by Dave Goulson
CHAPTER EIGHT
The Mating Habits of the Death-Watch Beetle
25 July 2010. Run: 39 mins 19 secs. Bit slow today, slight calf strain. I now creep quietly past the spaniel’s house. People: none. Dogs: 2. Butterfly species: 21. There was a young fox on the drive; it obviously hadn’t seen a runner before – its curiosity got the better of it, and it let me get very close. I also spotted a purple emperor butterfly sitting in a sunspot on the ground in the oak woods near L’ge Marenche. What a beauty, its wings flashing in the sunlight; how I would have liked to add one of those to my collection when I was a boy! Optimistically I used to try the time-honoured method of placing a rotting rat on woodland rides – this was supposed to attract them to feed on the exuding juices – but it never worked, perhaps because the nearest population was 100 miles away from Shropshire, where I lived at the time.
Tom lay awake and waited, in restless impatience. When it seemed to him that it must be nearly daylight, he heard the clock strike ten! This was despair … he lay still, and stared up into the dark. Everything was dismally still. By and by, out of the stillness, little, scarcely perceptible noises began to emphasize themselves. The ticking of the clock began to bring itself into notice. Old beams began to crack mysteriously. The stairs creaked faintly. Evidently spirits were abroad. A measured, muffled snore issued from Aunt Polly’s chamber. And now the tiresome chirping of a cricket that no human ingenuity could locate, began. Next the ghastly ticking of a death-watch in the wall at the bed’s head made Tom shudder – it meant that somebody’s days were numbered.
Mark Twain, The Adventures of Tom Sawyer (1876)
The very first time I slept in the farmhouse at Chez Nauche I was in for a shock, and the renewal of an old acquaintance. It was June 2003, and the night had been warm, so I woke early. While summoning the energy to get up, I heard a faint tapping. Tap-tap-tap-tap-tap, pause. Tap-tap-tap-tap-tap, pause. I knew that sound. Its source was an animal with which I had once been very familiar, having spent six months of my life studying it. It was a creature that I should not have been pleased to hear – at least not inside my house – but I couldn’t help but smile.
In early 1992 I was briefly unemployed, a dispiriting experience. I had finished my PhD on meadow brown butterflies the summer before, and had spent the autumn filling in as a replacement biology teacher at Eton College (an interesting time, but enough to convince me that schoolteaching was not my forte). I lived at the time in Didcot – someone had to – and going to the dole office in this slightly seedy town was depressing. I was madly applying for jobs all over the world, and I vividly remember being quizzed by an official as to my progress in finding work. She had a form to fill in, and one of the questions asked where I was looking for work. I explained that I had recently applied for jobs in the UK, Australia, the United States and Costa Rica, and suggested that she wrote down ‘the world’. She frowned, unhappy, for the next question was ‘Where else would you consider looking for work?’
Fortunately soon afterwards I heard about a short-term research post in the Zoology Department at Oxford University. When I was an undergraduate, I had spent a lot of time in the big, ugly, concrete zoology building on South Parks Road. The post was to work on the mating behaviour of the death-watch beetle, a creature about which I knew little at the time. Two zoologists, Martin Birch and Tristram Wyatt, had become interested in how death-watch beetles find their mates and had managed to obtain a small grant from the Association for the Study of Animal Behaviour. I applied, was interviewed by Martin and Tristram, and was delighted to be offered the job immediately.
Martin was once one of the leading experts in insect pheromones, but had tragically suffered brain damage in a car accident a few years earlier and was struggling to readjust. He had been a brilliant scientist and was still with-it enough to understand fully that parts of his brain weren’t working properly, and would get enormously frustrated and angry with himself when he couldn’t remember simple names or words. Nonetheless he was a lovely guy and great fun to work for.1
So what is so interesting about death-watch beetles? They are very small, a little over half a centimetre long, drab brown in colour and roughly bullet-shaped. They evolved to eat dead trees, and have since happily adapted to eating the timber beams in houses, which is lucky for them because in our modern, tidy world dead trees are never left lying around for long. Death-watch beetles grow incredibly slowly. From egg to adult takes up to thirteen years, reflecting the fact that there isn’t a lot of nutrition to be extracted from a dead tree, and that burrowing through it is hard work. Given enough time (two or three centuries usually suffice), they can do a lot of structural damage to a building; the roof of Westminster Hall in London nearly collapsed due to a very prolonged infestation. Unless you are interested in the preservation of ancient buildings, these beasts don’t sound terribly exciting. The clue to their interest lies in the name.
