A Sting in the Tale (7 page)

Using this set up, Ellington's team could measure the amount of oxygen used up by each flying bee. This in turn enabled them to calculate how much energy bees burn in flight: an estimate of about 1.2 kJh
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. That figure may not mean a lot, so let me contextualise by saying that a running man uses up the calories in a Mars bar in about one hour. A man-sized bumblebee (which would, I admit, be pretty terrifying) would exhaust the same calories in less than thirty seconds. Hummingbirds are often thought of as having exceptionally high metabolic rates, but a bumblebee's is roughly 75 per cent higher.

This simple fact explains an awful lot about the biology and conservation of bumblebees. They have to eat almost continually to keep warm; a bumblebee with a full stomach is only ever about forty minutes from starvation. If a bumblebee runs out of energy, she cannot fly, and if she cannot fly, she cannot get to flowers to get more food, so she is doomed – unless a small boy comes along and gives her a teaspoon of honey. With a stomach full of sugar she can start to fire up her flight muscles, shivering them to produce heat, and once she gets up to about 30°C, off she goes …

A bumblebee's dense furry coat has obvious advantages, but it can create problems when the weather is warm. Bumblebees cannot help but produce lots of heat when they fly, which can be difficult to get rid of if the air temperature is also high. This is probably why bumblebees are not common in Mediterranean countries, and why there are almost none in the tropics. If their body temperature exceeds 44°C they will die; as they approach this lethal limit their metabolism collapses and they become unable to fly. It is noticeable that those species of bumblebee that occur in warmer climates tend to have shorter fur, while those from high latitudes and altitudes tend to be very large and very furry. Hence the huge size and long shaggy coat of the world's largest bumblebee,
Bombus dahlbomii
, which inhabits the high Andes of South America.

Bumblebees do have a trick to help them lose heat in hot weather. As already mentioned, they normally keep their abdomen at a much lower temperature than their thorax, with the narrow waist connecting the two acting as a barrier. If the thorax starts to get too hot, the abdomen starts rhythmically to contract, sending surges of cool blood into the thorax and sucking back waves of hot blood. This heats the abdomen and so raises the surface area from which heat can be lost. Nonetheless, on very hot days in summer bumblebees will tend to stop foraging as noon approaches and recommence in the cool of evening.

Pumping heat from the thorax to the abdomen can also serve a very different purpose. The northernmost social insect in the world is a bumblebee known as
Bombus polaris
, which lives well within the Arctic Circle: large and unusually hairy, it can exist in regions where, even in the height of summer, air temperatures rarely exceed 5°C. Unlike other bumblebees, the
Bombus polaris
queens maintain a stable, high abdominal temperature (greater than 30°C) by pumping hot blood from their thorax to the abdomen. This enables them to develop eggs within their ovaries quickly, which is important in the very short Arctic summer. As the workers and males have no eggs to develop, their abdomens are substantially cooler.

The larger the bee, the easier it is for it to keep warm but the more prone it is to overheating in hot weather. This may explain why queen bees are so much larger than workers, for they are on the wing earlier, in the spring when the weather is cold. It may also explain why the worker bees that leave the nest to gather food tend to be larger, on average, than those that stay in the nest to look after the brood.

Moreover, bumblebees manage not only their own body temperatures, but also those of their brood and the nest. Dependent upon species, bumblebees have anywhere between two and seven months to complete their annual cycle. This would not be possible unless they speeded up the development of their offspring by keeping them warm, and as the grubs are flabby near-immobile creatures with very small muscles, they cannot warm themselves. Instead they are incubated by the queen (if they are the first batch of offspring), or by the workers. Once there are enough of them, the combined heat emanating from the workforce keeps the whole nest at a cosy 30°C or so without much difficulty. As with individual bees, overheating of the nest can be a problem in warm weather. If the nest becomes too warm, or if carbon dioxide levels climb too high, some workers will station themselves in the entrance and fan hot air out of the nest, acting like miniature air-conditioning units. Different bees have different temperature thresholds for beginning fanning behaviour; if the nest is slightly too hot, only one or two bees fan. If the temperature continues to rise, more and more join in. This very simple mechanism enables large nests to regulate their temperature very precisely, keeping it to within 1°C of 30°C day and night.

