Bats and Hibernation

There are approximately 17 species of bat in Britain; two belonging to the Rhinolophid family (the horseshoe bats) and the other 15 (or so) belonging to the Vespertilionidae family (or vesper bats meaning ‘evening’).  Some other species may be resident here at very low numbers or arrive here from abroad having been blown off course or transported in with goods such as timber.  Migration may also account for some species occurrence in the UK although this is still being investigated.

All British bats eat invertebrates, preferring prey such as chafers, beetles, moths, craneflies, caddis flies, lacewings, midges and mosquitos, small wasps and spiders.  A pipistrelle bat, one of the smallest UK species, can eat up to 3000 midges in one night!  Bats provide an extremely valuable pest control service.

Roosts

Bats use a range of structures and features for roosting, such as buildings, trees, fissures in quarries, caves, mines and bridges.  Some species prefer to roost in trees as opposed to buildings and vice versa.

tn_Noctule (stock image purchase)Bats use several different roosts throughout the year and the choice of roost site depends on a number of factors including the season, weather and locality to seasonal foraging habitat.  They often roost in groups (colonies) of the same species.  Summer roosts (including maternity roosts where bats gather to give birth and rear their young) tend to be warmer and very close to good summer foraging habitat so the pups are not left alone for long periods at a time.

Spring and autumn roosts are sometimes called ‘transitional roosts’ where the bats congregate in smaller numbers either before the summer maternity or after the winter hibernation periods.  These roosts can also be used for mating and socialising.  Spring roosts can also be used by pregnant bats as pre-maternity gathering sites.  Winter roosts, also called hibernation roosts, are used when the weather turns cold and the bats need to become torpid or hibernate to conserve energy when food resources (invertebrates) are scarce.

It should be noted that individual or small numbers of bats often use any of the above roost types throughout the year depending on their energy and social requirements.  They also return to the same roosts year after year so even when bats are absent from a roost, it is still considered to be ‘active’.  Roosts, in all their forms, are obviously very important to the survival of individual bat colonies and populations.

 Winter hibernation

 Like some other British mammals, such as dormouse and hedgehog, bats become torpid and/or hibernate over the winter months (usually between October and March/April).  The purpose of this is to increase the animal’s chance of survival over winter or during periods of cold and bad weather at other times of the year, i.e. when food tends to be in short supply.

In preparation for winter, bats build up their fat reserves, which act as insulation against cold and heat loss and also as the main energy store through the hibernation period.  Torpor is when a bat reduces its’ core temperature and metabolic rate and enters a state of relatively deep ‘sleep’.  Bats can wake up from torpor daily or every few days in the winter.  Hibernation is a more advanced state of torpor where the bat can remain in a very ‘deep sleep’ for a number of days or weeks at a time.  Bats often arouse from torpor/hibernation to perform a variety of functions, including regulating metabolic activity, sleep, defecating, urinating, drinking, mating, moving roost and foraging when the weather and temperature is suitable.

Bats are long lived animals and, depending on the species, can live between 10 and 20 years (occasionally over 30 years).  This is known from long-term studies of colonies where individuals are fitted with an arm ring

containing a unique identification number, which shows where and when they were ringed.  These studies can also give an idea of how far bats travel between roost sites.  The longest known distance for a horseshoe bat is 176 km, a considerable flight for an animal weighing only 17–34g!  Their ability to hibernate may contribute to the longevity of bats.

Hibernation sites

Historically bats used caves, rock fissures and trees for roosting.  More recently, bats have taken to roosting in a wide range of ‘artificial’ human-built structures such as houses (e.g. roof structures), cellars, mines, ice houses, grottoes, old railway tunnels and other underground structures including archaeological sites.

Many hibernation roosts are sites that offer a range of stable and variable environmental conditions (micro-climates) and many crevices and recesses.  Temperature and relative humidity regimes within hibernacula appear to be the most important factors in determining their use by bats (different species require subtly different conditions).  The quality of the surrounding habitat to support winter foraging is also a very important factor.

Protection of hibernation sites

When in torpor or hibernating, bats are very vulnerable to disturbance.  They cannot respond quickly to danger or stimuli and it can take some time to rouse and become awake enough to fly (up to 30 minutes depending on their state of torpor/hibernation).  The waking up process causes some of the valuable fat reserves to be used, so they either need to forage to replenish the fat reserves, or if it is too cold and they cannot forage, they may starve.  Disturbance of hibernating bats is, therefore, a significant risk to their survival.

In addition to creating and monitoring roosts, bat conservation has over the last 20 years or so also focused on protecting important hibernation sites.  Such protection may include the installation of grilles over cave, mine and tunnel entrances.  The grilles comprise metal bars that are spaced wide enough apart to enable bats to fly freely into and out of the roost whilst preventing people from gaining access.  These grilles have the added advantage of also protecting the bats from predation (lesser horseshoe and some other species sometimes roost close to ground level and predation by badgers and foxes is not unknown).

If you have an underground structure on your land or site, or if you know of one, and it is not currently being monitored by licensed bat ecologists, we would very much like to know about it.  We can provide information and advice and potentially undertake a survey to see if there are any bats present, identify the species and determine how they are using the roost.

tn_Barbastelle (stock image purchase)

Further reading

Altringham, J.D. (2003).  British Bats.  Harper Collins (London).

Bat Conservation Trust (2007).  Bat Survey Good Practice Guidelines. Bat Conservation Trust.  London.

Bristol University: http://www.bio.bris.ac.uk/research/bats.

Dietz, C., von Helversen, O. & Nill, D. (2009).  Bats of Britain, Europe & Northwest Africa.  A & C Black (London).

English Nature (2003).  Managing landscapes for the greater horseshoe bat.  English Nature.

English Nature (2004).  Bat Mitigation Guidelines.  English Nature.

Mitchell-Jones, A.J. & McLeish, A.P. (Eds) (2004).  Bat Workers’ Manual.  Joint Nature Conservation Committee.

Ransome, R.D. (1990).  The Natural History of Hibernating Bats.  Christopher Helm (Publishers) Ltd (Kent).

Swift, S. M. (1998). Long-eared Bats.  Poyser Natural History (London).

 

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