In past times, before hospitals were common, most folk died at home. Imagine a timber-framed cottage, grandad lying in bed in his nightshirt slowly dying from the palsy or some other indeterminate (at the time) but fatal disease, the family gathered around his bed in respectful silence. A faint drumming can be heard coming from the walls, from the ceiling. Five or six beats in quick succession, like someone drumming their fingers, or the tip of a pencil being tapped on a wooden table. Legend had it that this faint sound was the Devil, impatiently drumming his fingers as he waited for his chance to snatch the soul when it departed from the body. The noise was rarely heard at other times, but only because most households were rarely quiet enough except during a ‘death-watch’. Of course it was not the Devil – or at least not most of the time. As you might have guessed, it is actually the mating call of the death-watch beetle. These days few of us live in timber cottages, and those who do have almost certainly had the timbers treated against beetle infestations. The poor death-watch beetle has become a rare creature, so few of us have ever had the chance to hear its ardent call.
A death-watch beetle spends almost all of its long life as a small white grub, equipped with tough mandibles with which it chews its way very, very slowly through timber. After a decade or so it is fully grown, and in the autumn it forms a pupa. Pupae hatch into adult beetles the following spring. They live for just a few weeks as adults, and feed hardly at all. As with most insects, the adults exist to mate, and to lay their eggs. The beetles are slow-moving, clumsy and nearly blind. They have a poor sense of smell, so far as we can tell. Finding a mate presents something of a problem, to which they have evolved an unusual solution. They bang their head against the wood. Given their tiny size, they do so remarkably hard, making a noise loud enough for us to hear quite clearly, and sending vibrations through the surrounding timbers. Only the males bang their heads spontaneously. They pause in their wanderings, brace themselves with their forelegs and raise their heads, then swiftly strike their forehead five or six times within perhaps half a second, producing the characteristic drumming. If you place a death-watch beetle on a tin can it makes quite a racket.
The female makes exactly the same noise, but only in immediate response to a male, and only when she is a virgin. A solitary male drums occasionally as he walks. This was the noise I heard in Chez Nauche: Tap-tap-tap-tap-tap, with a long pause as the beetle ambled about, then tap-tap-tap-tap-tap once more. If a virgin female is about, the two beetles indulge in duets, drumming to each other. If several virgins are present, they will all reply to a male’s drumming, at which point the male becomes really quite excited, running about and beating his head repeatedly against the wood.
Now this had all been known for many years. My job was to work out how they found each other. You might think this a very trivial question – they can hear each other, so of course they can find each other easily, can they not? Well, no. Martin and Tristram had realised that these tiny beetles could not locate the source of a sound in the way we do. When we hear a noise, our brain automatically compares the volume in our
two ears and uses this to compute the rough location of the sound. It is not very accurate, but if the sound is repeated a few times, we can usually work out where it is coming from, more or less. Death-watch beetles hear with their feet, sensing the vibrations in the wood beneath them. Their feet are perhaps two or three millimetres apart, and so a vibration will not attenuate measurably between reaching one foot and the next. The beetles might obtain some directional information if they could detect the timing of the arrival of the noise at each foot, but quick calculations based on the speed of movement of vibrations through timber suggest that the vibrations would arrive at the first and last foot of the beetle just three-millionths of a second apart, which should be beyond the abilities of any nervous system to detect. So the replies of a female beetle tell a male that a virgin female is somewhere round about, but – frustratingly for him – they do not seem able to convey any information as to exactly where she is. Yet clearly death-watch beetles do mate, or else they would not still be with us. So do they just wander around and rely on blind luck to find each other (in which case isn’t all the drumming a bit of a waste of time?), or is there more to it? It was my job to find out.
The first task was simply finding some beetles to study. As I have said, these are no longer common creatures. Forests nowadays tend to be tidy affairs – dead trees are quickly cut down and disposed of, either because they might harbour tree pests and diseases or because they might fall on passers-by. This, combined with pesticide treatment of old buildings, means there are few places left to find creatures that live in dead wood. Fortunately Martin knew of an old church in north Oxfordshire, in the picturesque village of Steeple Aston, which had a large population of death-watch beetles. Every couple of days we would drive up there and search the floor and pews for beetles that had fallen from the ceiling above. We found it was easiest to borrow the broom that was propped in a corner and sweep the dust-bunnies and beetles into a pile, then sort through it. The church was always cool and the beetles always very lethargic, so it was an easy job to gather them up. We could often find twenty or thirty beetles in one visit. Back at the lab, I would place each in an individual plastic pot and keep them all in a cool room until I was ready for them.
I managed to borrow a huge disc of solid hardwood, a slice of rainforest tree, from the Forestry Department. This gave me a large, flat wooden arena on which to study my beetles. To start with, I would take a virgin female beetle – as ascertained by her willingness to reply to male calls – and place her in the centre of the disc. I would then place a male at the edge and simply watch to see what happened. The answer was, usually, not very much. Either the male would simply sit there or wander off without drumming, showing no interest in finding a mate, or he would drum, but the female would decide that she wasn’t in the mood and wouldn’t reply. Trying to get both a male and a female interested in each other at the same time was proving tricky, so I hit upon the idea of building an electronic beetle that could play the role of either male or female. It didn’t need to look like a beetle, just sound like one. After much fiddling about I ended up with a very crude gadget, the business end of which consisted of an old loudspeaker from a radio, at the centre of which I glued a plastic pipette tip. This was mounted in a clamp stand, so that the tip of the pipette touched my wooden arena. The remainder of the set-up was an untidy bundle of wires and electrical circuits which, at the touch of a button, would send a pulse of electrical signals to the speaker, which would turn them into sound, and this was transferred into vibrations in the wood by the pipette tip. After much adjustment I could mimic the drumming of a beetle and, what is more, if I so chose I could make my robotic beetle drum faster or slower, louder or more quietly, or with more or fewer beats than a real beetle.
