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  1. Bumblebees - Paperback - Dave Goulson - Oxford University Press
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It seems implausible that this is the result of sloppy parenting skills, the accidental neglect of some larvae at the expense of others; if having a workforce of uniform size were advantageous one would expect mechanisms to have evolved which would ensure an equitable distribution of food, or prevent larvae from pupating until they reached the required size. Given that larvae are reared in a controlled environment by a team of specialized workers, it seems far more likely that this size variation has an adaptive function; that colonies benefit from rearing workers of a range of sizes.

What might this benefit be? The most obvious comparable instance of size variation in the worker caste of social insects occurs in some ant and termite species. Here, size is related to behaviour, with individuals of particular sizes specializing in particular tasks; a phenomenon known as alloethism. For example, in leaf-cutter ants of the genus Atta, the largest workers are soldiers, specializing in nest defence against mammals; medium-sized workers forage for food, while the smallest workers tend the fungus gar- den and initiate alarm responses along trails near the nest Hughes et al.

Polyethism, the behavioural specialization of individual workers on particular tasks, is thought to be the key feature underlying the phenomenal ecological success of the eusocial insects Wilson The same can be said of humans; we each specialize in particular tasks, such as carpentry, hairdressing, farming, or the study of insects, in which we build up expertise. The benefits of such a system are obvious; if they are not clear, try asking an accountant to reshoe your horse. In bumblebees, there is disagreement in the literature as to whether they exhibit polyethism.

The traditional view is that individuals exhibit little behavioural specialization. They do not exhibit the clear age-based poly- ethism characteristic of honeybees A mellifera in which young workers do jobs in the nest and switch to foraging as they age , and workers regularly switch between foraging and performing tasks within the nest Free a; Van Doom and Heringa ; Cameron and Robinson However, this view is questionable; there is abundant evidence that bumblebee workers do exhibit polyethism.

Young adults only perform within-nest tasks and are more likely to become foragers as they become older Pouvreau ; O'Donnell et al. Wax in bumblebees is secreted on the underside of the abdomen, beginning on the second day after adult emergence but declining after the first week Roseler Since wax is only required within the nest, young workers are predisposed towards nest maintenance tasks.

In terms of age-related polyethism, the only difference between honeybees and bumblebees is that, in bumblebees, the age at which individuals switch to foraging is variable and some workers never become foragers. Young foragers generally collect nectar and tend to switch to collecting pollen as they age Free a , perhaps because collecting and Social Organization and Conflict 27 handling pollen is a more complex task than collecting nectar; Raine and Chittka a found that B.

Bumblebees probably do exhibit more behavioural plasticity than honeybees. Individuals can switch between tasks in response to colony requirements; for example, nest bees will switch to foraging if the foragers are experimentally removed or if nectar reserves are artificially removed Kugler ; Free a; Pendrel and Plowright ; Cartar Similarly, when nectar reserves are removed, foragers switch from pollen to nectar collection, and vice versa Free a; Cartar ; Plowright and Silverman Just as individual bees differ in the threshold temperature at which they begin incubating or fanning the brood Chapter 2 , individuals also differ in the threshold level of resources within the colony to which they respond Van Doom ; Cartar a.

Specialized foragers bring most food to the nest, while the majority of within- nest tasks are carried out by bees that primarily stay in the nest O'Donnell et al. Just as in humans, specialists are presumably more efficient at their tasks; workers that are primarily foragers occasionally do within-nest tasks, but they do so much less quickly than specialized nest bees Sakagami and Zucchi ; Cartar a; O'Donnell and Jeanne In addition to foraging and brood maintenance there is at least one other task that workers perform.

Large nests of B. Using marked bees in B. This species commonly uses bird nest boxes, as here, and naturally nests in holes in trees, from which it gets the common name of tree bumblebee. Photograph by Juliet Osborne. Thus bumblebee workers clearly do exhibit a range of behavioural specializations. Do these differences in behaviour relate to size i. It was long ago noticed that foragers of a range of bumblebee species appear to be larger, on average, than bees that remain in the nest CoMlle ; Sladen ; Meidell ; Richards ; Cumber a; Brian ; Free a.

