| Like male animals that have elaborate ornaments, females of many animals also exhibit remarkable ornamentation. Traditionally, biologists have paid little attention to female ornamentation since it is often regarded as non-functional or dismissed as a genetically non-adaptive byproduct of male ornaments. However, there is increasing evidence that female ornaments may be sexually selected by male mate choice. Fluorescence is known to occur when fluorophores absorb light at a shorter wavelength, usually ultraviolet light (UV; wavelengths<400nm), and then emit light at a longer wavelength. In recent years, fluorescence has been documented in a wide range of animals, including jumping spiders (Araneae:Salticidae). However, the function of the UV-induced fluorescence in animals has long been controversial. The salticid genus of Phintella is widely distributed in tropical Asia, and many species such as P. vittatata and P. bifurcilinea are known to produce UV-induced fluorescence on the palps of females, but not of males, exhibiting sexual dimorphism in fluorescence. What is the evolutionary significance of UV-induced female fluorescence in Phintella salticids? In this study, I used both P. vittatata and P. bifurcilinea as a model system to investigate the evolutionary significance of UV-induced female fluorescence in salticid spiders. In Chapter1, I tested the hypothesis that UV-induced female fluorescence is important in male mate choice in both P. vittatata and P. bifurcilinea by performing a series of binary male mate choice trials in which a males was given a choice between a fluorescent female and a fluorescence-blocked female by using a filter that can block UV light. My results showed that in both species of Phintella, males preferred fluorescent females over fluorescence-blocked females. The bright body color means a higher predation since bright colouration is more attractive to predators. To evaluate whether the fluorescent palps would increase the risk to be preyed, in Chapter2,I used a spider-eating salticid species, Portia sp., as predators to test whether Portia sp. would be more often attracted by the fluorescent females than non-fluorescent ones. The results showed that the presence of fluorescence had a significant effect on Portia’s prey preference:significantly more Portia were attracted to fluorescent females than to fluorescence-blocked females. The animal colouration has been envisaged to evolve as a compromise between conspicuousness and crypsis. On one way, colour signals should be much easier to be detected by the preferred mate but more difficult to be detected by the unintended receivers such as predators. In Chapter3, we tested this hypothesis by comparing female palp spectra of two species Phinetella and shrub leaves on which they live. Birds as the universal visually oriented predators are usually considered as selective agents on camouflage in arthropods. Although a majority of previous studies of cryptic protective colour patterns have been based on the visual system of human instead of on the predator’s, many predators can detect UV light in addition to the human visible spectrum. Therefore, a human interpretation of animal visual signals may be not adequate, and cryptic protective coloration should be assessed and quantified through a potential predator’s eyes. The results from Chapter3indicate that the principla spectrum component of the spiders and the leaves is significant different. And the color contrast of the vision model between the spiders and the leaves revealed that, the bird can’t distinguish the spiders from the leaves. But the aromatic color contrast revealed that, the bird can catch the sight of the P. Bifurcilinea but cant’t see the P. vittata. In sum, the crypsis of the UV-induced fluorescence of the spiders is not perfect. This study therefor opens a window for understanding the function of fluorescence in salticid spiders and also characterized the further research of the balanced evolution of animals’some sort of character. |
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