Is developmental instability sex-specific? Case study in two spiny lizards with different reproductive modes.

Developmental instability reflects an organism's ability to buffer genetic and environmental perturbations during growth and is commonly assessed through fluctuating asymmetry and canalization. Evidence from diverse animal groups suggests that developmental instability can be sex specific, often showing sex-biased sensitivities to developmental perturbations. Additionally, reproductive mode can shape developmental environments, potentially influencing how morphological variation is buffered during ontogeny. Yet, the relative importance of sex and reproductive mode in shaping developmental instability remains poorly studied. We tested whether developmental instability is sex specific in two spiny lizards with contrasting reproductive modes: the viviparous Sceloporus grammicus and the oviparous Sceloporus aeneus. We analyzed a curated subset of 69 adults (40 S. grammicus, 29 S. aeneus) from high-elevation sites in Estado de México, photographed the dorsal head, digitized 22 landmarks, and conducted Procrustes superimposition to extract shape variables. Fluctuating asymmetry was quantified using Procrustes distances, while canalization was assessed through comparisons of within-group variance-covariance matrices and principal component analysis. Fluctuating asymmetry did not differ between sexes in either species, and asymmetry variance was similar between males and females within species but differed between species. Canalization analyses revealed stronger buffering in S. grammicus males compared to both sexes of S. aeneus, while no sexual dimorphism in head shape was detected in either species. Together, these results suggest that developmental stability is more strongly associated with reproductive mode than with sex, and that size-related sexual differences do not translate into shape divergence. Our findings highlight how developmental environments linked to reproductive strategies, but not sex, can modulate morphological buffering.