Skull shape and size changes in different subpopulations of the California Sea Lion (Zalophus californianus) in Mexico

California sea lions (Zalophus californianus) are distributed along the Gulf of California and northeastern Pacific coast. Genetic studies have suggested the existence of 3 to 4 subpopulations in Mexico-1 on the Pacific coast of the Baja California peninsula (PC) and 3 in the Gulf of California-but the extent of this divergence is unclear, and it remains unknown if these subpopulations are morphologically distinct. In this context, we analyzed variation in skull size and shape of adult males between 5 and 13 years of age among 2 (north and central) of the 3 subpopulations of Z. californianus from the Gulf of California and the PC in Mexico. However, since the sample sizes for the 2 subpopulations in the Gulf of California were small, we merged all samples into 1 that we called the Gulf of California (GC) subpopulation. Artificial neural networks and geometric morphometrics were used to analyze skull images to quantify the extent to which these geographically separated subpopulations are undergoing morphological divergence. Our results find no significant differences in size in any view between the 2 subpopulations, but significant differences in the morphology of the dorsal, ventral, and lateral views of the skull between the 2 subpopulations. Overall, Z. californianus from the GC subpopulation have wider and lower skulls, extended back and outward with a more voluminous (bulky) nuchal crest, and narrow rostrum in comparison with skulls of Z. californianus from the PC subpopulation. Results concur with a previous genetic-based study, demonstrating that Z. californianus from both subpopulations in Mexico are diverging in their skull morphology and perhaps suggesting that they are experiencing different evolutionary pressures. California sea lions are marine carnivores that live along the coast of the North Pacific. In Mexico, their subpopulations are geographically isolated, and genetic evidence suggests that they are diverging. Artificial neural networks and geometric morphometrics were used to analyze California Sea Lion skulls from different Mexican regions, revealing morphological differences. The differences in skull form suggest that sea lions from the Gulf of California are experiencing different evolutionary pressures than those from the Pacific coast of the Baja California peninsula. El lobo marino de California, Zalophus californianus se encuentra ampliamente distribuido a lo largo de la costa nororiental del Pacifico y el Golfo de California. Los estudios geneticos han sugerido la existencia de 3 a 5 subpoblaciones en Mexico- una en la costa del Pacifico de la peninsula de Baja California (CP) y tres en el Golfo de California- pero el grado de divergencia no es claro, y se desconoce si estas subpoblaciones son morfologicamente distintas. En este contexto, analizamos la variacion del tamano y la forma del craneo de machos adultos de entre 5 y 13 anos entre 2 (norte y central) de las 3 subpoblaciones de Z. californianus del Golfo de California y la de la CP en Mexico. Sin embargo, dado que los tamanos de muestra de las 2 subpoblaciones del Golfo de California eran pequenos, juntamos todas las muestras en 1 sola a la que llamamos la subpoblacion del Golfo de California (GC). Se utilizaron tecnicas de morfometria geometrica y redes neuronales artificiales para analizar imagenes de craneos con el fin de cuantificar en que medida estas subpoblaciones separadas geograficamente, han divergido morfologicamente. Los resultados no muestran diferencias significativas en ninguna de las vistas entre las 2 subpoblaciones, pero si hubo diferencias significativas en la forma de las vistas dorsal, ventral y lateral de los craneos entre las dos subpoblaciones. En general, los ejemplares de Z. californianus de la GC tienen craneos mas anchos y bajos, 1 caja craneana alargada anteroposteriormente con 1 cresta sagital mas voluminosa (voluptuosa) y un rostro mas angosto en comparacion con los craneos de los Z. californianus de la CP. Nuestros resultados son consistentes con trabajos geneticos previos, demostrando que ambas subpoblaciones de Z. californianus en Mexico estan divergiendo en la forma de sus craneos, lo que sugiere que estan experimentando presiones evolutivas distintas.