In samples of 4, B. It seems that the difference in average size between foragers and nest bees comes about because large workers tend to switch from within- nest tasks to foraging at an earlier age, while the very smallest workers never switch to foraging Pouvreau Even in captive nests of B. Thus bumblebees do exhibit alloethism. Why then do larger workers tend to be foragers and small bees tend to look after the brood? In leaf-cutter ants, the explanation for alloethism is partially obvious; the large soldiers with their huge jaws are far better equipped to inflict damage on an attacking predator such as an armadillo than their smaller siblings [although the full explanation for alloethism in leaf-cutter ants is far more complicated than this; see Hughes et al.

For bumblebees, it is not immediately obvious why larger bees might be better suited to foraging. Rather than trying to explain why foragers are large, Free and Butler suggested an explanation as to why nest workers should be small; they argued that they would be better able to manoeuvre within the cramped confines of the nest. In a test of this hypothesis, Cnaani and Hefetz experimentally manipulated the size of nest work- ers of B. They demonstrated that larvae reached a larger size when tended by large workers, compared to when they were tended by an equal number of small workers.

However, this does not fully refute the hypothesis that small bees are better able to work within the nest, for a fairer com- parison would have been between larvae tended by an equal biomass consisting of either a few large workers or many small ones. One cannot help but suspect that, in this situation, many hands may well make light of the work and produce larger offspring. This experiment remains to be done, but even if it did find that small bees are advanta- geous within the nest, it would still not explain why large worker bees are reared at all.

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We also need to demonstrate a positive advantage for large size in foragers. A number of possible explanations for the larger size of foragers have been proposed. Free and Butler suggested that large workers could carry more forage. This is intui- tively obvious and subsequent experiments have confirmed it to be true.

Although larger foragers can carry more food, this does not explain why foragers are large. Presumably, the cost to the colony of rearing a worker is approximately pro- portional to its size. For example, for every worker of mg the colony could have reared two workers of mg. The single large bee or the two small bees would each be expected to bring back about 58 mg of forage per trip, but it seems likely that the single large bee would take much longer to do so for it would have to single-handedly visit twice as many flowers as each of the small bees.

Few studies have examined for- aging efficiency with respect to size. As noted earlier, Stout found that smaller workers of B. Morse b found no differences in the foraging speeds of large and small workers of B. To test whether large workers do bring back more food per unit time than small workers, we arranged B. The data demonstrated that larger bees are more efficient foragers when collecting nectar, but not pollen Fig. However, whether the greater efficiency of larger bees is sufficient to offset their greater rearing costs is doubtful. Why might larger bees be able to gather nectar more quickly?

Pouvreau sug- gested that larger workers are at an advantage in foraging because they have longer tongues and are able to feed on deeper flowers [the relationship between overall size and tongue length is proportional Medler ig62a,b; Pekkarinen ; Goulson et al. However, having a long tongue is not necessarily an advantage. Bees with short tongues can forage more quickly on shallow flowers Plowright and Plowright In fact, the most common bumblebee species in the United Kingdom are all relatively 30 Bumblebees Figure 3.

On the basis of 98 B. From Goulson et al. Trips were recorded as pollen gathering if pollen was visible in the pollen baskets of returning bees, a Nectar gathering trips, b Pollen gathering trips in which nectar may also have been gathered. If having a long tongue provided an automatic advantage, we would expect foragers to have evolved longer tongues, not a larger size.

Another possibility is that larger bees which have larger eyes may have greater vis- ual acuity, and so be better able to find flowers and reduce search times Spaethe and Chittka The visual acuity of bumblebees is greater than that of honeybees which are smaller Macuda et al.

Bumblebees - Paperback - Dave Goulson - Oxford University Press

There is some evidence that, in addition to size, larger workers have morphological adaptations that suit them to for- aging. For example, large workers of B. They also have disproportionately more Social Organization and Conflict 31 olfactory sensilla on their antennae i. Morse a suggested that large size may enable workers to forage over greater dis- tances.

