Origin Mechanism, Evolution Pressure And Impact On Male-male Competition Of Loss Of Sexual Traits Of Prinia Flaviventris In Breeding Season

Some species of cisticolidae Passeriformes show a special pattern, that graduate tail which is served as sexual trait, of breeding plumage is shorter than that of winter plumage. This special pattern may be contradict with sexual selection theory, because females prefer males with longer tail and males with shorter tail may be disadvantage in mating choice and male-male competition. This special pattern is rare and unclear, and may indicate a new breeding and survival strategy which is unknown. We advance a hypothesis that:females of these species choose mate while males with longer tail of winter plumage, and then males moult to shorter tail after mating. Shorter graduated tail which is better aerodynamic effect, could save energy and be good for breeding. From September 2008 to January 2010, we captured yellow-bellied prinia (Prinia flaviventris) and plain prinia(Prinia inornata) with mistnet at Nansha area Guangdong city, and feed them with mixture of Alphitobius diaperinus and bird feedstuff. We used cross choice chamber to test whether female preference exist, and used fight cage to test whether moult would affect the male-male competition result.Results of female preference experiments showed that female preference of yellow-bellied prinia was still exist, and females prefered males with longer tail. Loss of male sexual traits was not caused by female preference, but other forces overcomed the force of sexual selection. This result is consistent with our hypothesis that tail of winter plumage may be used for sexual selection. We conjecture that natural selection forces include prey and breeding may be the main reason why sexual traits are lost in breeding season.Experiment of male-male competition showed that:moulting males whose tail was longer, won the game as many times as unmoulted males whose tail is shorter, but moulting males had to try more attacks when they win the game. Our result means that moult would not affect the male-male competition, and male which starts moult early, would not loss the mate and territory, and cheaters would be handicapped for this reason. We conjecture that better aerodynamic of shorter tail would be good for fighting.Coefficient of variation of body traits of yellow-bellied prinia and plain prinia showed that:tail of breeding plumage were more variable than other traits of breeding plumage, and more variable than all traits of winter plumage in both two species. Tails of breeding plumage which were naturally selected, were more variable than tails of winter plumage which were sexual trait. We conclude that morphological variation is determined by strength of selection force, but not kind of selection force. Special pattern of morphological variation of these two species appeared under two preconditions. First, natural selection and sexual selection are sperated at time. And then, natural selection is more intense than sexual selection in breeding season.Symmetry degree of tail increased with length of tail, but skeletal traits just like tarsus and claw was reverse with tail pattern. Our result consistent with symmetry theory, and symmetry of sexual triats were determined by sexual selection.We conclude that loss of sexual traits of breeding season is not caused by female preference, and other forces which are more intense than sexual selection may be the main reason. Coefficient of variation of body traits also proved that evolution force of breeding season is more intense. Loss of sexual traits would not affect the male-male competition, and cheaters would be handicapped for this reason.Our research could answer the some aspects of our hypothesis, but there are also many questions about this pattern include moult mechanism, female plumage and aerodynamic of tail are still unclear. We will conduct future research on these sides and try to explain the origin and mechanism of this special pattern.