A Sting in the Tale (21 page)

As with most insects, the male bee has a powerful pair of claspers at the tip of his abdomen, with which he grips the female, and a tube-like endophallus which is inserted into the female and carries the sperm. Once his claspers are locked on, he lets go of the female with his legs and ‘lies back', legs folded in front of him as if meditating, with the only point of contact between himself and the female being the genitalia. Mating typically lasts for about half an hour. Sperm is transferred in the first minute or two, but after that the male pumps a sticky glue into the queen, forming a ‘mating plug', which prevents her from doing it again, and explains why queen bumblebees generally mate only once.

As you may have gathered, in general I think that bumblebees are among the most fascinating of creatures, and their mating habits are certainly intriguing and enigmatic, but in this one area I have to concede that their cousins the honeybees have them trumped. Unlike bumblebees, honeybee queens mate a dozen or so times, all during a single nuptial flight made early in life. Each young queen releases a pheromone which is enormously attractive to the males – known as drones – who eagerly swarm after her. They mate in mid-air, the male grasping the female's abdomen with his claspers and then explosively squirting sperm into her. This explosion produces an audible pop and ruptures the male's genitalia, usually causing fatal damage so that he falls to the ground and quickly expires, leaving behind fragments of his genitalia still attached to the queen. She continues her flight, and is swiftly mounted by a second male who scrapes off the remains of his predecessor before repeating the process. The new queen continues until she has stored enough sperm for a lifetime of egg production, at which point she returns to the nest, leaving behind a trail of dead lovers.

CHAPTER THIRTEEN

Does Size Matter?

I beg a million pardons. Abuse me to any degree, but forgive me: it is all an illusion about the bees. I do so hope you have not wasted any time for my stupid blunder – I hate myself, I hate clover and I hate bees.

Charles Darwin, letter to John Lubbock, 1862

In 2001 I took on a new PhD student named James Peat. James had been an undergraduate at Southampton, and my tutee. From a slightly posh family in Dorset, James was a bit of a charmer and a ladies' man, often sporting amusing facial hair. I'd always found him to be very entertaining, with a quirky, lateral view of the world, and I thought he might be just what my research group needed. Among other things, he is the only man I've ever known who insisted on making all of his own trousers.

I decided to let James loose on a topic which had intrigued me for many years. Bumblebees differ from their close relative the honeybee in that the workers are enormously variable in size. The largest workers are roughly ten times heavier than the smallest, yet all live alongside one another in a single nest. Most insects vary little in size: one adult peacock butterfly is much the same size as another; and the same can be said of grasshoppers, dragonflies or stag beetles, for example. Even the relatives of bumblebees – wasps, stingless bees and honeybees, – tend to be more or less uniform in size. The only group of insects which exhibits something similar to bumblebees are the ants. In some ant species, most notably leafcutters from the rainforests of South America, the workers vary enormously in size and this relates to the jobs that they perform. The biggest workers are monsters, with huge scimitar-shaped jaws suspended from large heads packed with muscle. Their job is to defend the nest against large vertebrate predators such as anteaters or insectivorous birds and lizards, and they can deliver a ferocious bite. The medium-sized workers forage for food, forming trails through the forest and up the trunks of trees to the foliage, where they snip semicircles of leaf to carry back to the nest. Collectively, they can strip a huge rainforest tree of leaves in a single night, so each night they set out in a different direction.

Tiny workers tend to the queen and brood, and also chew up the leaves brought in to the nest to feed to their fungus garden, in chambers deep underground. Leafcutter ants cannot digest leaves directly, so they have evolved a remarkable symbiosis with a species of fungus found only in their nests. The white, fluffy fungus turns unpalatable leaves into digestible fungal fronds, which are harvested by the tiny workers and fed to the rest of the ants in the nest. Some of the tiny workers also hitchhike on the backs of the foragers collecting leaves, and are thought to protect them from parasitoid wasps and flies, which would otherwise inject their eggs into the foragers.
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It clearly makes sense for different workers to have bodies suited to their jobs; the tiny workers would be hopelessly ineffective if called upon to attack an anteater, while the giant soldiers would not be small or nimble enough to care for the fungus garden. Does a similar line of reasoning explain why bumblebee workers vary so much in size?

