Plant defences against herbivores include direct defences such as secondary metabolites or physical structures (e.g. trichomes) as well as indirect defences mediated via mutualistic interactions with other organisms including ants. Production of both direct defences and rewards for mutualistic ants may be costly for a plant, and it has been suggested that trade-offs may exist between direct and ant-mediated defences. We have conducted a meta-analysis of 25 studies testing the above hypothesis and found a significant negative correlation between plant allocation to direct and antmediated defences. The strength of correlation was similar for across- and within-species comparisons, and for chemical and physical direct defences. However, trade-offs with direct defences were significant only in plants which offered to ants more costly rewards such as food bodies and/or domatia, but not in plants which attracted ants with relatively cheap extrafloral nectaries. Our results therefore support the hypothesis that plant investment in ant-mediated defences may reduce the requirement for direct chemical and physical defences, but only in plants which offer more costly rewards to their bodyguards. This journal is © 2012 The Royal Society.84628630Rico-Gray, V., Oliveira, P.S., (2007) The Ecology and Evolution of Ant-plant Interactions, , Chicago, IL: The University of Chicago PressHeil, M., Fiala, B., Linsenmair, K.E., Zotz, G., Menke, P., Maschwitz, U., Food body production in Macaranga triloba (Euphorbiaceae): A plant investment in anti-herbivore defence via mutualistic ant partners (1997) J. Ecol, 85, pp. 847-861. , (doi:10.2307/2960606)Janzen, D.H., Coevolution of mutualism between ants and acacias in Central America (1966) Evolution, 20, pp. 249-275. , (doi:10.2307/2406628)Heil, M., Fiala, B., Linsenmair, K.E., Boller, T., Reduced chitinase activities in ant plants of the genus Macaranga (1999) Naturwissenschaften, 86, pp. 146-149. , (doi:10. 1007/s001140050589)Dyer, L.A., Dodson, C.D., Beihoffer, J., Letourneau, D.K., Trade-offs in antiherbivore defenses in Piper cenocladum: Ant mutualists versus plant secondary metabolites (2001) J. Chem. Ecol., 27, pp. 581-592. , (doi:10.1023/ A:1010345123670)Letourneau, D.K., Barbosa, P., Ants, stem borers, and pubescence in Endospermum in Papua New Guinea (1999) Biotropica, 31, pp. 295-302. , (doi:10.1111/j.1744-7429.1999.tb00141.x)Rudgers, J.A., Strauss, S.Y., Wendel, J.F., Tradeoffs among anti-herbivore resistance traits: Insights from Gossypieae (Malvaceae) (2004) Am. J. Bot, 91, pp. 871-880. , (doi:10.3732/ajb.91.6.871)Heil, M., Delsinne, T., Hilpert, A., Schürkens, S., Andary, C., Linsenmair, K.E., Sousa, S., McKey, D., Reduced chemical defence in ant-plants? A critical re-evaluation of a widely accepted hypothesis (2002) Oikos, 99, pp. 457-468. , (doi:10.1034/j.1600-0706.2002.11954.x)Piovia-Scott, J., Plant phenotype influences the effect of ant mutualists on a polymorphic mangrove (2011) J. Ecol, 99, pp. 327-334. , (doi:10.1111/j.1365-2745.2010. 01728.x)Rehr, S.S., Feeny, P.P., Janzen, D.H., Chemical defence in Central American non-ant-acacias (1973) J. Anim. Ecol., 42, pp. 405-416. , (doi:10.2307/3294)Rosenberg, M.S., Adams, D.C., Gurevitch, J., (2000) METAWIN: Statistical Software for Meta-analysis, V. 2.0, , Sunderland, MA: SinauerKoricheva, J., Meta-analysis of sources of variation in fitness costs of plant antiherbivore defenses (2002) Ecology, 83, pp. 176-190. , (doi:10.2307/2680130)Gurevitch, J., Hedges, L.V., Meta-analysis: Combining the results of independent experiments (2001) Design and Analysis of Ecological Experiments, pp. 347-369. , (eds S. M. Scheiner & J. Gurevitch), New York, NY: Oxford University PressKoricheva, J., Nykänen, H., Gianoli, E., Metaanalysis of trade-offs among plant antiherbivore defenses: Are plants jacks-of-all-trades, masters of all (2004) Am. Nat., 163, pp. E64-E75. , (doi:10.1086/382601)Massad, T.J., Fincher, R.T., Smilanich, A.M., Dyer, L., A quantitative evaluation of major plant defense hypotheses, nature versus nurture, and chemistry versus ants (2011) Arthropod Plant Interact., 5, pp. 125-139. , (doi:10. 1007/s11829-011-9121-z)Schemske, D.W., Limits to specialization and coevolution in plant-animal mutualisms (1983) Coevolution, pp. 67-109. , (ed. M. H. Nitecki), Chicago, IL: University of Chicago PressRosumek, F.B., Silveira, F.A.O., De Neves, S.F., De Barbosa, U.N.P., Diniz, L., Oki, Y., Pezzini, F., Cornelissen, T., Ants on plants: A meta-analysis of the role of ants as biotic defenses (2009) Oecologia, 160, pp. 537-549. , (doi:10.1007/s00442- 009-1309-x)O'Dowd, D.J., Foliar nectar production and ant activity on a neotropical tree, Ochroma pyramidale (1979) Oecologia, 43, pp. 233-248. , (doi:10.1007/BF00344773)Katayama, N., Suzuki, N., Anti-herbivory defense of two Vicia species with and without extrafloral nectaries (2011) Plant Ecol., 212, pp. 743-752. , (doi:10.1007/ s11258-010-9862-2)