According to Free b , large bumblebee species tend to go on foraging trips of longer duration than smaller species and thus may cover larger distances. However, there have been no studies of the distance or duration of foraging trips in relation to size variation within species and, in general, very little is known of the foraging range of bumblebees see Chapter 6.

Similarly, we do not know how flight speed relates to size; if larger bees fly faster then this clearly would provide some advantage in foraging. The relative flight speeds of foragers in relation to their size have never to my know- ledge been examined. The limit is imposed by the rate at which energy is burned on the flight back to the nest; this must be less than the total amount of energy that can be contained in the honey stomach.

The energetic cost of foraging is approximately proportional to weight Heinrich b , and it has been shown that the amount of nectar that can be carried is proportional to weight. If the assumptions of Cresswell's model are correct then the maximum foraging distance of foragers should be independent of body size. Yet another possibility relates to predation. Foraging is a dangerous task that probably increases worker mortality in social insects Van Doom ; O'Donnell and Jeanne ; O'Donnell et al. Silva-Matos and Garofalo found that worker mortality in the tropical bumblebee B.

Similarly, longevity of workers of B. Estimates of worker longevity vary between species and studies, from However, mark recapture studies of B. Garofalo estimated the mean longevity of B. Foraging appears to reduce the ability of B. It seems likely that larger bees are less prone to predation, particularly by spiders, than small bees it is common to observe bumblebees caught temporarily in spider webs but they usually manage to break free.

Conversely, the conopid fly Sicus ferrugineus, which attacks bees while they are foraging on flowers, preferentially parasitizes large workers Schmid-Hempel and Schmid-Hempel a. If, overall, large bees do have a longer life expectancy as foragers, then sending larger bees out to forage may be the safest option 32 Bumblebees for the colony. No data are available on the longevity of foragers in relation to size; this would be an interesting and relatively straightforward area for study.

Perhaps one of the most promising candidate explanation for alloethism in bumble- bees relates to thermoregulation. Free and Butler pointed out that larger workers would be better able to forage in adverse weather. All bees are limited to foraging within a particular temperature range and, in general, the lower limit of this range shifts down- wards as body size increases Stone and Willmer For example, queens of B. Thus, larger foragers are presumably able to become active at lower ambient temperatures than small foragers but, conversely, they are more prone to overheating in warm wea- ther Heinrich a, b.

Indeed, these are arguments used to explain why bumble- bees are superior pollinators to honeybees in cool climates and why the distribution of bumblebees is largely confined to temperate regions. The nest itself is maintained at a more or less constant temperature, so individual-level thermoregulation is not an issue for bees working within the nest. Neat though this theory is, the only attempt to test whether workers of different sizes tend to forage in different weather conditions sug- gests that they do not Peat and Goulson We observed whether B.

If thermoregulation were the explanation for alloethism in bumblebees, one might predict that the queen should rear a few large foragers early in the season but that work- ers reared in summer should be smaller. Several studies have examined changes in worker size during the season, with variable results. Knee and Medler found an increase in worker size for three American species late in the season. Plowright and Jay found an increase in worker size as the season progressed in some species but not in others. Roseler describes an initial decline in the mean size followed by a general increase in B.

No clear pattern emerges and it seems that foragers are not, in general, larger early in the season. Of course the sizes produced may not be the optimum with regard to thermoregulation, particularly if the colony is constrained by a shortage of pollen. This is particularly likely to be the case in early spring when the queen has to single-handedly gather food for her offspring.

With regard to the first batch of workers, it is also possible that there is a trade-off between producing few large offspring, each of which would be well adapted to foraging in cool spring temperatures, or producing more smaller offspring, which would be poorer at thermoregulating. A Social Organization and Conflict 33 risk-averse queen might choose the latter strategy for it reduces the variance in the pro- portion of workers that might be lost to predation i. The hypotheses proposed to explain why foragers are larger than nest bees are not mutually exclusive, and a combination of factors is likely to be responsible.

