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Multiple models

More recently, the field has grown to include other areas of study, not all of which fit Darwin's definition of sexual selection. A "bewildering" range of models variously attempt to relate sexual selection not only to the fundamental questions of anisogamy and parental roles, but also to mechanisms such as sex ratios – governed by Fisher's principle, parental care, investing in sexy sons, sexual conflict, and the "most-debated effect", namely mate choice. Elaborated characteristics that might seem costly, like the tail of the Montezuma swordfish (Xiphophorus montezumae), do not always have an energetics, performance or even survival cost; this may be because "compensatory traits" have evolved in concert with the sexually selected traits.