The first step towards finding out the answer to this question was to determine if different-sized bees do different jobs. In fact it had long been suspected that the smallest bees tended to stay in the nest, and the bigger ones went foraging for food, but this had not previously been quantified. To do so, we simply reared up thirty buff-tailed bumblebee nests, then placed them out in the field and opened the doors to their boxes. These boxes consisted of an outer cardboard shell with a removable inner plastic box, and they were fitted with two doors, one which allowed bees both in and out, and the other with a valve which only allowed them in. We left the nests to settle down and forage naturally for four weeks, after which we returned in the middle of the day, when foragers were likely to be out doing their thing, and shut the doors. We then replaced the inner plastic box with an identical but empty one, and opened only the door which allowed bees in. Foragers returned home for the next few hours, loaded with food, but found that they had entered an empty nest from which there was no escape. James then had the tedious job of using Vernier callipers to measure the size – the width of the thorax – of all the bees from each nest, both those that were inside the nest when we sealed the door, and those that had been out foraging; about 4,500 bees in total, roughly one-third of which had been foraging.

This was not a perfect experiment; I am sure you will have realised that many foraging bees might have happened to be in the nest at the time we shut the doors. Nonetheless it showed very clearly that the very smallest workers were all in the nest, suggesting that they rarely if ever go foraging – in fact, unless you look inside a bumblebee nest you will never see these diminutive creatures. Medium-sized workers seemed to include a mix of nest bees and foragers, while the very largest workers were almost all foragers, just as had been suspected.

This is all very well, but why are foragers bigger than nest bees? It is easy to understand why ants which are to defend the nest must be large, but less easy to see why carrying food necessitates being large. You may well be thinking, ‘Surely a large bee can carry more?' It would be surprising if they could not, but it isn't as simple as that. Big workers presumably require more food to rear in the first place. Let's suppose, for the sake of argument, that it takes the same amount of food to rear one big worker or two small ones of half the weight. Further, let's suppose that the big worker could carry twice as much food. On each foraging trip, the two small workers would bring back the same weight of food between them as the larger bee. However, they would be back sooner as they would only have to visit half as many flowers to gain a full load – all else being equal – so they would be able to make more trips per day, and should be able to bring back more food in total. It would only be worthwhile to produce large foragers if they were somehow a lot better at gathering food than small ones, or if they had some other significant advantage of which we were unaware.

James set out to discover how efficient bees of different size were at gathering food. His experimental set-up was simple. We rigged up a buff-tailed bumblebee nest in my lab with a clear plastic tube attached to the nest door. When the door was opened, bees could walk down the tube, through a clear plastic chamber constructed over the pan of an electric balance, along a further tube, and out through the window to the outside world. We could individually identify every bee since we had carefully glued a numbered plastic disc to the thorax of each. This is an enormously fiddly job. The glue takes a minute or two to set, during which time the bee tries to dislodge the disc by hooking a foreleg over its back. As often as not it succeeds, and the process has to be repeated. Not infrequently the bee pushes the sticky disc forwards on to its head, effectively covering its eyes, and then blunders around bumping into things. I once tried using quick-drying superglue instead, but after ending up with a bee with its foot stuck to its back I quickly gave up.

As each bee left the nest over the pan of the balance, its number and weight were manually recorded. Initially, bees tended to fly across the chamber rather than walking over the pan, but this problem was solved by covering the chamber with transparent red plastic film; bees can't see red light very well so the chamber appeared dark, and bees won't fly in the dark, so they were forced to walk. Once they left the tube protruding from the lab window, the bees invariably hovered around for a minute or two, presumably trying to memorise the location, before disappearing off into the suburban gardens of Southampton. It was very exciting watching the first few bees depart, and we sat about eagerly awaiting their return.

