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Actual and potential tragedies: conflicts over female caste fate in Apis and Melipona bees. Tom Wenseleers & Francis Ratnieks University of Sheffield, UK. Stingless bees –Yucatan, Mexico. Stingless bees – S ão Paulo, Brazil. Why become a worker?. Worker s

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Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Actual and potential tragedies: conflicts over female caste fate in Apis and Melipona bees

Tom Wenseleers & Francis Ratnieks University of Sheffield, UK


Stingless bees yucatan mexico

Stingless bees –Yucatan, Mexico


Stingless bees s o paulo brazil

Stingless bees – São Paulo, Brazil


Why become a worker

Why become a worker?

Workers

Give up reproduction for the benefit of their mother queen

Darwinian puzzle

‘The sterile worker caste of the social Hymenoptera poses one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory.’

Darwin (1859) On the Origin of Species


E g honey bee

E.g. honey bee

  • benefit of becoming a queen:ability to head daughter swarm

  • SO WHY DO NOT MANY FEMALES OPT TO BECOME QUEENS?

  • females benefit from becoming a queen, but colony would suffer if all would do so“caste fate conflict”(colony mostly needs workers for swarming)

  • individual benefits but collective suffers = “tragedy of the commons”

Bourke and Ratnieks 1999 BES


Queen rearing in honey bees

Queen rearing in honey bees

Socially controlled, caste fate enforced


Exception melipona stingless bees

Q

Q

Q

Q

Q

Exception: Melipona stingless bees


Queens no larger than workers

Q

Q

Q

Q

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Queens no larger than workers...


In fact they are slightly smaller

...in fact they are slightly smaller

Melipona beecheii

mean = 57.1 mg

F3,480=76.3, p < 1E-13

mean = 48.2 mg

>66.1 mg

<26.6 mg

Wenseleers et al., in prep.


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Both castes reared from same cells


Mass provisioning

Mass provisioning


Melipona support predictions excess queens

Q

Q

Q

Q

Q

Melipona support predictions:excess queens


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Excess is killed


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Excess is killed


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Excess is killed


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Excess is killed


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Killing occurs quickly

Life expectancy adult workers = 48.5 days

Melipona beecheii

Queens killed within 25 hours after eclosing


Summary

Summary

  • social insect females benefit from developing as a queen

  • in Melipona, females have the ability to do this (’self determination’)

  • results in excess queen production

  • why do not all females develop as a queen?what limits exploitation within the group?


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

W.D. Hamilton (1936-2000)

Kin selection theory


Costs to kin can limit exploitation

Costs to kin can limit exploitation

  • when selfishness causescost to kin exploitationbecomesless profitable

  • queen overproduction causes depletion of workforce and has two costs to kin:reduced ability to swarmreduced production of males

  • prediction: less exploitation when group members are highly related

  • has never been tested


Factors determining kinship

Factors determining kinship

  • multiple mating by queen: reduces relatedness among sistersdoes not occur in stingless bees

  • worker laying

    • workers can sometimes produce sons

    • relatedness to worker’s sons = 0.75

    • relatedness to queen’s sons = 0.25

      can occur in stingless bees


Caste conflict model

Caste conflict model

  • female should become a queen with a probability of (1-Rf) / (1+Rm) (self determination)

    with Rf = sister-sister relatedness

    Rm = relatedness to males

    = 20% under single mating, all males queen produced

    = 14% under single mating, all males worker produced

  • assuming linear cost to total colony reproduction

  • higher/lower ratios with other cost functions

Ratnieks 2001 BES; Wenseleers & Ratnieks submitted


Test interspecies comparison

Test: interspecies comparison

PREDICTION less queen overproduction when males are worker’s sons, since costs are then to closer relatives(nephews, r = 0.75, rather than brothers, r = 0.25)


Male parentage in melipona

Male parentage in Melipona

% of males LOW > INTERMEDIATE> HIGH

workers’ sons

4 / 604 / OBS

Mean, 95% C.L.

#cols. / #males / study

* GLZ, p < 10-15

*

% of males workers’ sons

4 / 1,338 / GEN+OBS

16/ 505 / GEN+OBS

N.S.

