Tom Wenseleers & Francis Ratnieks University of Sheffield, UK 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 – São Paulo, Brazil

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

Bourke and Ratnieks 1999 BES 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

Tragedy of the commons Each 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

Queen rearing in honey bees Socially controlled, caste fate enforced

Exception: Melipona stingless bees Q

Queens no larger than workers...

...in fact they are slightly smaller Melipona beecheii mean = 57.1 mg F3,480=76.3, p < 1E-13 mean = 48.2 mg <26.6 mg >66.1 mg Wenseleers et al., in prep.

Both castes reared from same cells

Mass provisioning

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

Excess is killed A Melipona subnitida queen ecloses from her cell

Excess is killed Immediately afterwards, the workers aggress and kill the queen.

Excess is killed Sometimes several workers join in on the action.

Excess is killed Finally, 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.

Killing occurs quickly Life expectancy adult workers = 48.5 days Melipona beecheii Colony % of queens in brood combs % N % of queens among adults % N Life expectancy virgin queens 4 14 615 0.35 1131 25 hours 5 24 21 0.22 1843 8 3b 18 521 0.65 1226 35 T1 22 560 1.50 532 62 T2 23 732 0.49 612 19 T3 17 1184 0.00 1273 Queens killed within 25 hours after eclosing

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?

W.D. Hamilton (1936-2000) Kin selection theory

Costs to kin can limit exploitation when selfishness causes cost to kin exploitation becomes less profitable queen overproduction causes depletion of workforce and has two costs to kin: reduced ability to swarm reduced production of males prediction: less exploitation when group members are highly related has never been tested

Factors determining kinship multiple mating by queen: reduces relatedness among sisters does 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 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 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 % 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 Rio Grande do Norte, Brazil M. subnitida2 M. favosa4 Tobago, West Indies 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

Yucatan: Melipona beecheii Xunan Cab

Ah Muzencab

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 comparison % males workers’ 0% 34% 41% 95% sons predicted level HIGHEST > INTERMEDIATE > LOWEST of queen production 3 / 1 / 2,476 10 / 12 / 8,162 Cols. / months / indiv’s. * GLZ, p < 10-10 * % 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 Rio Grande do Norte, Brazil M. subnitida3 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 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 Rio Grande do Norte, Brazil M. subnitida2 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 MULTIPLE MOTHER QUEENS lowers relatedness should cause even greater queen overproduction

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 ?

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 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

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 high queen replacement takes 10 days in this period up to 70 queens are produced there are other ways to ensure a continuous stock of queens

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

What about other social insects? other swarming social insects: queen-worker size dimorphism army ants honey bees trigonine (non-Melipona) stingless bees caste fate enforced 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 Queen rearing in honey bees

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 control Queen rearing in trigonine bees

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

Q q b c a d w Fig. 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)

Plebeia remota dwarf queen 2 mm normal queen

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 queens produced are dwarf 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 !

Fig. 3. Excess dwarf queens (left) are killed by the workers.

Caste conflict in termites

Termite caste conflict when colony loses royal pair it may be replaced by replacement reproductives (‘neotenics’) in lower termites most individuals are totipotent any individual would like to be a replacement reproductive but just one pair is needed = another example of a ToC should result in excess production of replacements

Termite caste conflict model ESS is to molt into a replacement reproductive with a probability of 1-relatedness = 1 – 0.5 = 50% cf. Frank 1995 (assuming outbreeding) predicts development of excess replacements Wenseleers, Korb & Ratnieks, in prep.

Cryptotermes brevis 50% of all individuals develop as neotenics (Lenz et al. 1985) all but one pair killed as predicted by caste conflict theory

Development and killing of excess reproductives TERMOPSIDAE Porotermes adamsoni Mensa-Bonsu 1976 Lenz 1985 KALOTERMITIDAE Kalotermes flavicollis Ruppli 1969, Lüscher 1952, 1956, 1974 Neotermes connexus Myles & Chang 1984 Neotermes jouteli Nagin 1972 Cryptotermes brevis Lenz et al. 1985 RHINOTERMITIDAE Reticulitermes lucifugus Buchli 1956 Reticulitermes santonensis Wenseleers, Korb & Ratnieks, in prep.

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

What can we learn from all this?

Insight into conflict resolution THE SAME TENSION OCCURS IN HUMAN SOCIETY ! Insight into conflict resolution Self determination 20% queen production stingless bees Policing of caste fate 0.02% queen production honey bees Individual Freedom Causes a Cost to Society But females prefer to become queen with probability of 56% ! Efficient Society but No Individual Freedom

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

Acknowledgements Collaborators 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