They do not, however, address why there is so much size variation between foragers Fig. If it is in some way advantageous for foragers to be large and for nest bees to be small, why is there not a bimodal size distribution? It may be that having foragers of a range of sizes allows them each to specialize in flower types appropriate to their morphology and so improves overall foraging efficiency of the colony while minimizing intra-colony competition.

Different size classes do tend to visit different flower species Cumber a; Heinrich a; Morse b; Inouye a; Barrow and Pickard ; Johnson For example, Cumber a found that large workers of B. Overall, the mean corolla depths of the different flower species visited varies in accordance with the tongue lengths of different sized workers Prys-Jones Interestingly, Johnson found that it was only the large foragers of B. The smaller foragers visited both deep and shallow flowers.

Johnson suggests that this may be because the small bees were primarily nest bees that had been forced to forage due to a food shortage in their colonies, and thus they were inexperienced. An alternative explanation might be that small bees can oper- ate at a profit on a lower reward per flower because of their lower metabolic costs , so they can profitably visit deep flowers from which they cannot extract all of the nectar. Studies have also found that there are differences in the mean size of foragers engaged in gathering pollen versus those that gather nectar, but they do not agree in the direction of this difference.

Some studies have found that it is the larger foragers that tend to col- lect pollen, while the smaller foragers collect nectar Brian ; Free a; Miyamoto ; Pouvreau , while Goulson et al. It may be that the size of bees specializing in each of these tasks depends on which flowers are locally available. The structure of particular flowers suits bees of a particular size; for example, small foragers of B. Since these flowers only provide pollen, we might expect pollen gatherers to tend to be small in nests situated near to patches of flowering C.

Thus, the relative sizes of pollen and nectar gatherers may vary between nests and at different times of the year according to the flowers that are locally available. Having foragers of a range of sizes enables both resources to be gathered efficiently. It would be possible to test whether having foragers of a range of sizes is beneficial to bumblebee colonies by artificially varying the size distribution of workers in experi- mental nests.

Young workers can readily be moved between colonies, so that it would be possible to create colonies with only large or small workers, or with a range of sizes, and then measure nest growth and foraging efficiency.

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It's Eric the half a bee! Monty Python The sex of Hymenoptera is determined in an unusual way, using a system known rather dauntingly as parthenogenetic arrhenotoky Crozier and Pamilo As we have seen, fertilized eggs develop into diploid females they get one copy of each chromosome from each parent, as in most diploid organisms , while unfertilized eggs develop into haploid males they only have one copy of each chromosome.

Actually, it is slightly more complex than this. In many Hymenoptera including bumblebees , individuals are male if they are homozygous at one or more sex- determining loci Paxton etal. Heterozygotes at this locus are females. Since haploids are inevitably homozygous at all loci, all haploids are males.

But it is quite possible for a diploid to be homozygous at this particular loci; such individuals develop as males. In fact, use of a single sex- determining locus seems to be the norm in most Hymenoptera Cook and Crozier The fewer loci involved, the more likely it is that diploid males will occur.

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Diploid males appear to have very low fertility in bumblebees Duchateau and Marien In honeybees, diploid male larvae are consumed by workers, but in bum- blebees they are reared to adulthood Plowright and Pallet ; Duchateau etal. Their production is thus particularly undesirable because it places a burden on the col- ony for no gain. For any queen that is unfortunate enough to mate with a male carrying a copy of the sex- determining gene that is identical to one of her own two copies, half of her workers will develop as diploid males, and since they do no work then half of her workforce is effectively lost.

Such colonies are very unlikely to survive. Hence negative frequency dependent selection operates at the sex- determining locus; any allele that is common is selected against because queens carrying it are more likely to end up in matched-pair matings. Thus in a healthy outbred population we would expect a large number of rare alleles, rendering the probability of matched-pair matings remote.

In the only species studied to date, B. There is thought to be a trade-off between maximum colony growth and Social Organization and Conflict 35 reproduction, constrained by impending ecological often seasonal changes that will soon make conditions unsuitable for either. Limiting reproduction to the end of the cycle ensures that the largest possible workforce is present to rear reproductives Oster and Wilson In bumblebees, one might expect that declining floral avail- ability in the autumn would be the factor limiting colony development yet colonies of many species disband long before the end of the flowering season e.

