Tom Wenseleers Department of Biology University of Leuven, Belgium tom.wenseleers@bio.kuleuven.be

1. The evolution of conflict and cooperation in insect societies: towards greater realism in inclusive fitness models Tom Wenseleers Department of Biology University of Leuven, Belgium tom.wenseleers@bio.kuleuven.be

2. Conflict & cooperation insect societies: highly cooperative but conflicts over reproduction can occur these conflicts can be understood on the basis of Hamilton's inclusive fitness theory Ratnieks, Foster & Wenseleers Ann. Rev. Entomol. 2006

3. The evolutionary basisof conflict William D. Hamilton : inclusive fitness theory "The Genetical Evolution of Social Behaviour" (1964, J. Theor. Biol.) Robert Trivers & Hope Hare “Haplodiploidy and the Evolution of Social Insects" (1976, Science) Inclusive fitness theoryIndividuals selected to help relatives, potentiallyat a cost of more distant relatives Various relatedness asymmetries (e.g.between sisters and brothers) within insectsocieties are a source of conflict

4. Success stories of IF theory sex-ratios (nepotistic queen rearing) variation in male parentage andpatterns of queen and worker policing (mostly) but there are also some putative "exceptions" where empirical data do not fit basic relatedness predictions so well, and this has led to a certain amount of skepticism about IF theory Ratnieks, Foster & Wenseleers Ann. Rev. Entomol. 2006

5. Skepticism about IF theory "It is often said in research reports on social insects that some particular set of empirical data is “consistent with kin selection theory.” But the same can be said of almost any other imaginable result, and the particular connection of data to the theory remains unclear. Hence, kin selection theory is not wrong. It is instead constructed to arrive at almost any imaginable result, and as a result is largely empty of content. Its abstract parameters can be jury-rigged to fit any set of empirical data, but not built to predict them in any detail, nor have they been able to guide,with a few exceptions, research in profitable new directions." "the theory has contributed little or nothing not already understood from field and experimental studies" (E.O. Wilson BioScience 2008)

6. Large variation in male parentage found which is not causedby variation in relatedness. E.g. stingless bees: all species have single mated queens, yet worker reproduction varies massively, with 0-98% of all males being workers' sons in different species. Why the variation?

7. Costs & benefits • how can these so-called "exceptions" of kin selection theory be explained? • usually they stem from naive application of the theory, taking into account only relatedness, and not the costs & the benefits • the aim of my studies was to develop more realistic IF models that modelled costs & benefits in much more detail, allowing me to obtain much more accuratequantitative predictions • approach: Frank (1997): costs & benefits are not taken to be simple constants but are calculated by differentiating fitness functions

8. First series of models:caste determination

9. Caste determination Queens are usually larger than workers. This allows nestmates to force females to develop as workers by rationing their food intake.(trophic caste determination) Honey bee Stingless bees(trigonines) Army ants Bourke & Ratnieks 2001 Beh. Ecol. Sociob.; Wenseleers et al. 2003 J. Evol. Biol.

10. Honeybee: trophic caste determination Individuals cannot choose their own caste fate. Only 1 in 10,000 is allowed to become a queen.

11. Most stingless bees: trophic caste determination queen cell Individuals cannot choose their own caste fate. Only c. 1 in 5,000 is allowed to become a queen.

12. Exception: Melipona stingless bees queens and workers same size produced in identical, sealed cells caste fate cannot be enforced, and instead is expected to be determined by individual genotype caste fate conflict theory: larvae better of if they become queens leads to many larvae "selfishly" developing as queens(queen overproduction) “Power” to the individual larvae, social control impossible

13. Queen overproduction Melipona stingless beesca. 10% of the female larvae develop as queens represents a queen overproduction, since queens are needed only sparingly, to swarm or replace a failing mother queen consistent with the idea that larvae control their own caste development and that many selfishly develop as queens Ratnieks & Wenseleers Science 2006

14. Evidence for self determination (1) Melipona beecheii queens workers meansqueens: 11.4 mg workers: 11.6 mgGLM, F1=1.06, p=0.4 Wenseleers et al. Ethology 2003

15. Evidence for self determination (2) data from 413 combs from 8 different species: gynes are randomly distributed in combs D.A. Alves, V.L. Imperatriz-Fonseca, P. Santos-Filho & T. Wenseleers, unpublished data

16. Most excess queens killed... Wenseleers et al. Ethology 2003

17. ...a minority escapes being killed by parasitizing queenless colonies • Melipona scutellaris: some virgin queens escape being killed by leaving the colony and parasitizing unrelated queenless hives • if the mother queen dies in 25% (6/24) of the cases it is replaced by an unrelated queen coming from other queenright colony T. Wenseleers, D.A. Alves, T. Francoy, J. Billen & V.L. Imperatriz-Fonseca, unpublished data

