A Sting in the Tale (22 page)

It turned out the bigger foragers were, on average, more efficient at gathering food, bringing back more per unit time. The smallest of the bees that left the nest rarely returned with a net profit and so contributed very little to colony growth. This presumably explains why foragers tend to be large, but in doing so it raises another question: why are bigger bees better at foraging? I discussed this with James, and our first idea was that it might relate to keeping warm. To fly, a bee needs to keep her core temperature above about 30°C, and bigger bees ought to be better able to do this since they have a smaller surface area to volume ratio, but they might be prone to overheating on hot days. Bees working in the nest don't need to worry about keeping warm since they don't have to fly and in any case the nest itself is kept very cosy by insulation and the combined activities of the bees inside. If a bee becomes too cold when foraging she will be grounded, unable to return home, and at great danger of being eaten by a predator, so this might explain why small bees avoid foraging. James tried testing whether the smaller of the bees that went foraging were affected more badly by cold weather, but this didn't seem to be the case. He also went through the data set carefully to see whether smaller foragers tended to go out only on warmer days, but this didn't seem to be true either.

James and I didn't really get to the bottom of what made bigger bees better at foraging, but part of the answer has since been provided by a German researcher named Johannes Spaethe, working in the lab of Lars Chittka at Queen Mary University, London. He found that larger workers have much more acute vision, in part simply because their eyes are much bigger. This is likely to be enormously important when searching for particular types of flowers in the landscape, or when memorising and distinguishing between landmarks for navigation, but of no value whatsoever to a bee operating in the dark confines of the nest. He also found that having larger eyes enables bigger bees to fly in darker conditions, presumably helping them to get home if they find that it is starting to get dark when they are out collecting food. Spaethe went on to study the sensitivity of antennae to odours, and discovered that larger workers also have a better sense of smell. This would benefit them greatly when trying to locate or identify flowers by smell. Larger bees also tend to have bigger brains and so might have enhanced learning ability; this too would help in the complex job of traversing the landscape in search of food, although it has not yet been studied. Finally, larger bees might just be more likely to survive when out and about in a dangerous world. I have commonly seen bumblebees tear themselves from spiderwebs, and I imagine that this would be much harder for a small bee. Despite their large size, the life expectancy of foragers – at two to three weeks, roughly the same as a First World War pilot – is much lower than that of nest bees, which often survive for months. This difference might be much worse if the foragers weren't larger.

Interestingly, honeybees have a different strategy in this respect. Rather than divide up jobs according to size (workers are all the same size), honeybees divide up jobs by age. The younger bees stay in the nest, looking after the brood, while the older bees do the foraging. It seems likely that older bees are given the most hazardous job because they have less residual value to the colony; to take an extreme example, a very old bee is likely to die any day anyway so why not send her on a perilous mission, whereas a young bee has many weeks of work left in her, and it would be a greater loss to the colony if she were to die. There is a possible parallel with humans, although you might find this far-fetched. In the Hadza, a hunter-gatherer tribe of Tanzania, older, post-menstrual women gather a disproportionate amount of food, spending longer out in the savannah picking berries and digging for tubers than their daughters and granddaughters. In evolutionary terms, it may be a wise strategy for post-reproductive women to spare their children such arduous and potentially dangerous tasks and take the risks themselves, since they are unlikely to live too much longer and they are not able to reproduce again themselves. In evolutionary terms, granny's genes would rather she were eaten by a lion than that her grandchildren were eaten. I should add that I am not suggesting this is a conscious strategy, just as I do not suggest that worker bees consciously choose to help rear their sisters because they are aware of patterns of relatedness.
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Let us return to size variation in bumblebees. The question as to why foraging bumblebees tend to be large can be turned on its head by asking, why are nest bees small? It seems likely that the small bees are more nimble in the confined space of the nest, so that it benefits the colony to have small workers for within-nest tasks and big ones for food-gathering. If this were so, nests with workers of a range of sizes ought to fare better than nests with only large or only small workers. Such nests don't naturally exist, but James and I set out to create them by moving bees around. In many social insects, it is not possible to move workers between nests; for example, if one places a worker ant in a different nest she is usually swiftly attacked and killed. Social insects can usually detect the odour of non-nest mates, and are not welcoming to visitors. However, I had noticed that bumblebees from commercial nest boxes often enter the wrong nest when several are placed near each other, and they seem to be accepted. I presume that this is because they have been reared in identical conditions, and so perhaps they all smell much the same. Thus I thought that it might be possible to juggle workers between nests to create all-big or all-small colonies.