It is probably mounting pressure from parasitoids that become more abundant as the season progresses that curtails their development Schmid-Hempel etal. The early, pre-reproductive phase of colony development in bumblebees is generally harmonious, but in the later reproductive phase violent, even fatal, conflicts may occur between members of the colony. Some workers within a colony become more aggres- sive than others, both towards intruders and to their siblings, and these individuals tend to show a greater degree of ovarian development Free ; Foster et al.

These bees are generally nest bees rather than foragers, perhaps because foraging reduced opportunities for reproduction Foster etal. Late in the development of the col- ony, such workers will sometimes construct egg cells and lay their own unfertilized and thus male eggs. The foundress queen will retaliate by eating these eggs and then laying her own in the egg cells Free et al.

In turn, the workers may eat the queen's eggs, often doing so as she is laying them Van Honk et al. Egg-eating by workers has frequently been observed in a range of bumblebee species including B. Eggs are generally only eaten within the first 24 h after being laid Huber Perhaps after this time it becomes impossible for either the queen or workers to distinguish between their own eggs and those of others. Workers may also throw larvae out of the nest at this time, although the identity of these larvae has not been established Pomeroy Egg-eating leads to fights within the nest and, on occasion, the queen may be killed by her own workers Van Honk et al.

What causes this conflict? The answer lies at least in part in the unusual patterns of relatedness found within Hymenoptera due to their haplodiploid sex determination. In the vast majority of bumblebee species, the queen mates only once Estoup et al. Thus, we expect the queen to favour rearing her own sons rather than allowing her daughters to lay their own eggs.

The interests of the queen and of the workers are opposed; each would prefer to rear their own sons Hamilton ; Trivers and Hare Almost all studies of colony development and conflicts in bumblebees have been of B. In this species, the onset of conflicts within the colony known as the competition point appears to be closely correlated with the time when the colony commences rearing new reproductives Van der Blom ; Van Doom and Heringa ; Duchateau and Velthuis Up to this point, the foundress queen appears to produce a pheromone that induces diploid larvae to develop as workers rather than queens Roseler , If she dies or is removed, workers will often rear new queens and lay their own eggs earlier than would otherwise occur.

Miiller and Schmid-Hempel b monitored nests of B. Nests that had been briefly attacked produced significandy more males, suggesting that even this very brief suppression of queen dominance can lead to significant worker reproduction. In contrast, if Psithyrus remain in the nest, they suppress worker ovar- ian development to a similar degree to an undisturbed queenright colony Vergara etal.

Why does colony harmony break down at the competition point? What prevents workers from laying eggs earlier? Duchateau and Velthuis and Roseler hypothesize that worker aggression steadily increases until eventually the queen loses her dominance and ceases production of the pheromone. They argue that it is the phe- romone that inhibits worker reproduction. It has been experimentally demonstrated that the queen does cease pheromone production at this time.

Young female larvae placed with a queen taken from a colony before the competition point become workers, whereas if they are placed with a queen from a colony which has passed the competi- tion point they become queens Cnaani et al. As Bourke and Ratnieks point out, there is a flaw in the argument put forward by Duchateau and Velthuis and Roseler Suppression of worker reproduc- tion by a pheromone is unenforceable; selection would favour workers that ignored this signal and laid eggs anyway, if it were in their interests to do so.

Other authors have also disputed the suggestion that worker reproduction is inhibited by a queen pheromone and have provided some experimental evidence against this idea Keller and Nonacs ; Bloch etal. It seems unnecessary to argue that the switch to rearing reproductives should be forced upon the queen, since it is in her interests to do so at some point. Bourke and Ratnieks , suggest a subtly different interpretation. They hypothesize that the queen ceases pheromone produc- tion of her own volition rather than because she is being oppressed by the workers.

