Age-dependent association between testosterone and crown UV coloration in male blue tits (Parus caeruleus) |
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Authors: | A Peters K Delhey W Goymann B Kempenaers |
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Institution: | (1) Max Planck Institute for Ornithology, 82319 Seewiesen, Germany;(2) Present address: Vogelwarte Radolfzell, Schlossallee 2, 78315 Radolfzell, Germany |
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Abstract: | The proximate basis of sexual traits can suggest mechanisms maintaining honesty in signalling. A central role hereby has been
attributed to testosterone, although its importance for brightly coloured plumage has been questioned. We determined circulating
testosterone levels in male blue tits captured at the start of breeding and demonstrated an age-dependent relationship between
testosterone and male crown UV/blue coloration. In yearling males, testosterone increases with increasing ornamentation (higher
UV chroma, higher chroma, more UV-shifted hue), whereas in older males, this relationship is negative, with less UV-ornamented
males having higher testosterone. This pattern is robust since it occurred in 2 years, before and after egg laying, and in
males sampled during the day and during the night, despite a tenfold difference in testosterone levels. Since more UV-ornamented
young males gain higher within-pair paternity, while less UV-ornamented older males achieve more extra-pair matings, the results
imply that higher testosterone is associated with reproductive success and attractiveness in both age classes. We hypothesise
that this relationship could result from causal effects of testosterone on coloration or through associations with behaviour
and suggest ways to test these hypotheses. Our results caution against premature dismissal of a potential role for testosterone
in maintaining honesty of plumage signals. |
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Keywords: | Testosterone Sexual selection Plumage reflectance Structural UV colour Signal honesty |
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