Effects of nutrition on sexual ornaments and humoral immune responsiveness in adult male pheasants

Female pheasants may base their choice of mates on the expression of male ornaments such as the red chin wattle and the length of the spurs. Both these ornaments are predicted to be sensitive to access to carotenoids (and vitamins). Moreover, because carotenoids also enhance immune function, it has been suggested that there is a trade-off in the use of carotenoids for ornament expression and immune function. We fed adult male ring-necked pheasants (Phasianus colchicus) with food of different nutritional quality. This relieved males from restrictions of access to dietary carotenoids and allowed us to study the simultaneous effect on ornament expression (wattle size and redness, spur length) and immune responsiveness. Two treatment groups were given either a low or a high quality fodder between late February and late May, when males were 8-11 months old. As a result of the food-quality manipulation, males in the high nutrient group achieved significantly redder wattles whereas no significant effect was found for either wattle size or spur length. There was a non-significant tendency that the antibody response to one out of two novel antigens was affected by the experimental treatment, but the mean production of antibodies against the two antigens (diphtheria and tetanus toxoid) was not significantly related to the experimental treatments. Over the experimental period, the change in humoral immune responsiveness was positively related to the increase in spur length, but not related to the change in wattle colouration. We suggest that wattle colour may be an honest indicator of males’ ability to find or assimilate dietary carotenoids, but we found no firm evidence that the mechanism maintaining honesty is a trade-off between the use of carotenoids for ornament colouration and general immune function. In contrast, our study lends some support to the idea that spur length, a sexually selected trait, may reflect humoral immunocompetence in male pheasants.