After about half an hour, the first bee appeared back at the window, with large balls of pollen on its legs. Unfortunately it didn't seem able to find the end of the plastic tube, so it flew backwards and forwards along the long row of glass windows. It was soon joined by others, and quickly a cloud of confused bees built up outside the window. It became apparent that there was a flaw in our plan. The lab was on the third floor of a very sizeable and ugly concrete building, with seven floors in all. Viewed from the outside, there were seven rows of identical windows, one above the other, and each row was perhaps 50 metres long. All looked much the same, and the little plastic tube protruding from the corner of one of them was exceedingly inconspicuous. Our poor bees knew their nest was there somewhere, but couldn't find the correct window. Before long the frustrated bees started coming in any open window, often a floor or two above or below my own, invading offices, biochemistry labs and so on. Since the bees all had numbered discs attached it didn't take anyone long to realise who was to blame, and we soon found ourselves bombarded with phone calls and having to run around the building with nets and pots to catch our strays. We shut the nest door so that no more bees could go out, put back as many as we could recover, and sat down to discuss what to do. Eventually we decided on attaching a large orange funnel to the nest tube, providing an eye-catching target for returning bees. To make life even easier for them, Ben Darvill, then an undergraduate student, somehow acquired a traffic cone and gaffer-taped it to the outside of the building just below the correct window. How he got it up there I never found out, since it was too large for him to have put it out through the window. When I left Southampton five years later it was still there.

The funnel and traffic cone served their purpose and bees began foraging from the nest, running out through the plastic tubes and returning laden with pollen or nectar. Once this was all working, James and a team of undergraduate helpers set about recording and weighing every bee that left or came back into the nest. Someone had to be by the nest at all times during daylight hours, so they set up a rota between them. The nest lasted for about one month, after which we replaced it with a second, and eventually a third, so the team, between them, spent three months sitting at the lab window with notebook in hand. The difference between the outgoing and incoming weights of each bee allowed us to calculate how much food they had gathered on their trip, and since we also knew how long this had taken them, we were able to calculate their efficiency in terms of forage gathered per minute. Some bees were notably more dynamic than others, sprinting down the tubes, returning within a few minutes with full loads, then immediately setting off again. Others ambled along the tube, stopping occasionally, sometimes getting to the exit funnel and then turning back as if they'd changed their mind and decided they didn't really fancy going foraging after all. The keen bees were enormously impressive. Bumblebees carry pollen on their legs and nectar in their honey stomach, a chamber which, when full, almost fills their abdomen. Some of them brought back up to 150 milligrams of food, close to their own body weight, having gathered it in twenty minutes or less, and they did so many times during the day.

Bees got better at foraging with experience. On their first trip from the nest, many came back weighing less than when they set out, presumably because they had flown about burning energy and failed to find any food. Over time their success rate increased, and on average it took about thirty or so trips before they reached peak efficiency. Presumably during this time they were learning about how to navigate through the landscape and experimenting with different flowers to see which were most rewarding. On their first foraging trips bees tended to collect nectar, usually returning with no signs of pollen on their legs, but as they got older and more experienced they turned to collecting more pollen. I suspect that this is because drinking nectar is much easier than gathering pollen, for the latter requires the bee to brush pollen from the anthers with her hairy legs, then comb the grains from her body into her pollen baskets, using a little nectar to glue them together. An experienced bee makes this look easy, but it must take quite a bit of practice.

A few largish bees never went foraging but sat for most of their time just inside the door to the colony, or just outside in the tube. We dubbed these ‘guard bees', since honeybee colonies have individuals that sit in the nest entrance and try to prevent intruders, but in truth we don't really know if these bees were actually doing any guarding. They never did anything when foragers came past them – often literally walking over them – perhaps because all the bees coming in were legitimate members of the nest. If a cuckoo bee had attempted to get in, or a worker from another nest hoping to lay a few eggs, they might have sprung into action.