Presentation on theme: "Stingless bees –Yucatan, Mexico"— Presentation transcript:
0 Tom Wenseleers & Francis Ratnieks University of Sheffield, UK Actual and potential tragedies: conflicts over female caste fate in Apis and Melipona beesTom Wenseleers & Francis Ratnieks University of Sheffield, UK
3 Why become a worker? Workers Give up reproduction for the benefit of their mother queenDarwinian 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
4 Bourke and Ratnieks 1999 BES E.g. honey beebenefit of becoming a queen: ability to head daughter swarmSO 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
5 Tragedy of the commonsEach individual gains by pursuing interests that increase returns relative to neighbours but decrease the value of the common goods. If all succumb to the temptation of free-riding, the outcome is a collective disaster William Forster Lloyd 1832
13 Melipona support predictions: excess queens A piece of uncapped comb of Melipona subnitida clearly reveals that queens are produced in excess – up to 20% of all females develop as queens
14 Excess is killedA Melipona subnitida queen ecloses from her cell
15 Excess is killedImmediately afterwards, the workers aggress and kill the queen.
16 Excess is killedSometimes several workers join in on the action.
17 Excess is killedFinally, the workers dump the dead queen corpse and leave it to decompose in the colony. The fact that queens are killed by the workers shows they are produced in excess.
18 Killing occurs quickly Life expectancy adult workers = 48.5 daysMelipona beecheiiColony% of queens in brood combs % N% of queens among adults % NLife expectancy virgin queens4146150.35113125 hours524210.22184383b185210.65122635T1225601.5053262T2237320.4961219T31711840.001273Queens killed within 25 hours after eclosing
19 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 productionwhy do not all females develop as a queen? what limits exploitation within the group?
21 Costs to kin can limit exploitation when selfishness causes cost to kin exploitation becomes less profitablequeen overproduction causes depletion of workforce and has two costs to kin: reduced ability to swarm reduced production of malesprediction: less exploitation when group members are highly relatedhas never been tested
22 Factors determining kinship multiple mating by queen: reduces relatedness among sisters does not occur in stingless beesworker layingworkers can sometimes produce sonsrelatedness to worker’s sons = 0.75relatedness to queen’s sons = 0.25can occur in stingless bees
23 Caste conflict modelfemale should become a queen with a probability of (1-Rf) / (1+Rm) (self determination)with Rf = sister-sister relatednessRm = relatedness to males= 20% under single mating, all males queen produced= 14% under single mating, all males worker producedassuming linear cost to total colony reproductionhigher/lower ratios with other cost functionsRatnieks 2001 BES; Wenseleers & Ratnieks submitted
24 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)
25 Male parentage in Melipona % of males LOW > INTERMEDIATE > HIGHworkers’ sons4 / 604 / OBSMean, 95% C.L.#cols. / #males / study** GLZ, p < 10-15% of males workers’ sons4 / 1,338 / GEN+OBS16 / 505 / GEN+OBSN.S.*13 / 108 / GENRio Grande do Norte, BrazilM. subnitida2M. favosa4Tobago, West IndiesM. beecheii1Yucatan, MexicoM. quadrifasciata3Various sites, Brazil1 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 1999All species singly mated: Peters et al 1999, Contel & Kerr 1976, Paxton et al 2001, Kerr 1975, Kerr et al 1962
30 M. beecheii caste ratios Max. = 21% Average = 14.6%95% C.L.Prop. of queens produced10 cols. 8,162 ind.Moo-Valle, Quezada-Euan and Wenseleers 2001 Insectes Sociaux
31 Test: interspecies comparison % males workers’ % % % % sonspredicted level HIGHEST > INTERMEDIATE > LOWESTof queen production3 / 1 / 2,47610 / 12 / 8,162Cols. / months / indiv’s.* GLZ, p < 10-10*% of queens produced6 / 2 / 3,9899 / 11 / 2,806N.S.*78 / 10 / 13,514Mean, 95% C.L.M. beecheii1Yucatan, MexicoM. beecheii2Yucatan, MexicoM. favosa5Tobago, West IndiesRio Grande do Norte, BrazilM. subnitida3M. quadrifasciata4Various sites, Brazil1 Darchen & Delage-Darchen 1975; 2 Moo-Valle et al 2001; 3 Koedam et al 1999, 2002; 4 Kerr 1950; 5 Sommeijer et al 2002
32 Test: interspecies comparison % males workers’ % % % % sonspredicted level HIGHEST > INTERMEDIATE > LOWESTof queen productionData from months with maximum queen production only** GLZ, p < 10-10% of queens producedN.S.*Mean, 95% C.L.M. beecheii1Yucatan, MexicoM. favosa4Tobago, West IndiesRio Grande do Norte, BrazilM. subnitida2M. quadrifasciata3Various sites, Brazil1 Moo-Valle et al 2001; 2 Koedam et al 1999, 2002; 3 Kerr 1950; 4 Sommeijer et al 2002
33 Future test: Melipona bicolor MULTIPLE MOTHER QUEENS lowers relatednessshould cause even greater queen overproduction
34 SummaryMelipona females selfishly exploit colony by developing as queenscauses “tragedy of the commons”: queen overproductionreduced exploitation when costs are to close kin (workers’ sons)
