Bats are the only mammal many people will see in a cave.
During summer bats move around and have a selection of roosting sites, often old trees, buildings etc. They may use caves to roost during this time and the females may use them as a nursery for the young. If food is scarce they sometimes go into torpor (controlled lowering of body temperature) much like a hibernating bat. An active bat will have a temperature of about 39 degrees and a heart beat of 1000 per minute. Bats live up to 15 years, they generally eat large insects.

HIBERNATING BATS
Bats spend more time in torpor as food becomes scarce. They traditionally hibernate from October to late April as their food source (insects) disappears, but with warmer winters more insects are around and the hibernation period is becoming more irregular. In late autumn they put on weight (not by eating more but by spending more time in torpor) so that their fat reserves will last them for up to five months of hibernation. Once a suitable hibernation site has been found (the air flow, humidity and temperature must be fairly constant – cooler than their summer roosting sites) they crawl into a crevice (only some species hang upside down). In caves they may be far underground where temperature varies little, but in a position where they are able to sense subtle changes in air temperature signifying changes outside. Heavy bats prefer warm roosts, skinny bats cooler roosts. Old wise bats regulate their temperature within a narrower band than youngsters and seem to roost singly.
In hibernation a bat may breathe once an hour, have a temperature as low as 2C, and a pulse of 10 per minute. Reducing its temperature means it uses less energy.
By November they may only wake when it is warmer and they can feed. Each day throughout hibernation the bats pulse rate slowly rises until it seems to make an unconscious decision (in mid afternoon) either to carry on raising its pulse and wake up, or allow it to drop again and return to hibernation. Every 10-15 days throughout winter they will wake to feed, urinate, just shuffle their feet, drink or fly off to roost somewhere else (up to 30 miles away). As winter moves by the bats fat reserves dwindle and they choose progressively colder roosts. In November they may go for a 12C roost, by March they prefer a 6C roost. In April they spend less time asleep and choose cool roosts close to feeding grounds, after digestion the bat allows its temperature to drop quickly.
If they are disturbed, they slowly wake up – 30 minutes after disturbance the heart rate will have gone from about 20 to 600 beats per minute and the body temperature from about 10C to 36C. This must use a surprising amount of energy, energy the bat cannot afford to use up unnecessarily as the valuable fat reserves are depleted with little hope of replenishment. Too many disturbances will mean that the fat reserves will not last the winter and they literally starve to death. In a state of hibernation bats are so torpid that they appear dead, but it takes very little to disturb them. Bats are quite specific on what temperature they want to hibernate at. Heat from cavers’ bodies will cause a rise in temperature which is detected by the bat. In a large or draughty passage this effect will be negligible, but in a small chamber or with a group of cavers the effect may be enough to rouse the bat. Heat generated by a caver is probably the most common cause of bat disturbance underground. Shining bright lights on the bat and the noise of a flashgun may be sufficient to wake it up.
Many bats in Britain are on the northern limit of their distribution - a small climate change could have major impacts.
Of the 16 species of British bat, 13 are regularly recorded in the Mendip district, with different species using caves to varying degrees.

The HORSESHOE BATS have a cone shaped nose leaf and hang by their hind feet with their wings wrapped around themselves.   Both species of horseshoe bat are relatively common in Mendip caves and will be the most commonly noted bats in caves as they tend to hang in the open in passages and chambers.  The Lesser Horseshoe bat (Rhinolophus hipposideros) is 15-19mm in length and weighs just 5-9 grams, making it one of our smallest bats. In winter their roost in the cave is usually in open or exposed positions, and not in clusters. They prefer cooler sites than the Greater Horseshoe and can often be seen sharing the same winter roosts but in more exposed parts of the cave ceiling. They are small and very agile and can be found deep underground. The coolest sites in a cave are near the floor so these bats may be found low down on boulders near your feet as well as hanging on the walls and ceiling. Following a decline in the last 100 years the population of lesser horseshoes now appears to be stable.  Lesser Horseshoes are known to roost in the caves in Cheddar Gorge and Burrington Combe, in the smaller caves in Ebbor Gorge and in the caves on eastern Mendip, close to Stoke St. Michael.
In summer they emerge from their roost 30 minutes after sunset to feed in sheltered deciduous woodland.
The Greater Horseshoe bat (Rhinolophus ferrum-equinum) prefers warm roosts. Their summer nursery roosts include a few caves where they congregate, tightly packed, in high domed chambers to raise the temperature to 30C, but this is increasingly rare in Britain. In autumn, as the nursery breaks up, the adults seek out cooler areas while the young may stay until October. Males may roost underground in summer.  It prefers temperatures of 5-11C and it is quite common for it to wake and feed on warmer winter evenings. On the Mendip Hills the Greater Horseshoe is known to roost in much the same locations as those mentioned above for the Lesser, although in Ebbor Gorge it appears that the Greater Horseshoes are found in Wookey Hole, whilst the Lesser roosts in the smaller caves.  Nationally the Greater Horseshoe’s numbers have declined massively and it could be vulnerable to extinction in Britain.  Two Special Areas of Conservation have been set up on the Mendips due to the large colonies of horseshoe bats, the largest ones use mines and other artificial cavities. The Mells Valley has a large Greater Horsehoe maternity roost.  The North Somerset and Mendips SAC was set up due to the large number of mines and caves in the area providing good roosting potential for both species.