The ability of established bumblebee colonies to keep warm is most impressive. I once was looking for the kindest way to dispose of a colony of Turkish buff-tailed bumblebees – factory-reared bees which could not be released into the wild in the UK as they are not native here – and decided that freezing them was probably the best option. I placed the nest in its entirety in a domestic freezer at -30°C. The next day I came back to find the colony very much alive and buzzing loudly; the workers had gathered into a tight clump over the brood and were presumably shivering at maximum capacity. The queen was hidden in their centre, and seemed quite unperturbed.

CHAPTER FOUR

A Brief History of Bees

Let us travel back in time 135 million years. The vast supercontinent of Gondwana was beginning to break up, with South America drifting off to the west of Africa, and Australia moving majestically off to the east. Antarctica decided to head south, dooming all but the most adaptable of its inhabitants to an eventual icy grave. The South Atlantic and Indian Oceans were slowly forming.

At this ancient time, an era known to geologists as the Cretaceous, the continents were clothed in green forests of tree ferns, cycads, huge horsetails, and conifers such as pines and cedars. This was the height of the reign of the dinosaurs, although not the species that are so well known to schoolchildren the world over: amongst the trees, herds of vast herbivores such as Iguanodon grazed, standing on their hind legs to reach higher foliage; heavily armoured, tank-like species such as Gastonia bulldozed through the undergrowth; and packs of ferocious meat-eaters such as Utahraptor hunted their prey. The air swarmed with primitive insects including oversized dragonflies and early butterflies, and this was also the heyday of the pterosaurs, the largest animals ever to fly above the earth, with wingspans up to 12 metres. Much smaller dinosaurs had also taken to the air; feathers, probably first evolved to help these little creatures keep warm, became elongated on their forelegs to allow gliding and, eventually, active flight. These were the first birds. Our own ancestors at this time were rather unimpressively small, rat-like creatures skulking in the undergrowth, nervously coming out at night to nibble on insects, seeds and fallen fruit. If we could travel to this ancient land, we might be too concerned with the dangers posed by the larger wildlife to notice that there were no flowers; no orchids, buttercups or daisies, no cherry blossoms, no foxgloves in the wooded glades. And no matter how hard we listened, we would not hear the distinctive drone of bees. But all that was about to change.

Sex has always been difficult for plants, because they cannot move. If one cannot move, then finding a suitable partner and exchanging sex cells with them poses something of an obstacle. The plant equivalent of sperm is pollen, and the challenge facing a plant is how to get its pollen to the female reproductive parts of another plant; not easy if one is rooted to the ground. The early solution, and one still used by some plants to this day, is to use the wind. One hundred and thirty-five million years ago almost all plants scattered their pollen on the wind and hoped against hope that a tiny proportion of it would, by chance, land on a female flower. This is, as you might imagine, a very inefficient and wasteful system, with perhaps 99.99 per cent of the pollen going to waste – falling on the ground or blowing out to sea. As a result they had to produce an awful lot.

Nature abhors waste, and it was only a matter of time before the blind stumbling of evolution arrived at a better solution in the form of insects. Pollen is very nutritious. Some winged insects now began to feed upon it and before long some became specialists in eating pollen. Flying from plant to plant in search of their food, these insects accidentally carried pollen grains upon their bodies, trapped amongst hairs or in the joints between their segments. When the occasional pollen grain fell off the insect on to the female parts of a flower, that flower was pollinated, and so insects became the first pollinators, sex facilitators for plants. A mutualistic relationship had begun which was to change the appearance of the earth. Although much of the pollen was consumed by the insects, this was still a vast improvement for the plants compared to scattering their pollen to the wind.

