Scientists have found that the intensity of colour in a male cichlid fish is an indicator of its parasite load (the number of parasites an individual is carrying) and that such parasite-mediated sexual selection may have led to the divergence of two species of Lake Victoria cichlids.
Martine Maan, Anne van Rooijen, Jacques van Alphen and Ole Seehausen investigated the relationships between the intensity of colour in male individuals of the Lake Victoria cichlid species Pundamilia pundamilia and P. nyererei and the overall health of the fish (as measured by the parasite load) and published the results of their research in the latest issue of the Biological Journal of the Linnean Society.
The authors examined male individuals of both species collected from Makobe Island (south-eastern Lake Victoria; western Speke Gulf) for both colour and parasite load.
Mature males of P. nyererei are predominantly red, and the degree of redness in an individual (which the authors name redscore) was calculated by determining the area of red in the dorsal, anal and caudal fins and dividing this by the total fin area.
Mature males of P. pundamilia are predominantly blue, and the degree of blueness of an individual (which the authors name bluescore) was similarly calculated by counting the number of blue-reflecting scales on the body and adding this number to twice the number of blue-reflecting dorsal fin membranes (each membrane between two spines or rays occupies approximately twice the area of one scale).
Parasite load was determined by dissecting individuals and checking and counting the numbers and diversity of parasites in the skin, fins, gills, abdominal cavity, gonads, liver, and gastrointestinal tract.
The authors found that low parasite loads are associated with high body redscores in P. nyererei and high body bluescores in P. pundamilia. Coupled with the fact that females of P. pundamilia and P. nyererei use male nuptial coloration in interspecific mate choice, the authors conclude ur results suggest that parasite-mediated sexual selection within each species could cause divergent selection between the species on male coloration and parasite resistance.
Previous research by Maan and co-authors showed that female nyereri can select the best sexual partners by choosing the males with the deepest red colouration and the largest territories, as the fish have the least parasites. (See Deeper red and bigger territory means fewer parasites, News, 10.8.2006.)
For more information, see the paper: Maan, ME, AMC van Rooijen, JJM van Alphen and O Seehausen (2008) Parasite-mediated sexual selection and species divergence in Lake Victoria cichlid fish. Biological Journal of the Linnean Society 94, pp. 53"60