologists of the future must interpret what this incongruous, Tardis-like installation among the early Victorian masonry might signify.
Habitat enhancement, Narnia style
Over the last 18 months we've recorded bat droppings in the wardrobe, but no roosting or hibernating bats so far. This winter our hibernation counts were 8 lesser horseshoe bats in January (temperature 2-3 degrees C) and 4 lesser horseshoes in February (temp. 9-10 degrees C); also a single Natterer's bat, with its snowy white tummy fur. Our monitoring results will contribute to national bat recording schemes. These numbers sound low, but, perhaps surprisingly, this is the 9th highest known winter roost for lesser horseshoe bats in Devon; this, in a part of the country which is a stronghold for both greater and lesser horseshoes.

It reflects how much is unknown, and still to be discovered. Horseshoe bats are keenly light averse, and will avoid streetlit areas if they can - so why have they adopted this city centre, urban site? Usually at our annual Pips & Pints bat walks in May each year, we pick up occasional lesser horseshoe calls on the bat detectors, amongst the circling pipistrelles. When we carried out some harp trapping here at the park a couple of years ago with Dr Fiona Mathews from the University, we caught all three pipistrelle species (common/bandit, soprano, and the bigger, rarer Nathusius' pipistrelle), and a lesser horseshoe; yet why don't many of the crevice-roosting species, with the exception of our single Natterer's bat this year, seem to overwinter in the Catacombs?

And how common is this phenomenon of urban horseshoe bats, let alone urban overwintering bats, when built-up areas would be expected to be unsuitably warmer than surrounding countryside? (See Tania Esteban's blog for a discussion of urban bat ecology). There seem to be few examples from the UK: the only urban horseshoe bats we've come across up to now are here in Exeter, and in Taunton; any other records would be of great interest.

If you want to find out more about Exeter's bats, please join us for our annual Pips & Pints event this May, a joint event run by DWT Exeter Local Group, Exeter City & East Devon Bats, and Devon Mammal Group.
Exeter LG (link to event details will be here in due course)
Exeter City & East Devon Bats
Devon Mammal Group

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