To start with, insects had to seek out the unimpressive brown or green flowers amongst the surrounding foliage. It was now to the advantage of plants to advertise the location of their flowers, so that they could be more quickly found and to attract insects away from their competitors. So began the longest marketing campaign in history, with the early water lilies and magnolias the first plants to evolve petals, conspicuously white against the forests of green. The first pollinators may have been beetles, which many water lilies still rely on to this day. With this new reliable means of pollination, insect-pollinated plants became enormously successful and diversified. Different plants now began vying with one another for insect attention, evolving bright colours, patterns and elaborate shapes, and the land became clothed in flowers. In this battle to attract pollinators, some flowers evolved an additional weapon – they began producing sugar-rich nectar as an extra reward. As these plants proliferated, so the opportunities for insects to specialise grew, and butterflies and some flies evolved long, tubular mouthparts with which to suck up nectar. The most specialised and successful group to emerge were the bees, the masters of gathering nectar and pollen to this day.

All bees feed more or less exclusively on nectar and pollen throughout their lives. While many other insects such as butterflies and hoverflies feed on flowers as adults, very few do so as young too. Flowers are sparsely distributed in the environment, and immature insects cannot fly from one to another as only adult insects have wings. The innovation unique to bees is that the adult females gather the food for their offspring, so that their larvae do not need to move at all. The larval stage is maggot-like, legless and generally rather feeble, being defenceless and capable of only very limited movement. They are entirely dependent on the food provided by the adult bees.

The first bees evolved from wasps, which were and remain predators today. The word ‘wasp' conjures up an image of the yellow-and-black insects that often build large nests in lofts and garden sheds and which can be exceedingly annoying in late summer when their booming populations and declining food supplies force them into houses and on to our picnic tables. Actually, there are enormous numbers of wasp species, most of whom are nothing like this. A great many are parasitoids, with a gruesome lifestyle from which the sci-fi film
Alien
surely took its inspiration. The female of these wasps lays her eggs inside other insects, injecting them through a sharply pointed egg-laying tube. Once hatched, the grubs consume their hosts from the inside out, eventually bursting out of the dying bodies to form their pupae. Other wasp species catch prey and feed them to their grubs in small nests, and it is from one such wasp family, the Sphecidae, that bees evolved. In the Sphecidae the female wasps stock a nest, usually an underground burrow, with the corpses, or the paralysed but still living bodies, of their preferred prey. They attack a broad range of insects and spiders, with different wasp species preferring aphids, grasshoppers or beetles. At some point a species of sphecid wasp experimented with stocking its nest with pollen instead of dead insects. This could have been a gradual process, with the wasp initially adding just a little pollen to the nest provisions. As pollen is rich in protein, it would have provided a good nutritional supplement, particularly at times when prey was scarce. When the wasp eventually evolved to feed its offspring purely on pollen, it had become the first bee.

Exactly how long ago this happened we do not know for insects rarely form fossils, and so we have to piece together their history from sparse information. Occasionally, insects become trapped in tree resin which fossilises to amber, beautifully preserving them for eternity. Crawling insects such as ants seem to have become trapped most often, but it seems that bees were rarely so foolish and examples of bee fossils are particularly few. The oldest known bee in amber is about 80 million years old, and is of a type known as a stingless bee, similar to species that live today in South America. These are advanced social bees that live in vast colonies, so it is a pretty good guess that the earliest bees were on the wing long before this.

A rather different source of information on the evolution of insects is provided by analysis of DNA sequences, which allow us to make educated guesses as to how long ago different evolutionary lineages diverged. Studies of the similarity of the DNA in wasps and bees suggest that the first bees appeared about 130 million years ago, 50 million years before the first known fossil bee, and probably very shortly after the first flowers evolved in the Cretaceous.

Over the millennia, bees have adapted to feeding on flowers in various ways. Many species have become hairy, which helps them to brush pollen from flowers, and also to hold it in flight. In the leafcutter bees, for instance, the pollen is stored among dense hairs on the underside of the abdomen, so that the bees often appear to have bright yellow bellies. In bumblebees and honeybees, stiff bristles on the hind legs form a basket into which pollen is placed. If one is going to visit flowers for their pollen it makes sense to also collect their nectar, for this is a great source of sugar to sustain flight. Nectar is expensive for plants to produce, and therefore many flowers evolved over time to hide their nectar, ensuring that only the insects most likely to provide them with a reliable pollen delivery service can reach it. Many bees evolved longer and longer tongues to make it easier for them to reach nectar hidden within flowers; some now have tongues longer than their bodies.
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