Abstract
Although parasites may impair the expression of tail ornaments in birds, the importance of parasitism in driving the evolution of the initial stages of tail ornamentation is not well understood. Parasites could have negatively affected the expression of nonexaggerated, functional traits before these evolved ornaments, or they could have played a relevant role only after tails became ornamental and hence too costly to produce. To shed light on this issue, we studied the correlation between the abundance of feather mites (Acari, Proctophyllodidae) and the size, quality, growth rate and symmetry of tail feathers of blackcaps (Sylvia atricapilla), a non-ornamented passerine. Tail length was not correlated with mite load, yet blackcaps holding many mites at the moment of feather growth (fledglings) had lighter and more asymmetric feathers that grew at relatively lower rates. In blackcaps whose mite load was measured one year after feather growth (adults), only the negative correlation between mite intensity and feather symmetry remained significant. Changes in mite load since the moult season could have erased the correlation between condition-dependent feather traits and current parasite load in adults. Our results support the idea that different traits of non-ornamental feathers can signal parasite resistance. Therefore, parasitism could have played a central role in the evolution of tail ornamentation ever since its initial stages. © 2002 The Linnean Society of London. Biological Journal of the Linnean Society, 2002, 76, 481–492.
Highlights
Since Hamilton & Zuk (1982) introduced the idea that parasites may affect mate choice, parasitism has become an important issue for understanding the evolution of ornaments (Clayton, 1991a; Zuk, 1992; Andersson, 1994; Møller, 1994)
Date and time of capture had no influence on mite load, and neither mite abundance was associated with the body condition of their hosts measured as body mass relative to body size (ANCOVA with body mass as the dependent variable, age and population as factors, and mite abundance and tarsus length—a measure of body size—as covariates, variables Log10transformed to linearize relationships: age F1,68 = 4.93, P = 0.030, population F1,68 = 4.78, P = 0.032, body size F1,68 = 16.85, P < 0.001, mite abundance F1,68 = 0.001, P = 0.98, all interactions between independent variables with P > 0.25)
The health status of blackcaps at the moment of feather development was apparently related to the expression of their feather characters, as shown by the negative correlations found between mite load and feather growth rate, quality and symmetry in fledglings
Summary
Since Hamilton & Zuk (1982) introduced the idea that parasites may affect mate choice, parasitism has become an important issue for understanding the evolution of ornaments (Clayton, 1991a; Zuk, 1992; Andersson, 1994; Møller, 1994). Central to this hypothesis is that an intense parasite infection may impair the host body condition (Lehman, 1993; Lochmiller et al, 1993; Møller et al, 1998), reducing the expression of ornaments that are costly to produce (Zuk, 1992; Hill & Montgomerie, 1994; Veiga & Puerta, 1996).
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