18. Levels of queen production M. quinquefasciata M. seminigra M. pseudocentris M. beecheii M. interrupta M. bicolor SELF DETERMINATION Queens reared in worker cellsExcess queens reared “anarchy” M. melanoventer M. quadrifasciata M. subnitida M. marginata M. scutellaris M. fuliginosa M. asilvae M. rufiventris M. favosa M. trinitatis M. compressipes Trigona amalthea TROPHIC CASTE DETERMINATION Queens reared in queen cellsOptimal # of queens reared Trigona ventralis Trigona ruficrus S. postica S. bipunctata Tetragonisca angustula Apis mellifera 0.01% 0.10% 1.00% 10.00% 100.00% % of females reared as queens D.A. Alves, V.L. Imperatriz-Fonseca, P. Santos-Filho & T. Wenseleers, unpublished data

19. Can we predict the absolute levels of queen production?

20. Various mechanistic explanations (1) Kerr (1950) proposed a 2-locus 2-allele system for Melipona females heterozygous at both loci develop into queens would result in 25% queens system could be generalized to 3 or 4 loci, resulting in 12% or 6% of queens yet the Kerr hypothesis does not explain why only Melipona would have this caste determination mechanism, nor why queen production should be so high, or why it should be 6%, 12% or 25%.(proximate, not an ultimate explanation)

21. Various mechanistic explanations (2) S. Jarau & M. Ayasse (Sunday) larvae in cells that contain a high amount of geraniol develop as queens interesting, as it provides a theory about what cue larvae could use to decide to develop as either a queen or a worker but does not explain why larvae would put their threshold at a certain level again, provides a proximate, but not an ultimate explanation develop as workers develop as queens threshold Frequency distribution 9-25% develop as queens Geraniol content of a given cell

22. Inclusive fitness model • assume larvae can control their own caste fate, and develop with a genetically determined probability into either a queen or a worker • assume that relative colony productivity (swarm and male production) decreases linearly withqueen overproduction (mean prob. that larvae develop as queens) • what is the evolutionarily stable probability of developing as a queen? • result model: single mating (stingless bees): 14-20% of all larvae selected to develop as queens depending on male parentage Wenseleers et al. 2003 J. Evol. Biol.

23. Quantitative fit In terms of absolute quantitativefit neither the model of W. Kerrnor my model is fully satisfactory Observed levels of queenproduction generally below 14% or 20% Kerr model model Wenseleers et al. D.A. Alves, V.L. Imperatriz-Fonseca, P. Santos-Filho & T. Wenseleers, unpublished data

24. More detailed model original model assumed that colony productivity drops to zero when all females develop as queens more detailed model: colony is unable to grow and produce new swarms when the rate at which new workers are produced equals the rate at which they die this results in swarm production dropping to zero when c. 40% of all females develop as queens (based on literature data on worker mortality, the % of eggs laid that are female and the number of new cells provisioned per day per worker) depending on parameters, this results in an ESS whereby 5-20% of all females should develop as queens: excellent quantitative fit to observed data only if worker mortality is close to zero does the ESS reduce to that obtained in the original, simpler model T. Wenseleers, unpublished

25. Conclusion inclusive fitness models can be developed to make very concrete predictions, in this case about what % of female larvae should develop as queens in different bee species the quantitative fit to empirical data increases as the model is made more realistic, e.g. implementing the details of how colonies grow and reproduce, etc...

26. Second series of models:worker reproduction

27. Worker reproduction Workers can lay unfertilised male eggs and would be expected to benefit from doing so since they are always more related to sons than to brothers. Causes a queen-worker conflict over male parentage. worker queen

28. Queen policing Queen selected to prevent workers from reproducing since she is more related to sons than grandsons. Common bumblebee Courtesy of the BBC series “Life in the Undergrowth”

30. Worker policing Workers can also prevent each other from reproducing by eating each others eggs, and are selected to do so particularly when they are collectively most related to the queen's sons (i.e. under multiple mating) or when suppressing worker reproduction increases colony productivity or makes the colony sex-ratio more female biased Ratnieks & Visscher Nature 1989

31. Worker policing German wasp Vespula germanica Bonckaert et al. Beh. Ecol. 2008

32. Inclusive fitness model • assume that workers in a colony become egg-layers with a genetically set probability • assume that relative colony productivity decreases as the % of egg-laying workers goes up (due to a shortage of foragers) and that worker reproduction does not change the sex-ratio • what is the evolutionary stable probability to become an egg-laying worker? • if there is no policing: low relatedness should result in greater selfishness, and a greater % of workers laying eggs • presence of policing: reduces the benefit of laying eggs Wenseleers, Helantera & Ratnieks 2004 J. Evol. Biol.; Wenseleers et al. 2004 Am. Nat.

