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Bacteria and the evolution of honest signals. The case of ornamental throat feathers in spotless starlings

AuthorsRuiz-Rodríguez, Magdalena ; Tomás, Gustavo ; Ruiz-Castellano, Cristina ; Soler, Juan José
Keywordsbacterial load
feather-degrading bacteria
sexual selection
Sturnus unicolor
uropygial gland
Issue DateMay-2015
PublisherJohn Wiley & Sons
CitationFunctional Ecology 29: 701-709 (2015)
Abstract1. Mechanisms guaranteeing reliability of messages are essential in understanding the underlying information and evolution of signals. Micro-organisms may degrade signalling traits and therefore influence the transmitted information and evolution of these characters. The role of micro-organisms in animal signalling has, however, rarely been investigated. 2. Here, we explore a possible role for feather-degrading bacteria driving the design of ornamental throat feathers in male spotless starlings (Sturnus unicolor). We estimated length, bacterial load, degradation status and susceptibility to degradation by keratinolytic bacteria in those feathers, compared with non-ornamental adjacent feathers in males, as well as to throat feathers in females. In addition, the volume of the uropygial gland and its secretion was measured and the secretion extracted. We also experimentally evaluated the capacity of each secretion to inhibit growth of a keratinolytic bacterium. 3. The apical part of male ornamental throat feathers harboured more bacteria and degraded more quickly than the basal part; these patterns were not detected in female throat feathers or in non-ornamental male feathers. Moreover, degradation status of male and female throat feathers did not differ, but was positively associated with feather bacterial density. Finally, the size of the uropygial gland in both males and females predicted volume and the inhibitory capacity of secretion against feather-degrading bacteria. Only in males was uropygial gland size negatively associated with the level of feather degradation. 4. All results indicate differential susceptibility of different parts of throat feathers to keratinolytic bacterial attack, which supports the possibility that throat feathers in starlings reflect individual ability to combat feather-degrading bacteria honestly. This is further supported by the relationship detected between antimicrobial properties of uropygial secretion and the level of feather degradation. 5. Our results suggest that selection pressures exerted by feather-degrading bacteria on hosts may promote evolution of particular morphologies of secondary sexual traits with different susceptibility to bacterial degradation that reliably inform of their bacterial load. Those results will help to understand the evolution of ornamental signals.
Publisher version (URL)http://dx.doi.org/10.1111/1365-2435.12376
DOIdoi: 10.1111/1365-2435.12376
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