*

13 / 108 / GEN

M. favosa4

Tobago, West Indies

M. subnitida2

Rio Grande do Norte, Brazil

M. beecheii1

Yucatan, Mexico

M. quadrifasciata3

Various sites, Brazil

1 Paxton et al 2001; 2 Contel & Kerr 1976; Koedam et al 1999, 2002; 3 da Silva 1977; Toth et al 2002; 4 Sommeijer et al 1999

All species singly mated: Peters et al 1999, Contel & Kerr 1976, Paxton et al 2001, Kerr 1975, Kerr et al 1962


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Yucatan: Melipona beecheii


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Ah Muzencab


M beecheii caste ratios

M. beecheii caste ratios

Max. = 21% Average = 14.6%

95% C.L.

Prop. of queens produced

10 cols.8,162 ind.

Moo-Valle, Quezada-Euan and Wenseleers 2001 Insectes Sociaux


Test interspecies comparison1

Test: interspecies comparison

% males workers’ 0% 34% 41% 95% sons

predicted level HIGHEST > INTERMEDIATE> LOWEST

of queen production

3 / 1 / 2,476

10 / 12 / 8,162

* GLZ, p < 10-10

Cols. / months / indiv’s.

*

% of queens produced

6 / 2 / 3,989

9 / 11 / 2,806

N.S.

*

78 / 10 / 13,514

Mean, 95% C.L.

M. beecheii1

Yucatan, Mexico

M. beecheii2

Yucatan, Mexico

M. favosa5

Tobago, West Indies

M. subnitida3

Rio Grande do Norte, Brazil

M. quadrifasciata4

Various sites, Brazil

1 Darchen & Delage-Darchen 1975; 2 Moo-Valle et al 2001; 3 Koedam et al 1999, 2002; 4 Kerr 1950; 5 Sommeijer et al 2002


Test interspecies comparison2

Test: interspecies comparison

% males workers’ 0% 34% 41% 95% sons

predicted level HIGHEST > INTERMEDIATE> LOWEST

of queen production

Data from months with maximum queen production only

*

* GLZ, p < 10-10

% of queens produced

N.S.

*

Mean, 95% C.L.

M. beecheii1

Yucatan, Mexico

M. favosa4

Tobago, West Indies

M. subnitida2

Rio Grande do Norte, Brazil

M. quadrifasciata3

Various sites, Brazil

1 Moo-Valle et al 2001; 2 Koedam et al 1999, 2002; 3 Kerr 1950; 4 Sommeijer et al 2002


Future test melipona bicolor

Future test: Melipona bicolor

MULTIPLE MOTHER QUEENSlowers relatedness

should cause even greater queen overproduction


Summary1

Summary

  • Melipona females selfishly exploit colony by developing as queens

  • causes “tragedy of the commons”: queen overproduction

  • reduced exploitation when costs are to close kin (workers’ sons)


Alternative explanations for excess queen production in melipona

Alternative explanations for excess queen productionin Melipona ?


1 kerr s theory of genetic caste determination

1. Kerr’s theory of genetic caste determination

Kerr (1950) proposed

2-locus 2-allele system

for Melipona

females heterozygous at both loci develop into queens

results in 25% queens


Different levels of explanation

Different levels of explanation

  • not an alternative hypothesis –different level of explanation (Alcock 1993) :Kerr’s hypothesis suggests HOW the observed caste ratios could come about (PROXIMATE)Caste conflict theory explains WHY the caste ratios are as observed (ULTIMATE)

  • cf. XY-sex determination as an efficient mechanism to attain optimal 1:1 sex ratioin mammals


2 insurance against queen loss

2. Insurance against queen loss?

  • queen are overproduced to ensure that continuous stock of queens is present

  • bet-hedging argument

  • queen overproduction is far too highqueen replacement takes 10 daysin this period up to 70 queens are produced

  • there are other ways to ensure a continuous stock of queens


Queen stocks kept in prisons

Queen stocks kept in prisons

In Trigonini stingless bees, e.g. Plebeia remotaensures that continuous stock of queens is present without having to overproduce them


What about other social insects

What about other social insects?