I experimented with a giant home-made pooter. Pooters were invented as a device for picking up small insects without harming them; such creatures are all too easy to squash with forceps or fingers. Ordinarily, a pooter consists of a small glass bottle with a cork in the top. The cork has two holes in it, through which two long, flexible, transparent tubes are pushed. Importantly, the end of one of these tubes, where it protrudes below the cork inside the bottle, is covered with fine mesh or netting. To use a pooter, one tube is placed in the mouth – this must be the one with the mesh attached – and the other tube is pointed at the insect. A swift suck on the former usually results in the insect shooting up the latter in a rush of air and ending up, unscathed, in the bottle. It is best to avoid doing this with ants as they squirt out formic acid when frightened which is not kind to the lungs and results in a lot of coughing, but otherwise this is an excellent device for handling tiny beasts. It had occurred to me that a super-sized pooter based on a large jam jar and tubes large enough to suck up bumblebees might be just the thing for efficiently moving bumblebees between nests. I constructed one, and tried it out.
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I had a nest in our darkroom which was equipped with red lighting so that the bees would not fly when I removed the lid. Several workers were scurrying around nervously on top of the nest, aware of the disturbance. When handling bumblebee nests it is important not to breathe on them as this causes the bees to become very agitated, presumably because they think they are under attack from a large mammal such as a badger. I carefully pointed one tube at a bee, and sucked hard on the other. The bee clung on, confused by the sudden draught. I gave the mightiest suck I could manage, and was starting to go blue in the face when the bee suddenly rocketed up the tube and into the jar. Pleased with my success, I paused to get my breath back, then tried a second bee. Unfortunately I had forgotten the all-important netting on the tube going to my mouth. As I sucked, the bee that was already in the jar shot up the tube and into my mouth. Before I could spit it out it stung me, causing my lower lip to swiftly swell to a ridiculous size. As you might imagine, James and my other students made fun of me mercilessly for the rest of the day.

My giant pooter never really caught on, but nonetheless James did manage to juggle bees between nests to create four different experimental treatments: nests with thirty large workers, nests with thirty small workers, nests with about sixty small workers of the equivalent total weight as thirty large workers, and nests with thirty workers of assorted sizes. These he placed out on the university campus to see how they fared. We predicted that the nests with only thirty small bees would be poor at foraging and so grow very little. The nest with thirty large bees ought to be good at collecting food but poor at looking after their young. Comparing nests with sixty small bees versus thirty bees of twice the size ought to reveal whether two small bees are better than one big one. Finally, we predicted that the nests with a mix of large and small bees, as occur naturally, would perform best of all.

When the bees had had three weeks of looking after themselves, James gathered in the nests and counted how many young they had reared. Our predictions were wrong. The nests with only big workers reared the most offspring, performing better than nests with a natural mix or with the equivalent weight of small worker bees. Taken at face value, this suggests that bumblebees have got it wrong. They shouldn't bother rearing any small workers, but should just concentrate on producing big ones, even if this means having fewer of them. This may be true – no organism is perfectly adapted to its environment – but I suspect that the results might be misleading. The weather during James's experiment was awful, with near-constant rain, and none of the nests were thriving. Perhaps larger bees are better at coping with such adverse conditions. It may serve the queen's interests to have mainly small workers in the nest, for they would be easier for her to dominate and prevent from laying their own, male, eggs. Perhaps mixing bees from different nests is hopelessly unnatural and causes strife within the nest, disrupting its normal functioning. We still don't fully understand the roles that bees of different sizes play in bumblebee nests.