33. Theoretical prediction: queenright cololonies 15 % of laying workers set mainly by effectiveness of policing not by relatedness 10 ESS % of laying workers 5 0.5 0.6 0.7 0.8 0.9 1 Effectiveness of policing (P) Wenseleers et al. J. Evol. Biol. 2003

34. Theoretical prediction: queenless colonies 60 % of laying workers higher when relatedness is lower 50 linear cost function 40 ESS % of laying workers 30 20 concave cost function 10 0 0.4 0.5 0.6 0.7 Relatedness among workers Wenseleers et al. J. Evol. Biol. 2003

35. Explaining variation in the % of egg laying workers • comparative study of 10 species (9 wasps+honeybee) with variable % of egg laying-workers • correlate worker egg-laying with relatedness and the effectiveness of the policing system

36. Effect of policing Asian paper wasp 30 saxon wasp level of selfishness red wesp % of egg-laying workers 10 tree wasp Norwegian wesp median wesp 5 hornet German wasp common wasp honeybee 0 Wenseleers & Ratnieks Nature 2006 policing reduces the incentive to selfishly lay eggs 30 50 70 80 90 95 98 99 100 effectiveness of the policing

37. Effect of relatedness Asian paper wasp Polistes chinensis 25 25 saxon wasp Dolichovespula saxonica tree wasp red wasp D. sylvestris Vespula rufa 7.5 7.5 Norwegian wasp D. norwegica 5 5 median wasp D. media 2.5 2.5 German wasp Vespula germanica level of selfishness hornet Vespa crabro % of egg-laying workers common wasp 0.75 0.75 Vespula vulgaris 0.5 0.5 0.25 0.25 0.075 0.075 honeybee Apis mellifera opposite to basic relatedness prediction but in line with species with low relatedness having more policing (red) Wenseleers & Ratnieks Nature 2006 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.6 0.7 0.7 genetic relatedness

38. RELATEDNESSLOW HIGH 100 100 ANTS MIEREN BEES BIJEN WESPEN WASPS Workers most related tothe sons of other workers werksters meest verwantmet zonen koningin→ worker policing % adult males producedby workers Workers most relatedto the sons of the queen 10 10 1 1 t-test, p=0.0000000001 n=90 species 0 0 - - 0.15 0.15 - - 0.10 0.10 - - 0.05 0.05 0.00 0.00 0.05 0.05 0.10 0.10 0.15 0.15 relatedness difference between workers’ and queen’s sons Wenseleers & Ratnieks Am. Nat. 2006

39. In queenless colonies: basic relatedness prediction recovered 40 honeybee 35 german wasp 30 common wasp Asian paper wasp 25 level of selfishness % of egg-laying workers red wasp 20 tree wasp 15 median wasp saxon wasp 10 hornet norwegian wasp 5 0 in queenless colonies:no policing and the basic relatedness prediction is recovered 0.3 0.4 0.5 0.6 0.7 Wenseleers & Ratnieks Nature 2006 genetic relatedness

40. Supports worker control Large difference between % of egg laying workers in QR and QL colonies for species with worker policing (red, where in presence of the queen workers are selected not to lay eggs); small difference for the other species (green).Supports the idea that workers respond to the queen signal in their own best interests ("worker control") and that they are not coerced by the queen. queenrightqueenless 35 30 25 20 % of egg-laying workers 15 10 5 0 0.3 0.4 0.5 0.6 0.7 relatedness among workers

41. What about the variation in specieswith single-matedqueens? RELATEDNESSLOW HIGH 100 100 ANTS MIEREN BEES BIJEN WASPS WESPEN werksters meest verwantmet zonen koningin→ worker policing % adult males producedby workers 10 10 1 1 0 0 - - 0.15 0.15 - - 0.10 0.10 - - 0.05 0.05 0.00 0.00 0.05 0.05 0.10 0.10 0.15 0.15 relatedness difference between workers’ and queen’s sons

42. Variation in male parentage not linked to policing • e.g. stingless bees: worker reproduction usually not policed, no variation in relatedness (r = 0.75) • yet worker reproduction varies massively: 0-98% of all males workers' sons • possible explanation: colony-level cost: if workers deposit a male egg in a cell it will reduce the number of workers produced since worker-laid eggs will also compete with female eggs laid by the queen

43. Inclusive fitness model • assume workers replace a random queen-laid egg with an own egg with a genetically set probability • since they will end up replacing some female worker-destined eggs with male eggs this will result in a reduced colony productivity (differential equation model) • determine the ESS probability for a worker to replace a random queen-laid egg with an own egg • prediction: worker reproduction should be more commonif colony produces a lot of workers, i.e. if the queen lays mostly female eggs (smaller colony-level cost)

44. Prediction supported

45. Conclusion inclusive fitness models can be developed to make very concrete and accurate predictions, such as about what % of female larvae should develop as queens in different bee species, what % of the workers should lay eggs or what % of the males should be workers' sons the quantitative fit to empirical data increases as the model is made more realistic, e.g. by implementing the details of how colonies grow and reproduce, etc... Wilson's critique is not warranted!

46. Acknowledgements F.LW. Ratnieks D.A. Alves V.L. Imperatriz-Fonseca wasp work: F.L.W. Ratnieks, F. Nascimento, A. Tofilski,M. Archer, N. Badcock, W. Bonckaert, T. Burke, K. Erven, H. Helantera, L. Holman, K. Vuerinckx stingless bee work: V.L. Imperatriz-Fonseca, D. Alves, T. Francoy, M. Ribeiro, J. Quezada