The Secret of Secrets: Carbonic Anhydrase Concentration in Lizards' Femoral Gland Secretions Is Tuned to Environmental Conditions

Among the modalities of animal communication, the chemical channel is the only one that allows signalers and receivers to communicate without being necessarily in the same place at the same time. This asynchrony may influence signal design, as its effectiveness depends on adapting to predictable environmental conditions. Many lizard species scent-mark territories by depositing waxy secretions made of protein-lipid mixtures from specialized epidermal glands. These secretions, once left on a substrate, face environmental fluctuations, and their lifespan depends on the mixture's ability to tolerate such changes. Since some proteins in these secretions are enzymes, we hypothesize they may serve homeostatic functions, enabling the mixture to actively respond to external conditions. Accordingly, we can expect that (1) enzymes remain active in secretions; and (2) their abundance varies across an environmental gradient, tuning the homeostatic ability to the predictable external conditions. We tested these predictions using carbonic anhydrase (CA), an enzyme found in many lizard secretions that regulates pH by catalyzing the reaction between water and carbon dioxide. Protonography confirmed CA activity in proteins extracted from femoral gland secretions from eight Podarcis lizard species and Psammodromus algirus. We also analyzed CA abundance in males (N = 70) from 12 Ps. algirus populations, and its correlation with the environmental gradient (geography, topography, climate). Model comparison revealed a credible relationship between CA concentration and geography as well as bioclimatic variables in Ps. algirus. Taken together, these findings suggest CA plays an enzymatic function that may help stabilize the internal chemical environment of secretions, potentially enhancing the scent-mark's lifespan and effectiveness under varying environmental conditions.