Surprising fact: bats make up almost a quarter of the mammal species found in the UK. Who would have thought it? Though seldom seen by many, we have seventeen species of breeding bat hurtling through our night-time airspace. Being nocturnal, hidden during the day and flying mostly out of sight at night, bats tend not to be high on the list of UK mammals people are aware of, and their tendency to crop up in horror stories and at Halloween parties gives them a certain reputation. It’s a bit unfair really, given they are totally amazing. And as I stood in the wood at dusk this week, feeling the draft as bats flew past my face, hearing the beat of their wings and watching their aerial agility, I was reminded of this absolutely fascinating world of bats.
A good place to start: bats are mammals – and they can fly. That’s pretty mind bending in itself. They are, in fact, the only mammals in the world capable of powered flight, and they have managed this with surprisingly little structural change from a typical mammal; they have a backward bending knee which helps them in landing and in hanging upside down, and they have elongated finger bones from which the flight membranes are stretched (bats’ Latin name Chiroptera translates as hand-wing).
With these two relatively minor adjustments to the mammalian structure, bats can fly, but they have a few other sneaky adaptations which aid them in their flight. Their wings are just brilliant; a double layer of skin enclosing the elastic and muscle fibres, nerves and blood vessels needed to keep this living aerofoil in tip-top condition. The bat wings I have touched have felt soft and warm, so incredibly delicate (the closest I can come up with in comparison is a new puppy’s ear, but the point is, the wings are a living part of the bat unlike a bird’s wing which is predominantly made of feathers). A bats wing can heal and grow and is sensitive and reactive. Then there is the bat’s muscles; up to twelve percent of the bats lean body mass is made up of the chest muscles used to power flight. The bats wings are beating between nine and fifteen times per second, so oxygen needs to be kept flowing to these muscles. Bats are able to increase their oxygen uptake by twenty times from resting to flying, and if that still isn’t enough, the muscles are rich in a pigment (myoglobin) which stores oxygen for when the blood supply can’t keep up with the muscle’s demand for it; the bats blood in itself can carry fifty percent more oxygen than that of typical land mammals… bats are seriously well evolved for flight.
There is a bat species somewhere that will fill almost any major feeding niche – some are carnivores, some eat fruit, some feed on blood, others on nectar. There are bats that specialise in eating fish, and others that go for pollen. But the majority of bats are insectivorous and that includes all of the bats found in the UK. Which brings me to another of the UK bats incredible adaptations: echolocation.
Not all bats use echolocation, but most do, and in particular, those bats trying to catch insects while both the bat and the insects are on the wing. Echolocation is the process of creating sound and then analysing the pattern of reflection of that sound as it bounces off surfaces – in this case, insects and the bats surroundings. Bats have large sensitive ears and growths of skin on their faces which focus in the reflected ultrasonic noises, helping the bat work out precise information about the location, speed and direction of movement of their prey. Imagine the processing power of the brain that interprets that sound and is rapid enough to convert it into information the bat can use to catch a flying insect while on the move itself…
In the UK, bats tend to spend the day in hollow trees or in undisturbed parts of buildings, places where they are protected from direct sunlight. Seasonally, they will occupy the same roosts, though these could be a regular sequence of different roosts over the course of a year. Bats hibernate in places that are buffered against rapid temperature change, but that have a high relative humidity such as caves or tunnels. Many of these roost and hibernation sites have disappeared over the last few decades, this being one of the reasons that bats are probably the most endangered animals in the UK at the moment. The other reason is declining food sources – insecticides have reduced insect numbers and contaminated the remaining insects so the build-up of toxins within the bat as it eats the insects either poisons the bat, or interferes with its reproduction; herbicide use has reduced plant biodiversity which has a knock-on effect on insect numbers too.
Evolutionarily speaking, it’s hard to see where bats have come from. They don’t fossilise well, but the probability is that they have evolved from a nocturnal, insectivorous, tree-dwelling ancestor which glided from branch to branch and rested upside down. There is certainly an element of the primeval as we creep through the wood at dusk, on the look out for the wood’s more hidden inhabitants. A few days ago we were given a treat. As we walked down the path from camp on the way home, we were suddenly surrounded by bats. They were using our path as a feeding corridor, flitting backwards and forwards at head height. Without a bat detector (which makes the bat calls audible to us) it is impossible to say for certain, but these were probably pipistrelle bats, the most common and widespread of the British bat species (we think we have four species in the wood). It was such a special experience; I can’t remember being that close to bats in flight before. We stood and were part of the bats world for a short time, watching and hearing the bats flash past – they were so fast, it was hard to get a clear picture of them, more an impression of deft winged acrobats, of speed and an ability to change direction with almost impossible precision and angles. There was no chance of keeping track of a single individual; their speed and unpredictability was truly phenomenal.
A few months ago we put up forty bat boxes throughout the wood, places where bats can roost during the day. Is it a co-incidence that there are more bats in the wood than we have ever seen in one place here before? Maybe, but maybe not. Bats are astonishing, and hopefully in managing the wood with bats in mind, we will have more chances to meet these wonderful creatures up close – and perhaps next time we will have our bat detector with us!
Note: The photo at the top is of a whiskered bat, a species we think we have in the wood. This is a rescue bat which had a successful recovery and release in our garden.