As the temperature starts to drop we are all dusting off our winter coats and donning our scarves in order to brave the cold. Similarly, the bats of the UK have begun to fatten up in the face of the coming winter months. Autumn is a crucial time in the bats’ calendar, balancing mating and eating in preparation for hibernation. There are 17 breeding species of bat in the UK, and mating usually occurs in the autumn months, but the females do not give birth until spring. There are two known methods for this delayed birthing. The female either stores the sperm over the winter or fertilisation takes place but the fertilised egg is not implanted into the uterine wall until the mother wakes up after hibernation. Mating can be very energy intensive; finding a mate can often mean flying great distances in order to increase the pick of genetic material, however post hibernation the most pressing thing will be to feed to replenish the used energy reserves.
Given the nocturnal nature of bats, much of their behaviour and ecology remains a mystery to us, making ongoing studies very exciting to any bat enthusiasts. Recently, there have been a number of studies into the phenomenon of autumn swarming that has been observed in the cave dwelling bats of the UK. This activity is thought to be a social gathering, of mostly males, as a means of meeting a mate with different genetic material to ones in closer proximity. Many of the individuals have travelled great distances, but do not use the location for roosting. The uneven female male divide is thought to be because females will only need to be mated with once, but the males will return in order to mate with as many females as possible.
All UK bats are insectivorous, eating midges, moths, and beetles, among other things. Flying is very energy intensive and so they need to eat a lot of bugs to get enough energy; common pipistrelles (which only weigh between 3.5 and 8.5g) can eat up to 3,000 insects a night! Most prey is caught and eaten on the wing, but there are a number of different hunting methods; some species using open mouths, some scooping into the tail membrane, and some using their feet. However, all bats use echolocation to ‘see’ their prey, firing high-frequency sounds out into the air and using the sound that bounces back to locate and identify prey. The highest pitch sound that human ears can detect is 20kHz but a bat can detect noises up to 110kHz! The noctule is the loudest UK bat, and creates sounds which are four times louder than the noise limit that is legally allowed in a nightclub!
So how do they store energy for hibernation when it takes so much for flying and mating? Well first they will just eat more, additionally they begin to enter a hibernation-like state known as torpor. This lowers their breathing and body temperature, consequently reducing their metabolic rate and energy consumption. Bats might remain in this state for just an evening or a couple of days if it is very cold; but will continue to fly, feed and mate on the warmer nights. When the temperature drops and the food reserves run out they will enter full hibernation. The ideal winter roost for a bat must maintain a constant, cool temperature, as a rise in temperature could wake them from their torpid state, which ideally they only want to do in spring when insects begin to emerge. Underground caves are a common choice, however it is still unknown where a lot of the UK bats go in winter, common pipistrelles for instance are not all accounted for in the winter months!
The secretive world of bats can feel so special when you are lucky enough to observe it in some way. Obviously, this can be difficult as we cannot see or hear them but there is a way to take a peak! With a bat detector it is possible to go out and not only determine if bats are there, but also what species, and even listen to their conversation, whether it be hunting or socialising. Bat specialists have even been able to determine what certain calls mean. Most counties have a local bat group where it is possible to go out and observe behaviour, and play an active role in the conservation of these mysterious, protected creatures.
For more information about bats visit the Bat Conservation Trust website.
Words by Asia Roberts-Yalland
Asia Roberts-Yalland 27. Based in East Anglia, works with seeds and plants. BSc Biology, MSc Managing the Environment from Aberystwyth University. Environmentalist and nature lover.
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