There is one final twist to this tale. When out and about watching bees it soon becomes obvious that different species of bumblebee visit different species of flower. For example, almost the only visitors to foxgloves are garden bumblebees, plus the occasional common carder bumblebee, as only these species have a long enough tongue to reach the nectar. Similar but more subtle differences in floral preferences can be found
within
species, according to variation in size, not least because this relates to tongue length. Although foraging bees tend to be larger than nest bees, there is a lot of size variation within the foragers. In buff-tails, the tongues of the smallest foragers are just 4 millimetres long, while those of the largest are nearly twice this length.
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James caught and measured the size of hundreds of wild, foraging buff-tails in and around Southampton, and found that the average size and tongue length varied greatly depending on which flower he caught them on. White clover, bramble and oilseed rape attracted the smallest bees, greater knapweed and field beans the largest. The latter two flowers have deep corollas. He also recorded how long it took bees of different sizes to extract the nectar from particular flowers. Lo and behold, large bees with long tongues were quicker at getting nectar from deep flowers than smaller bees, but the converse was true on shallow flowers, where small bees came into their own.

James noticed that the floral preferences of bees of a particu-lar size were also influenced by the sturdiness of the flower. The biggest workers are pretty chunky beasts, the largest insects likely to land on most flowers. Some flowers simply cannot cope with their weight. Occasionally, a big buff-tailed bumblebee will land on a white clover, at which point the stalk of the flower often gives way and it collapses to the ground. The bee is left struggling to feed while lying on her back under the flower, which looks neither comfortable nor particularly efficient. They usually give up on white clover pretty quickly and go to find something else to feed on. In contrast, other flowers have very sturdy stems: viper's bugloss and knapweed flowers can easily support the weight of large bees, and so provide them with a more attractive proposition.

All of this means that it makes absolute sense for a colony to produce foragers ranging in size, for this allows it to efficiently exploit a broad range of flowers in the surrounding area: small, short-tongued bees for shallow and weak-stemmed flowers, and bigger, longer-tongued bees for the sturdier and deeper flowers.

You might by now be wondering as to the relevance of the Darwin quotation at the start of this chapter. It was taken from a letter to his friend John Lubbock on 3 September 1862. The context is that the day before, Darwin had written to Lubbock with a request:

‘I write now in great Haste to beg you to look (though I know how busy you are, but I cannot think of any other naturalist who wd be careful) at any field of common red clover (if such a field is near you) & watch the Hive Bees: probably (if not too late) you will see some sucking at the mouth of the little flowers & some few sucking at the base of the flowers, at holes bitten through the corollas. All that you will see is that the Bees put their Heads deep into the head & rout about. Now if you see this, do for Heaven sake catch me some of each & put in spirits & keep them separate. I am almost certain that they belong to two castes, with long & short probosces. This is so curious a point that it seems worth making out. I cannot hear of a clover field near here. Pray forgive my asking this favour, which I do not for one moment expect you to grant, unless you have clover field near you & can spare 1/2 hour.'

Darwin had been watching honeybees foraging in a field of red clover near Southampton, where he had recently been staying, and had noticed that some were robbing the flowers through holes chewed at the base, while others were feeding from the conventional entrance to the flower. Darwin was familiar with ant species in which castes with different morphology perform different jobs, and he came to the notion that this might also apply to honeybees; he supposed that those feeding conventionally had long tongues, while the short-tongued bees robbed flowers from the side. He then fired off the request to Lubbock to test this idea for him. It seems that after sending this request, Darwin himself must have decided to sample some bees and measure their tongues. He realised that he had sent his friend on a wild goose chase – as we now know, honeybee workers are all of much the same size and tongue length – and so he sent his angst-ridden apology the very next day, finishing with, ‘I hate myself, I hate clover and I hate bees.' It seems a little over the top, for so far as we know Lubbock never found time to look into this in the day between the arrival of the two letters. Of course, had Darwin asked his friend to carry out the same exercise with bumblebees, he might very well have found a positive result.