letters to nature 784 NATURE | VOL 405 | 15 JUNE 2000 | www.nature.com jotunitic chilled margins 8,9 . Chilled margins of similar composition are also found around the Hidra anorthosite body 7 . Batches of primitive jotunitic magma derived by partial fusion of a ma®c lower crust of Gothian to post-Gothian age (1,550±1,400 Myr ago) are therefore the best candidates for parental melts, not only for the massif-type anorthosites but also for the associated noritic and ilmenite-rich intrusions in the Rogaland anorthosite province. The variations in parental magma composition inferred for other anorthosite provinces probably depend on the exact nature of the underlying crust. The Rogaland anorthosites were emplaced post-orogenically ,60 Myr after the latest recorded Sveconorwegian deformation, coinciding with post-orogenic granitoid magmatism elsewhere in south Norway and western Sweden. Seismic and gravimetric pro- ®les along the south coast of Norway have shown that Sveconorwe- gian Moho-ramp tectonics cause the intrusion of crustal tongues into the contemporary mantle 28 . Melting of these crustal tongues was recently proposed to account for the anorthosite formation 29 and currently provides us with the most plausible setting for lower crustal melting and genesis of the parental magmas for the Rogaland massif-type anorthosites. M Received 4 January; accepted 25 April 2000. 1. Scha Èrer, U., Wilmart, E. & Duchesne, J. C. The short duration and anorogenic character of anorthosite magmatism: U-Pb dating of the Rogaland complex, Norway. Earth Planet. Sci. Lett. 139, 335±350 (1996). 2. Emslie, R. F. in The Deep Proterozoic Crust in the North Atlantic Provinces (eds Tobi, A. C. & Touret, J. L. R.) 39±60 (Reidel, Dordrecht, 1985). 3. Longhi, J., Vander Auwera, J., Fram, M. S. & Duchesne, J. C. Some phase equilibrium constraints on the origin of Proterozoic (massif) anorthosites and related rocks. J. Petrol. 40, 339±362 (1999). 4. Scoates, J. S. & Frost, C. D. A strontium and neodymium isotopic investigation of the Laramie anorthosites, Wyoming, USA: Implications for magma chamber processes and the evolution of magma conduits in Proterozoic anorthosites. Geochim. Cosmochim. Acta 60, 95±107 (1996). 5. Longhi, J., Fram, M. S., Vander Auwera, J. & Montieth, J. N. Pressure effects, kinetics, and rheology of anorthositic and related magmas. Am. Mineral. 78, 1016±1030 (1993). 6. Mitchell, J. M., Scoates, J. S. & Frost, C. D. High-Al gabbros in the Laramie Anorthosite Complex, Wyoming: implications for the composition of melts parental to Proterozoic anorthosite. Contrib. Mineral. Petrol. 119, 166±180 (1995). 7. Demaiffe, D. & Hertogen, J. Rare earth element geochemistry and strontium isotopic composition of a massif-type anorthositic-charnockitic body: the Hidra Massif (Rogaland, SW Norway). Geochim. Cosmochim. Acta 45, 1545±1561 (1981). 8. Robins, B., Tumyr, O., Tysseland, M. & Garmann, L. B. The Bjerkreim±Sokndal Layered Intrusion, Rogaland, SW Norway: Evidence from marginal rocks for a jotunite parent magma. Lithos 39, 121± 133 (1997). 9. Duchesne, J. C. & Hertogen, J. Le magma parental du lopolithe de Bjerkreim±Sokndal (Norve Áge me Âridionale). C. R. Acad. Sci. 306, 45±48 (1988). 10. Morse, S. A. A partisan review of Proterozoic anorthosites. Am. Min. 67, 1087±1100 (1982). 11. Menuge, J. F. The petrogenesis of massif anorthosites: a Nd and Sr isotopic investigation of the Proterozoic of Rogaland/Vest-Agder, SW Norway. Contrib. Mineral. Petrol. 98, 363±373 (1988). 12. Ashwal, L. D. & Seifert, K. E. Rare-earth-element geochemistry of anorthosite and related rocks from the Adirondacks, New York, and other massif-type complexes: Summary. Geol. Soc. Am. Bull. 91, 105± 107 (1980). 13. Simmons, E. C. & Hanson, G. N. Geochemistry and origin of massif-type anorthosites. Contrib. Mineral. Petrol. 66, 119±135 (1978). 14. Taylor, S. R., Campbell, I. H., McCulloch, M. T. & McLennan, S. M. A lower crustal origin for massif- type anorthosites. Nature 311, 372±374 (1984). 15. Stein, H. J., Morgan, J. W., Markey, R. J. & Wiszniewska, J. A Re-Os study of the Suwalki Anorthosite Massif, Northeast Poland. Geophys. J. 4, 111±114 (1998). 16. Demaiffe, D., Weis, D., Michot, J. & Duchesne, J. C. Isotopic constraints on the genesis of the Rogaland anorthositic suite (Southwest Norway). Chem. Geol. 57, 167±179 (1986). 17. Emslie, R. F., Hamilton, M. A. & The Âriault, R. J. Petrogenesis of a Mid-Proterozoic anorthosite- mangerite-chanockite-granite (AMCG) complex: Isotopic and chemical evidence from the Nain Plutonic Suite. J. Geol. 102, 539±558 (1994). 18. Walker, R. J., Carlson, R. W., Shirey, S. B.