I put the robotic beetle at the centre of the arena and began placing males on the edge. Whenever one drummed, I would reply immediately and, as far as I could tell, the beetles were completely fooled into thinking that the reply came from an eager virgin female. If the male was initially sleepy or inactive, a couple of drums from the robot usually stimulated it into life. In this way I could see how good the males were at finding the ‘female’. It soon became clear that they were pretty hopeless. They would wander around, drumming their heads and listening for a reply, often taking hours to approach the location of the robot. I filmed their behaviour and spent many hours recording their tracks, in an attempt to discern any kind of pattern.
The answer turned out to be fairly straightforward and unexciting. When a male beetle is getting replies from a female, he tends to walk a short way and then drum his head again. He listens carefully to the reply. If it is louder than the previous one, he knows he is going in roughly the right direction, and so he keeps going, pausing at intervals to repeat his call. If he is going in the wrong direction, her reply will get quieter – when this happens, he tends to turn round and head back the way he came. If she stops replying entirely (suggesting that he has walked so far from her that she can no longer hear him), he tends to turn round and go back. By this very crude mechanism, he should on average get slowly closer to her. I wrote this up as a scientific article and sent it off – my job was done. But I still had a few weeks of funding left, and I had become intrigued by something else.
When a male finds a real, virgin female he invariably clambers straight on to her back and attempts to mate with her. The males are not very bright, so they often climb on to the female backwards (to be fair, the front and back ends of the female don’t look much different). He encourages her to mate by continuing to drum, but now of course he is whacking his head against the back of the female’s head or, if he is the wrong way round, against her backside – an interesting form of foreplay. At the same time he extrudes his genitalia and probes away hopefully. The interesting (but perhaps not entirely surprising) thing is that often this less-than-subtle approach is not successful. Mating can only happen if the female is willing, because she also has to extrude her genitalia from under her carapace. If she refuses to do so, there is nothing the male can do about it, but continue to drum away and prod at her with his penis. If this doesn’t get him anywhere, after a few minutes he usually gives up and wanders off to find a more obliging female.
I noticed fairly quickly that the larger males were more likely to succeed in charming a female than the smaller, weedier ones. I took to weighing the males before and after mating, and discovered something that seemed quite astonishing to me. On average, during copulation the males were transferring 13 per cent of their body weight to the female, and up to 20 per cent in some extreme cases. Exactly what the material was that was being transferred I never found out, but presumably it was sperm and associated juices. This is roughly the equivalent of a human male producing fourteen litres of sperm in one go – not an entirely pleasant prospect. Large male beetles tended to produce more, so by choosing the heaviest males to mate with and rejecting the small ones, the females were ensuring that they received particularly large packets of sperm. Why might the females do this?
It is not uncommon in other insects for males to transfer nutrients to the females during mating, as well as sperm. In some creatures, such as hanging flies and scorpion flies, the male catches a prey item and uses it to persuade the female to have sex with him, a so-called nuptial gift. The larger and more impressive the prey item, the more likely she is to be impressed. In some crickets, the male impregnates the female with sperm during mating, but also glues a large, sticky ball of nutritious gloop to her bottom, which she promptly eats. In all of these cases the food helps the female to produce more or larger eggs, which with luck will be fertilised by the male, so the nutrients go to benefit his own offspring. This nuptial gift might be particularly important in creatures like death-watch beetles where the adults don’t get to feed much, if at all, so the females have no other source of nutrition. In this situation it might be vital for the female to choose a male that could provide the most nutrition, since this would help her to produce the maximu
m possible number of offspring. Obviously female death-watch beetles get an excellent opportunity to judge the weight of a potential mate when he is sitting on top of her, whacking her in the back of the head.
I experimented with playing tricks on the females. I found that I could fool a female into mating with even the smallest and weediest male, simply by putting a bit of Blu-tack on his back. The Blu-tack made the male heavier and that was all it took, although the poor female then received a most disappointing package.
One interesting side-effect of the large nuptial gift produced by male death-watch beetles is that it makes them weigh substantially less than they did before, which in turn makes them much less attractive to females. Even the biggest male could rarely persuade a female to mate with him if he had mated before, however hard he tried. The only way I could make him appealing to females was once more to make him heavier with a blob of Blu-tack. And once again the females must have felt short-changed, because whatever was being transferred to the female, the male could produce very little of it when he was mating for the second time. So it seems that in natural situations most male death-watch beetles probably only ever get to mate once, and then they have shot their bolt – unable to seduce another female because they weigh too little.