The Evening or VESPER BATS are a family that includes Pipestrelles and Myotis, these have a muzzle similar to a dog and prefer to hang against a surface like a wall or tree. These bats fold their wings alongside their bodies. If you see a small bat flying outside before dark it is probably a Pipestrelle.
Pipestrelles (Pipestrellus spp.) are Britain’s most common bats but are rarely found in caves.  There are three species of Pipestrelle the Common, Soprano and Nathusius’, which are very difficult to tell apart.  The first two species might be found in caves, mines and tunnels in the winter.
Barbestelle (Barbestella barbastellus) - rarely seen, it uses caves and mines for hibernation.
Brown long eared bats (Plecotus auritus) hunt in dense vegetation. This bat and the Barbestrelle prefer cold caves to hibernate – down to 0C. This means they are more likely to be found hibernating near the entrances to caves.  The Grey long eared bat (Plecotus austriacus) is a rare species, not generally recorded on Mendip.
Noctule (Nyctalus noctula) – Britain’s largest bat, if you see a large bat flying in the open out before dark it is probably a Noctule. They roost almost exclusively in tree holes in the UK but occasionally enter caves in autumn.
Leisler’s bat (Nyctalus leisleri) generally prefers holes in trees and buildings and does not often frequent caves, although might be found in crevices in winter.
The Serotine (Eptesicus serotinus) is a large bat, which generally inhabits buildings and hollow tress but which has been recorded as hibernating in caves and mines. One has been recorded hibernating in the entrance to Goatchurch, along with Greater and Lesser Horseshoes.

Myotis bats prefer to hibernate at temperatures between 2 and 5C. They may be seen swarming round cave entrances in autumn. Many Myotis bats hibernate in crevices including wriggling into loose scree/boulders – so a bat motionless by your feet might be hibernating, not dead! Natterer's bats (Myotis nattereri) hibernate in caves, mines, cellars and tunnels, usually squeezed in a crack or crevice. They hunt in dense vegetation. Daubenton's bat (Myotis daubentoni) hibernates in caves, mines, tunnels in crevices as well as on open walls. They hunt low over open water and so are found near such habitats. Whiskered bats (Myotis mystacinus) hibernate singly in caves on open walls and in crevices. The closely related Brandt's bat (Myotis brandti) appears to have similar habits to the Whiskered bat but is not generally known from the Mendips.  The rare Bechstein’s Bat (Myotis bechsteini) hibernates in caves, mines, cellars and tunnels but is the third species not widely recorded on the Mendip Hills.  The Alcathoe bat has recently been confirmed as living in the UK. It is very similar to Brandt's and Whiskered and so presumably can be found in caves. 

Bat guano is important in the cave ecosystem, although in the caves of Mendips it is rare to find a large bat roost and hence significant guano deposits. Bats eat half their weight in insects each day producing large amounts of nitrogen rich guano. This guano is food for beetles and other forms of cave life.  Bats often carry parasites such as ticks, fleas and mites.

BATS AND THE LAW
It is illegal to intentionally or recklessly disturb a bat while it is occupying a structure or place that it uses for shelter or protection. Under the Habitats Regulations it is an offence to deliberately disturb a bat (this applies anywhere, not just at its roost). If more than one bat is involved, the fine is £5,000 per bat.
Bat workers acknowledge the important help they get from cavers to know what is going on in the roosts and hibernaculums used by bats that are inaccessible to non-cavers. Cavers can assist bat conservation groups by not disturbing the bats and by reporting their findings. See the Bat Conservation Trust for more information on bats.

OTHER MAMMALS
You may see evidence of badgers, foxes, otters, mink or rats using a cave entrance for shelter.