By Helen Hodson
At this time of year adult bats are turning their attention to finding a mate. Having spent the summer roosting separately, male and female bats are now starting to gather at mating roosts. Females have spent the last few months rearing their pups in maternity colonies while the males roosted individually or in small groups. Summer is when these flying mammals are most noticeable as colonies can sometimes include hundreds of individuals who are each busy catching thousands of insects every night.
By the end of August the pups had learnt to fly and catch their own food and the maternity roosts started to disperse. Mating begins during autumn and can continue into the winter. Males give out special calls to attract females and some species also emit a special scent. Females have a unique ability to store sperm over winter and prevent fertilisation until spring, when the embryo will develop at a rate which is dependent on food availability and temperature.
In September bats are also busy feeding and building up their fat reserves to see them through four months of hibernation. As UK bats only feed on night-flying insects, this is a critical time, as their food source will be scarce during winter. By the end of October bats will start to seek out suitable hibernation sites. These tend to be caves, mines, cellars or disused service tunnels; anywhere with a constant low temperature where they are less likely to be disturbed. In Sussex there are several disused tunnels, ice houses and other underground sites which are important hibernation roosts and are monitored each year. Natterer’s, Daubenton’s and Brandt’s bats are most commonly seen during these surveys, but occasionally an individual greater horseshoe also turns up.
By November bats will be entering periods of torpor, where their heart rate drops, body temperature lowers and metabolic rate slows. Reliant on their fat reserves for energy, they become very vulnerable to disturbance. These fascinating creatures will begin to emerge from hibernation during March and the cycle of life begins again.