& Boyd, F. R. Os, Sr, Nd and Pb isotope systematics of southern African peridotite xenoliths: Implications for the chemical evolution of subcontinental lithospheric mantle. Geochim. Cosmochim. Acta 53, 1583±1595 (1989). 19. Asmerom, Y. & Walker, R. J. Pb and Os isotopic constraints on the composition and rheology of the lower crust. Geology 26, 359±362 (1998). 20. Esser, B. K. & Turekian, K. K. The osmium isotopic composition of the continental crust. Geochim. Cosmochim. Acta 57, 3093±3104. 21. Johnson, C. M., Shirey, S. B. & Barovich, K. M. New approaches to crustal evolution studies and the origin of granitic rocks: what can the Lu-Hf and Re-Os isotope systems tell us? Trans. R. Soc. Edinb. 87, 339±352. 22. Esperanc Ëa, S., Carlson, R. W., Shirey, S. B. & Smith, D. Dating crust-mantle separation: Re±Os isotopic study of ma®c xenoliths from central Arizona. Geology 25, 651±654 (1997). 23. Saal, A. E., Rudnick, R. L., Ravizza, G. E. & Hart, S. R. Re±Os isotopic evidence for the composition, formation and age of the lower continental crust. Nature 393, 58±61 (1998). 24. Frick, L. R., Lambert, D. D. & Cartwright, I. Re±Os dating of metamorphism in the Lewisian Complex, NW Scotland. Goldschmidt Symp. (Heidelberg) J. Conf. Abs. 1, 85 (1996). 25. Versteeve, A. J. Isotope geochronology in the high-grade metamorphic Precambrian of Southwestern Norway. Geol. Surv. Norway Bull. 318, 1±50 (1975). 26. Michard, A., Gurriet, P., Soudant, M. & Albarede, F. Nd isotopes in French Phanerozoic shales: external vs. internal aspects of crustal evolution. Geochim. Cosmochim. Acta 49, 601±610 (1985). 27. Lambert, D. D., Foster, J. G., Frick, L. R., Ripley, E. M. & Zientek, M. L. Geodynamics of magmatic Cu-Ni-PGE sul®de deposits: New insights from the Re-Os isotope system. Econ. Geol. 93, 121±135 (1998). 28. Andersson, M., Lie,J. E. & Husebye, E. S. Tectonic setting of post-orogenic granites within SW Fennoscandia based on deep seismic and gravity data. Terra Nova 8, 558±566 (1996). 29. Duchesne, J. C., Lie Âgeois, J. P., Vander Auwera, J. & Longhi, J. The crustal tongue melting model and the origin of massive anorthosites. Terra Nova 11, 100±105 (1999). Supplementary information is available on Nature's World-Wide Web site (http://www.nature.com) or as paper copy from the London editorial of®ce of Nature. Acknowledgements Financial support for Re±Os isotopic studies at Monash University derive from the Monash University Research Fund, the Australian Crustal Research Centre, and the Australian Research Council. The research reported here was supported by the Norwegian Research Council. Correspondence and requests for materials should be addressed to H.S. (e-mail: henrik.schiellerup@geo.ntnu.no). ................................................................. Unrelated helpers in a social insect David C. Queller*, Francesca Zacchi*, Rita Cervo², Stefano Turillazzi², Michael T. Henshaw*, Lorenzo A. Santorelli² & Joan E. Strassmann* * Department of Ecology and Evolution, Rice University, PO Box 1892, Houston, Texas 22251-1892, USA ² Dipartmento di Biologia Animale e Genetica, Universita Á di Firenze, Via Romana 17, 50125 Florence, Italy .............................................................................................................................................. High-resolution genetic markers have revolutionized our under- standing of vertebrate mating systems 1 , but have so far yielded few comparable surprises about kinship in social insects. Here we use microsatellite markers to reveal an unexpected and unique social system in what is probably the best-studied social wasp, Polistes dominulus. Social insect colonies are nearly always composed of close relatives 2,3 ; therefore, non-reproductive helping behaviour can be favoured by kin selection, because the helpers aid repro- ductives who share their genes 4 . In P. dominulus, however, 35% of foundress nestmates are unrelated and gain no such advan- tage. The P. dominulus system is unlike all other cases of unrelated social insects, because one individual has nearly 0 0.2 0.4 0.6 0.8 1.0 Survival 0 100 200 300 Cell number Polygynous Monogynous n=54 n=48 Fraction surviving Number of cells n=23 n=19 Figure 1 Success of colonies begun by one foundress and multiple foundresses (monogynous and polygynous, respectively) measured for 1995 colonies through mid- August, when reproductives are being produced. Polygynous colonies are more successful both for survival (P , 0.01, test for equality of percentages) and cell number (P , 0.01, t-test). Relative numbers of monogynous and polygynous colonies in the survival study are representative of its source population. The cell number data excludes failed nests and nests rebuilt after predation. Bars show standard error (s.e.). © 2000 Macmillan Magazines Ltd