  • other swarming social insects: queen-worker size dimorphism

    • army ants

    • honey bees

    • trigonine (non-Melipona) stingless bees

  • caste fateenforced via food control

  • results in few queens being produced

  • makes individuals work for the benefit of society and develop as a worker, even when not in best interests of individuals themselves


Policing of caste fate food control

Policing of caste fate: food control

Queen rearing in honey bees


Honey bee

Honey bee

  • multiply mated: Rf=0.3, Rm=0.25

  • females would like to become queens with prob. of (1-Rf) / (1+Rm) = 56%

  • only 0.02% actually become queens

  • strong divergence between individual and colony optimum

  • females are coerced into a working role


Policing of caste fate food control1

Policing of caste fate: food control

Queen rearing in trigonine bees


Evasion of caste policing dwarf queens

Evasion of caste policing:dwarf queens

  • observations

    • occur in ants and trigonine bees

    • same size as workers

    • produced in excess

    • can reproduce, although usually less fecund

  • hypothesis

    • selfish strategy to overcome worker feeding control?

  • support

    • overproduced relative to normal queens


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

a

c

a

Q

b

q

d

d

q

Q

w

Q

q


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Plebeia remota

dwarf queen

2 mm

normal queen


Frequency of dwarf queens

Frequency of dwarf queens

  • overproduced relative to normal queens

    • E.g. Schwarziana quadripunctata

    • 1 in 85 worker cells (1.2%) yield dwarf queens

    • only 1 in 620 females reared as normal queens (0.16%)

    • i.e. 88% of all queensproduced aredwarf queens & produced in 7-fold excess relative to normal queens

  • excess queens are killed by workers as in Melipona

  • as predicted by caste conflict theory !


Caste conflict in termites

Caste conflict in termites


Development and killing of excess reproductives

Development and killing of excess reproductives

Wenseleers, Korb & Ratnieks, in prep.


Summary2

Summary

  • social insect caste system provides scope forconflict

  • caste conflict may cause significant costs to the society(Melipona – queen overproduction)

  • coercion is more effective than kinship in suppressing caste conflict


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

What can we learn from all this?


Insight into conflict resolution

THE SAME TENSION OCCURS IN HUMAN SOCIETY !

stingless bees

honey bees

Self determination20% queen production

Policing of caste fate

0.02% queen production

Insight into conflict resolution

Individual Freedom Causes a Cost to Society

But females prefer to become queen with probability of 56% !

Efficient Society but No Individual Freedom


References

References

  • Bourke A.F.G., Ratnieks F.L.W. 1999. Kin conflict over caste determination in social Hymenoptera. Behavioral Ecology & Sociobiology 46: 287-297.

  • Moo-Valle H., Quezada-Euán J.J.G., Wenseleers T. 2001. The effect of food reserves on the production of sexual offspring in the stingless bee Melipona beecheii (Apidae, Meliponini). Insectes Sociaux 48: 398-403.

  • Ratnieks F.L.W., Monnin T., Foster K.R. 2001. Inclusive fitness theory: novel predictions and tests in eusocial Hymenoptera. Annales Zooogici Fennici 38: 201-214.

  • Ratnieks F.L.W. 2001. Heirs and spares: caste conflict and excess queen production in Melipona bees. Behavioral Ecology & Sociobiology 5: 467-473.

  • Wenseleers T., Ratnieks F.L.W., Billen J. 2002. Conflict over caste fate in social insects: a tragedy of the commons examined. Submitted.

  • Wenseleers T., Ratnieks F.L.W. 2002. Tragedy of the commons in bees. Submitted.

  • PDF reprints and talk at www.shef.ac.uk/projects/taplab/twpub.html


Actual and potential tragedies conflicts over female caste fate in apis and melipona bees

Acknowledgements

  • CollaboratorsV-L. Imperatriz-Fonseca, M. de F. Ribeiro, D. de A. Alves (SP, Brazil)H. Moo-Valle, J. Quezada-Euán and Luis Medina-Medina (Dept. of Apiculture, Merida, Mexico)R. Paxton (Tübingen, Germany)

  • FundingBritish CouncilFWO-VlaanderenVlaamse LeergangenEU ‘INSECTS’ and ‘Social Evolution’ NetworksMarie Curie Fellowship


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