Workers do not lay eggs before this time because it is not in their interests to do so; worker reproduction early in the colony cycle would slow colony growth because males do not work and reduce production of full sisters who are more closely related to workers than their sons. Social Organization and Conflict 37 Interestingly, there is evidence that ovarian development in young workers is pre- vented by the presence of the queen and, also, by the presence of dominant workers in the absence of a queen Roseler and Roseler ; Roseler et al.

Queens and dominant workers seem able to recognize, and are more aggres- sive towards, workers with developed ovaries Van Doom and Heringa ; Duchateau ; Roseler and Van Honk Ovarian development in workers is reversible: Some col- onies switch from rearing workers to rearing reproductives at a relatively early stage in colony development, about 10 days after the emergence of the first adult workers Duchateau and Velthuis ; Duchateau et al.

These colonies produce mostly males. Other colonies switch later, about 24 days after emergence of the first workers, and these tend to produce mainly new queens. In both colony types, the competition point occurs about 31 days after emergence of the first workers. Thus in early- switching colonies, the competition point does not occur until about 21 days after the queen com- mences laying male eggs. In contrast, in late-switching colonies, the competition point occurs about 7 days after the first eggs are laid that are destined to become new queens Duchateau and Velthuis Bourke and Ratnieks suggest that the timing of the switch is under the control of the queen.

In early-switching colonies, she commences laying male eggs while presuma- bly continuing to release the pheromone that prevents female offspring from developing into queens. They argue that workers are unable to detect that they are rearing males until the male larvae are about 10 days old 15 days after the eggs were laid. Since it is prob- ably not in the interests of workers to lay their own eggs before the switching point and they are not able to detect that this has occurred for 15 days, we would expect a substan- tial delay between the point at which reproductives are first produced and the onset of the competition point in early-switching colonies.

Attempts to test whether workers are indeed only able to detect male larvae at an age of 10 days, by adding male larvae of vary- ing ages to colonies, suggests that this is not so Lopez-Vaamonde et al. Regardless of the age of the larvae, worker reproduction did not begin until 19 days after male larvae were added.

The reason why workers do not begin egg laying earlier remains obscure, since it would appear to be in their interests to do so. Once the workers detect the presence of male larvae, they could throw them out of the nest and attempt to replace them with their own offspring. However, it seems that they rarely do so, and this may be because earlier emerging males can be expected to 38 Bumblebees enjoy higher mating success providing that there are queens available for them to mate with Bulmer Since the sons of workers would be at least 15 days younger than those of the queen, their expected mating success may be much lower.

Also, the queen's sons are nearing the completion of their development by the time workers begin egg laying timing of development taken from Duchateau and Velthuis If the workers lay their own eggs at this point, they will not have hatched until after the oldest of the queen's sons have pupated.

Thus they will not be in direct competition for food and so, for the workers, there is little or nothing to be gained from destroying the first male lar- vae that they detect. Consider now late-switching colonies. Here the switch is presumably determined by the queen ceasing to produce the pheromone that prevents female larvae from develop- ing into queens Cnaani etal.

Experimentally increas- ing the number of workers in the nest can bring forward the production of new queens Bloch , suggesting that the queen's decision as to when to cease pheromone pro- duction is flexible and dependent on the availability of a sufficiently large work force to rear new queens which require more food than workers. Larvae are sensitive to the queen's pheromone at about days old days after their eggs were laid. The workers commence laying their own eggs about 7 days after the first eggs that are des- tined to become queens are laid Duchateau and Velthuis As Bourke and Ratnieks point out, this corresponds precisely with the presumed time at which the queen ceases pheromone production.

Neat though these explanations for the timing of onset of the competition point are, there are some anomalies that require further investigation: Bloch found that in a few early- switching colonies, the competition point preceded the switching point. Bourke and Ratnieks put this down to worker error. As Bourke and Ratnieks concede, their hypothesis falls 6 days short of explain- ing the 21 day lag generally observed between the switching point and the competi- tion point in early- switching colonies. These 6 days cannot be explained by the time needed for ovary development in workers, for Duchateau and Velthuis dem- onstrated that the ovaries of some workers are fully developed before the switching point and in late-switching colonies egg laying by workers occurs very promptly.

