Recently we have shown that the free energy for pore formation induced by antimicrobial peptides contains a term representing peptide-peptide interactions mediated by membrane thinning. This many-body effect gives rise to the cooperative concentration dependence of peptide activities. Here we performed oriented circular dichroism and x-ray diffraction experiments to study the lipid dependence of this many-body effect. In particular we studied the correlation between lipid's spontaneous curvature and peptide's threshold concentration for pore formation by adding phosphatidylethanolamine and lysophosphocholine to phosphocholine bilayers. Previously it was argued that this correlation exhibited by magainin and melittin supported the toroidal model for the pores. Here we found similar correlations exhibited by melittin and alamethicin. We found that the main effect of varying the spontaneous curvature of lipid is to change the degree of membrane thinning, which in turn influences the threshold concentration for pore formation. We discuss how to interpret the lipid dependence of membrane thinning.