Protein components of femoral gland secretions in a polymorphic lizard (Podarcis muralis)

In animal intraspecific communication, the information conveyed by a signal is determined by the social function of the signal itself, and allows predicting its pattern of variability: an identity signal will exhibit a high inter-individual variation, an ontogenetic stability, a strong genetic determination, and no relation with quality traits. A strategy signal will share with the previous the genetic determination, and the independence from the condition or quality of the individual, but it will be discrete and multimodal. On the opposite, quality cues will be continuous traits, less variable than identity signals, and strongly related to condition and quality of the signaller. The chemical channel is the most ancient one, and the only one able to continue signalling also in the absence of the signaller. Such property becomes very important in decision-making processes, when territorialism or multiple strategies occurred in a population. Lizards are good models to study chemical communication. Most species have a set of epidermal glands (femoral glands) which produce waxy secretions used as cues in social context. They are a mixture of lipids and proteins, the former used to communicate individual quality and condition, the latter, far less studied, maybe involved in signalling. The proteins nature, and some preliminary observations has suggested that they may convey identity-related information. The aim of my research is to assess if proteins from femoral glands are actually used as signals and if they convey identity- or strategy-information. As study species I chose the Common Wall lizards (Podarcis muralis), a small lacertid presenting a ventral colour polymorphism. I first investigated the intra- and inter-populations patterns of variability of the protein assemblage, using one-dimensional electrophoresis, to demonstrate that signal is variable enough to support the identity-signal hypothesis. I then moved to the comparison of the protein patterns of the three main colour morphs. I used two-dimensional electrophoresis to obtained a finer resolution, and spectrometric analysis to identify proteins. I expected to find a morph-specific protein composition, according to the strategy-signal prediction. To obtain an experimental evidence for the communication role of proteins, I set up a behavioural test in neutral arenas in order to demonstrate that lizards can decrypt the information encoded into proteins alone. Male behaviour was observed in presence of the protein scent of its own, of that by an unfamiliar male, and a control: a treatment effect would have been interpreted as the prove that proteins were detectable and informative. Finally, I investigated if and how the lipids and proteins co-varied along the activity season, to verify the prediction that only lipids, as quality-signals, would have shown a variation in their composition. Results from the first three steps of the research agreed with the hypothesis that proteins are identity- and strategy-signals: the among-individuals variation was large, and accompanied by a genetic correlation with clade and population of origin (identity-signal); colour morphs had their own protein pattern, with specific spots in the two-dimensional electrophoresis maps (strategy-signal); lizards responded differently to the proteins from an unfamiliar males (detectable and informative signals). The final step introduced some unpredicted responses: while some parts of the protein signal were, as expected, seasonally stable, some others were not, and varied according to the lipids content. This outcome requires a more complex hypothesis about the protein roles, which will remain speculative until a clear protein identification will be attained. Unfortunately, the identification attempts I performed during the different research steps failed, due to the lack of specific databases against which to match spectrometry data. So, further work should focus on this specific point.