Micro-organisms include important degrading agents such as bacteria and fungi that digest the keratin (Burtt & Ichida 1999) modifying colour-based signals (Shawkey et al. Thus, these signals may require additional maintenance costs related to defending them from exposure to the physical and biological environment. Any signal based on morphological structures is prone to deterioration due to abrasion (Burtt 1986) or damage induced by degrading organisms. Reliability of signals is mainly thought to depend on costs associated with their production, showiness and/or maintenance (Maynard-Smith & Harper 2003), and an important part of maintenance costs is related to resources allocated to prevent signal degradation. Those results will help to understand the evolution of ornamental signals.Ī variety of morphological, behavioural and physiological signals of animals that evolve in social contexts (Westneat 2012) allows individual senders to inform about certain characteristics that will influence decisions of receivers (Endler 1993 Ruxton & Schaefer 2011). 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.This is further supported by the relationship detected between antimicrobial properties of uropygial secretion and the level of feather degradation. 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.Only in males was uropygial gland size negatively associated with the level of feather degradation. Finally, the size of the uropygial gland in both males and females predicted volume and the inhibitory capacity of secretion against feather-degrading bacteria.
Moreover, degradation status of male and female throat feathers did not differ, but was positively associated with feather bacterial density. 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.We also experimentally evaluated the capacity of each secretion to inhibit growth of a keratinolytic bacterium. In addition, the volume of the uropygial gland and its secretion was measured and the secretion extracted. 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. Here, we explore a possible role for feather-degrading bacteria driving the design of ornamental throat feathers in male spotless starlings ( Sturnus unicolor).The role of micro-organisms in animal signalling has, however, rarely been investigated. Micro-organisms may degrade signalling traits and therefore influence the transmitted information and evolution of these characters. Mechanisms guaranteeing reliability of messages are essential in understanding the underlying information and evolution of signals.Con todo es necesaria algo de regeneración vegetal y se han establecido pequeños cercaditos inaccesibles a las vacas para que en ellos crezcan acebuches nuevos y algo de hierba y matorral. El suelo está muy desprovisto de vegetación por la gran carga ganadera de la zona, pero ésto es fundamental para algunas especies que requieren áreas muy áridas como Anthus Campestris, la bisbita campestre u Oenanthe Oennanthe, la collalba gris. No sólo sus usos son heterogéneos, también lo son sus ecosistemas: hay una laguna temporal irrigada por el arroyo majaberraque, una zona de pastizal abierto, un bosque de acebuches adehesados y un pinar con árboles de gran envergadura. La reserva es a la vez paraje protegido, lugar de explotación de ganado vacuno extensivo y coto de caza. El batir de alas de la especie es bastante potente y característico. Al acabar el baño podemos oír cómo se marchan volando lastrados por el peso del agua que mojas las plumas de sus alas.
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