'The Males Have To Make Themselves Stand Out'
Fish blind to feminine charms
Dalton, Brian E. & Thomas W. Cronin, N. Justin Marshall & Karen L. Carleton, 2010: The fish eye view: are cichlids conspicuous?, Journal of Experimental Biology, v.213, pp.2243-2255. doi: 10.1242/jeb.037671
The extent of animal colouration is determined by an interplay between natural and sexual selection. Both forces probably shape colouration in the speciose, rock-dwelling cichlids of Lake Malawi. Sexual selection is thought to drive male colouration, overcoming natural selection to create conspicuous colour patterns via female mate choice and male–male competition. However, natural selection should make female cichlids cryptic because they mouthbrood their young. We hypothesize that as a result of both sexual and natural selection, males will have colours that are more conspicuous than female colours. Cichlid spectral sensitivity, especially in the ultraviolet, probably influences how colours appear to them. Here we use simple models of the trichromatic colour space of cichlid visual systems to compare the conspicuousness of male and female nuptial colours of nine species. Conspicuousness of colours was evaluated as their Euclidian distance in colour space from environmental backgrounds and from other colours on the same fish. We find in six of the nine species that breeding males have colours that are statistically more conspicuous than female colours. These colours contrast strongly with each other or with the backgrounds, and they fall within a range of spectra best transmitted in the habitat. Female colour distances were sometimes smaller, suggesting that females of some species are more cryptic than males. Therefore, selection can differentially act to generate male colours that are more conspicuous than those in females. However, in two species, females had colours that were more conspicuous than male colours, suggesting that other selective forces and possibly sexual conflicts are acting in this system.
Key words: sexual selection, speciation, colour, signal, vision, cichlid
Abbreviations: Brock, brown rock background • Erock1-3, excrement-covered rock backgrounds 1-3 • S3m, Thumbi West Island spacelight background at 3 m depth • S7m, Thumbi West Island spacelight background at 7 m depth
Cover: Closely related cichlids of Lake Malawi differ in male nuptial colouration, while females often appear less colourful to the human eye (from top, females appear in odd rows with conspecific males in even rows below). Dalton et al. (pp. 2243−2255) show that, according to cichlid visual systems, territorial males often have more conspicuous colours than conspecific females, supporting the hypothesis that sexual selection overpowers natural selection in the evolution of male, but not female, cichlid colouration. However, conspicuous female colours in several species raise questions about their behavioural role and the evolutionary processes creating them. Photos courtesy of Justin Marshall, Ad Konings and Brian Dalton.
You look at a fish in an aquarium. You know what you see. But what does the fish see through that milky round eye?How does the fish see colour and space and contrast?Neurobiologists at the Queensland Brain Institute have been poking around inside the eye of the cichlid fish.
Professor Justin Marshall has discovered fish don't see colours the same way humans do!
The research, which features on the cover of the latest edition of the Journal of Experimental Biology, focused on cichlids in Lake Malawi in southern Africa. Scientists found cichlids could see ultra-violet colors, making their sight much better than that of humans, who can only see the basic red, green and blue spectrums.
The scientists also studied the cichlid's sexual communication and found that while female cichlids are brightly colored to the human eye, they are relatively inconspicuous to their potential mating partners. Conversely, the males appear brightly colored to the females and stand out in much the same way as a bowerbird.
"The males have to make themselves stand out. It's definitely worth it because it shows the female they are fit and healthy," Professor Marshall said.
Further, the neurobiologist said cichlids provided a fascinating research model for scientists, as they had evolved very quickly over the last million years.
"Understanding the eyes and the way in which they're put together and what advantage they give to the animal's sensory systems is very important for future discoveries," Professor Marshall said.