It is not clear why workers should lay their own eggs in late-switching colonies. Also, given the strong male bias in the ratio of males to new queens found in many bumblebee populations Beekman and Van Stratum and the probable early male advantage discussed earlier, the expected reproductive success of worker-produced males in late-switching colonies is very low. Bourke and Ratnieks suggest that competition between worker-produced males and future queens is minimal since the colony has plentiful resources at this time.

Lopez -Vaamonde Social Organization and Conflict 39 etal. This does of course beg the question as to why the queen does not lay more diploid eggs, and so increase production of new queens? If the colony has sufficient resources to rear worker-laid males, then it could presumably rear more future queens instead. Overall, Bourke and Ratnieks' hypothesis fits the available data reasonably well and is certainly the closest we have yet come to a full explanation for the reproductive strate- gies adopted by bumblebee nests. It would be very useful to obtain data on other species, since almost all studies to date have focussed exclusively on B.

Identification of the queen pheromone would be invaluable, for it would enable experimental manip- ulations to test various aspects of the hypothesis [unfortunately, analysis of queen exo- crine secretions suggests that there are at least candidate compounds; Hefetz et al. Also, at present, there are few data on the proportion of bumblebee males that are produced by workers and as to how skewed parentage of worker-produced males is towards the more dominant individual workers. Such data could be obtained easily using established microsatellite markers Estoup et al.


It appears to be the result of a gradual process whereby conflict between the queen and workers steadily increase, resulting in a loss in queen condition and, sometimes, ultimately leading to her death. Why is this in the interests of the workers? Bourke considers the conflicting pres- sures on workers with regard to matricide in detail, although at this time it had not become apparent that B. In colonies specializing in male production, one would expect competition to be most fierce. However, in early- switching colonies the queen generally lays some diploid eggs which develop into queens towards the end of colony development, so that matricide still has a cost to workers in terms of lost sis- ters.

Unfortunately, no data are available on the frequency of matricide in early-switching versus late-switching colonies. The optimum strategy for workers may well depend on the condition of the queen, as well as her decision to switch early or late. If her reproductive potential has been reduced through injury or parasitism, this may favour matricide. Since injury may occur 40 Bumblebees during queen-worker conflict over male production, there may be positive feedback; conflict reduces the queen's condition which, in turn, pushes the optimum worker strategy towards further conflict leading to matricide.

If the queen's condition becomes sufficiently low it may actually benefit her to die, rather than to continue fighting with her daughters who in her absence will produce more of her grandsons Bourke - 3. A male bias is unexpected in social Hymenoptera.

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Kin selection theory predicts that in a col- ony founded by a monogamous queen, workers should favour a 3: Neither the queen nor the workers should bene- fit from a male-biased sex ratio. So why are bumblebee sex ratios seemingly male biased? Possible explanations have been considered in depth by Bourke and Beekman and Van Stratum The answer must be linked to the frequency with which colonies adopt an early- switching or late -switching strategy, because the former produce mostly males and the later mostly new queens. Bourke and Ratnieks argue that queens adopt an early-switching, male-producing strategy with a probability of 0.

If the queen chooses to adopt a strategy of producing males then the workers have no choice but to comply, since they cannot lay their own diploid eggs Bulmer If half of all colonies specialize in male production, then it is in the interests of both the queens and workers in remaining colonies to specialize in queen produc- tion Bourke and Ratnieks A very similar system is thought to operate in the ant Pheidole desertorum Helms Choose your country or region Close. Ebook This title is available as an ebook. To purchase, visit your preferred ebook provider. Bumblebees Behaviour, Ecology, and Conservation Second Edition Dave Goulson Provides an authoritative and accessible account of bumblebee biology Builds on the reputation of the first edition, providing a fully revised and comprehensive successor Incorporates recent advances in active fields of research, including taxonomy, immunity to disease, and foraging behaviour Focuses on practical issues such as conservation strategies, management of bumblebees for crop pollination, and the possible impacts of bumblebees as non-native invasive species New to this Edition: Considerable expansion of most chapters following new work in the area.

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