Parents of teenagers know just how much preening goes on before a date, and animals are no different: male peacocks display their impressive tails, blue footed boobies flash their feet and male red grouse get out an impressive pair of red combs above their eyes. Francois Mougeot from the Estación Experimental de Zonas Áridas in Spain is intrigued by the messages conveyed by the elaborate sexual ornaments that birds have evolved. ‘I am interested in which sort of quality these ornaments emphasise and how does it work so that it gives a reliable message,’ explains Mougeot. Knowing that the immune systems of birds infected with parasites increase the levels of damaging oxidising agents in the birds' blood, and that these oxidising free radicals could be mopped by a red pigment, carotenoid, which is also found in the birds' combs, Mougeot and his collaborator, Stuart Piertney from the University of Aberdeen, wondered whether the eye combs of healthy uninfected birds would reflect their fitness relative to infected individuals. The duo decided to take a close look at some male red grouse combs (p. 400).
At the time that Mougeot began the study, he was working in Aberdeen, so he drove down to the British Army base at Catterick Moor in North Yorkshire, to trap some male grouse. Dazzling the birds at night with a bright light, Mougeot, Jesus Martínez-Padilla and Lucy Webster attached radio transmitters to the animals, collected a blood sample, photographed and measured the size of the combs and waited until the birds had defecated so that they could assess the number of parasites that each bird was carrying. Then they gave half of the birds a drug to clear the parasites from their intestines and left the rest of the infected birds untreated.
After releasing the birds, the trio returned to the moor two weeks later to find out how the birds had done. Tracking them down at night by following their radio signals, the team photographed and measured the combs again and collected another blood sample from each bird before removing the transmitter and returning the birds to the moor.
So how had the birds' combs faired? The combs of the uninfected birds were clearly brighter and larger than the birds that were still infected with parasites. And when Jonathan Blount in Exeter, UK and Lorenzo Pérez-Rodríguez in Ronda de Toledo, Spain, analysed the blood samples they could see that the uninfected birds were suffering significantly less oxidative damage than they had before their infection was treated. The team also noticed that the birds that reduced their oxidative damage most had brightened their combs the most.
‘Oxidative stress does mediate the relationship between parasites and the coloration,’ says Mougeot; so the brightness of male red grouse combs seems to be a reliable indicator of the bird's fitness and could tell females how strong a potential suitor's immune system is. Mougeot also suggests that the carotenoids could play a role in the birds' defence systems. He explains that carotenoids could act as antioxidants so that, instead of diverting carotenoids to brighten their combs, the infected birds have dim combs because they retain carotenoids in the blood to combat infection-induced oxidative stress. Alternatively, the carotenoid levels of infected birds could be lower than healthy birds because the carotenoids are damaged by oxidation, dimming their combs. Either way, the combs of infected birds are dimmer and probably a turn-off for red grouse eligible bachelorets.