35 Alternative explanations for excess queen production in Melipona ?
36 1. Kerr’s theory of genetic caste determination Kerr (1950) proposed2-locus 2-allele systemfor Meliponafemales heterozygous at both loci develop into queensresults in 25% queens
37 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 ratio in mammals
38 2. Insurance against queen loss? queen are overproduced to ensure that continuous stock of queens is presentbet-hedging argumentqueen overproduction is far too high queen replacement takes 10 days in this period up to 70 queens are producedthere are other ways to ensure a continuous stock of queens
39 Queen stocks kept in prisons In Trigonini stingless bees, e.g. Plebeia remota ensures that continuous stock of queens is present without having to overproduce them
40 What about other social insects? other swarming social insects: queen-worker size dimorphismarmy antshoney beestrigonine (non-Melipona) stingless beescaste fate enforced via food controlresults in few queens being producedmakes individuals work for the benefit of society and develop as a worker, even when not in best interests of individuals themselves
41 Policing of caste fate: food control Queen rearing in honey bees
42 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 queensstrong divergence between individual and colony optimumfemales are coerced into a working role
43 Policing of caste fate: food control Queen rearing in trigonine bees
44 Evasion of caste policing: dwarf queens observationsoccur in ants and trigonine beessame size as workersproduced in excesscan reproduce, although usually less fecundhypothesisselfish strategy to overcome worker feeding control?supportoverproduced relative to normal queens
45 QqbcadwFig. 1. (a) In the trigonine stingless bees, queens are normally reared from special royal cells constructed near the periphery of the comb (Q). The other, smaller cells yield males and workers. (b) However, in the stingless bee Schwarziana quadripunctata, approx. 1% of all females in small cells cheat on their intended caste fate and become miniature queens (q) rather than sterile workers. The female in the larger royal cell (Q) is a normal queen. (c) Just like normal queens (c), these dwarf queens can succesfully reproduce and head colonies (d). (scale bars = 5mm, a and b and c and d are the same scale)
47 Frequency of dwarf queens overproduced relative to normal queensE.g. Schwarziana quadripunctata1 in 85 worker cells (1.2%) yield dwarf queensonly 1 in 620 females reared as normal queens (0.16%)i.e. 88% of all queens produced are dwarf queens & produced in 7-fold excess relative to normal queensexcess queens are killed by workers as in Meliponaas predicted by caste conflict theory !
48 Fig. 3. Excess dwarf queens (left) are killed by the workers.
50 Termite caste conflict when colony loses royal pair it may be replaced by replacement reproductives (‘neotenics’)in lower termites most individuals are totipotentany individual would like to be a replacement reproductivebut just one pair is needed = another example of a ToCshould result in excess production of replacements
51 Termite caste conflict model ESS is to molt into a replacement reproductive with a probability of 1-relatedness = 1 – 0.5 = 50% cf. Frank (assuming outbreeding)predicts development of excess replacementsWenseleers, Korb & Ratnieks, in prep.
52 Cryptotermes brevis50% of all individuals develop as neotenics (Lenz et al. 1985)all but one pair killedas predicted by caste conflict theory
53 Development and killing of excess reproductives TERMOPSIDAEPorotermes adamsoniMensa-Bonsu Lenz 1985KALOTERMITIDAEKalotermes flavicollisRuppli 1969, Lüscher 1952, 1956, 1974Neotermes connexusMyles & Chang 1984Neotermes jouteliNagin 1972Cryptotermes brevisLenz et al. 1985RHINOTERMITIDAEReticulitermes lucifugusBuchli 1956Reticulitermes santonensisWenseleers, Korb & Ratnieks, in prep.
54 Summary social insect caste system provides scope for conflict caste conflict may cause significant costs to the society (Melipona – queen overproduction)coercion is more effective than kinship in suppressing caste conflict
56 Insight into conflict resolution THE SAME TENSION OCCURS IN HUMAN SOCIETY !Insight into conflict resolutionSelf determination 20% queen productionstingless beesPolicing of caste fate0.02% queen productionhoney beesIndividual Freedom Causes a Cost to SocietyBut females prefer to become queen with probability of 56% !Efficient Society but No Individual Freedom
57 ReferencesBourke A.F.G., Ratnieks F.L.W Kin conflict over caste determination in social Hymenoptera. Behavioral Ecology & Sociobiology 46:Moo-Valle H., Quezada-Euán J.J.G., Wenseleers T The effect of food reserves on the production of sexual offspring in the stingless bee Melipona beecheii (Apidae, Meliponini). Insectes Sociaux 48:Ratnieks F.L.W., Monnin T., Foster K.R Inclusive fitness theory: novel predictions and tests in eusocial Hymenoptera. Annales Zooogici Fennici 38:Ratnieks F.L.W Heirs and spares: caste conflict and excess queen production in Melipona bees. Behavioral Ecology & Sociobiology 5:Wenseleers T., Ratnieks F.L.W., Billen J Conflict over caste fate in social insects: a tragedy of the commons examined. Submitted.Wenseleers T., Ratnieks F.L.W Tragedy of the commons in bees. Submitted.PDF reprints and talk at
58 AcknowledgementsCollaborators V-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)Funding British Council FWO-Vlaanderen Vlaamse Leergangen EU ‘INSECTS’ and ‘Social Evolution’ Networks Marie Curie Fellowship