Who did that? Self and colour-morph discrimination from faeces in the common wall lizard

Faeces may serve as complex chemical cues in terrestrial vertebrates, able to convey to conspecifics diverse information about the individual who left them. While lizards are known to scent-mark territories with faecal pellets, the specific roles of different chemical fractions in social communication remain poorly understood. Specifically, faecal hydrophilic components may include peptides and proteins of the individual itself, which are hypothesized to convey identity-related information, in analogy with what occurs in femoral gland secretions. This study investigated whether the hydrophilic extract of faeces allows the common wall lizard ( Podarcis muralis ) to discriminate between its own scent and that of unfamiliar conspecifics. Using automated trajectory analysis, we monitored the behavioural responses of 36 adult males to three stimuli : distilled water (CTRL), the aqueous extract of own faeces (SELF), and that of an unfamiliar male (OTHER). Results indicated that unfamiliar scents elicited significantly higher exploration of the central, exposed area of the experimental arena compared to CTRL and SELF treatments. Furthermore, this response was specifically modulated by the donor’s ventral colour morph; lizards showed increased activity when exposed to “homomorphic” stimuli (same morph) rather than heteromorphic ones. These findings suggest a modular organization of faecal chemical cues in lacertids, where the hydrophilic fraction primarily conveys stable identity-related information (self-recognition and morph) mirroring the functional partitioning observed in femoral gland secretions and highlights the complexity of lizard semiochemical communication.