in the same period. More recently, researchers have doubted whether Bateman was correct. Honest signalling The handicap principle of Amotz Zahavi, Russell Lande and W. D. Hamilton, holds that the male's survival until and through the age of reproduction with seemingly maladaptive traits is taken by the female as a signal of his overall fitness. Such handicaps might prove he is either free of or resistant to disease, or that he possesses more speed or a greater physical strength that is used to combat the troubles brought on by the exaggerated trait. Zahavi's work spurred a re-examination of the field and several new theories. In 1984, Hamilton and Marlene Zuk introduced the "Bright Male" hypothesis, suggesting that male elaborations might serve as a marker of health, by exaggerating the effects of disease and deficiency. Male intrasexual competition Male–male competition occurs when two males of the same species compete for the opportunity to mate with a female. Sexually dimorphic traits, size, sex ratio, and the social situation may all play a role in the effects male–male competition has on the reproductive success of a male and the mate choice of a female. Larger males tend to win male–male conflicts. Males take many risks in such conflicts, so the value of the resource must be large enough to justify those risks. Winner and loser effects further influence male behaviour. Male–male competition may also affect a female's ability to select the best mates, and therefore decrease the likelihood of successful reproduction. 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. Toolkit of natural selection Sexual selection may explain how characteristics such as feathers had survival value at an early stage in their evolution. The earliest proto-birds such as Protarchaeopteryx had well-developed feathers but could not fly. The