Morphological conservatism and natural history in two sympatric Amerotyphlops species (Serpentes: Typhlopidae), from the Atlantic Forest of Northeastern Brazil

Blindsnakes (Scolecophidia) are among the most morphologically conservative reptiles, yet the extent and nature of their phenotypic variation is poorly documented. We provide an integrative assessment of allometry and trophic ecology in two sympatric species of Amerotyphlops from the Atlantic Forest of northeastern Brazil, A. brongersmianus and A. paucisquamus . Based on 100 individuals, we combined morphometric, reproductive, dietary, and phenological data to discuss how fossoriality influences body shape and ecological strategies. A. brongersmianus exhibited greater overall robustness, with females displaying significant allometric shifts in snout and trunk width, whereas A. paucisquamus showed somatic isometry (including body mass), contrasting with strong positive allometry in reproductive organs. Regarding sexual dimorphism, neither species exhibited statistically significant differences in body proportions after correction for multiple comparisons, although A. brongersmianus presented higher SSD indices. Reproductive allometry revealed divergent strategies, contrasting the negative scaling of testis width in A. brongersmianus with the marked positive scaling of testicular dimensions in A. paucisquamus . Furthermore, while A. brongersmianus lacked significant bilateral asymmetry, A. paucisquamus exhibited directional asymmetry in testis width. The species displayed distinct phenologies associated with rainfall, yet shared an exceptionally narrow myrmecophagous niche, feeding almost exclusively on ant brood. Morphological differences in cranial width and trunk robustness align with distinct prey assemblages and soil use, although these differences fall within a narrowly conserved morphofunctional space, implying fine-scale ecological partitioning. Our results show that morphological stasis in Amerotyphlops may coexist with modest, non-pervasive adaptive adjustments along constrained functional axes, highlighting how subtle developmental and ecological modulations sustain evolutionary persistence in fossorial lineages. • Sympatric Amerotyphlops species differ in body size and robustness. • External sexual dimorphism is absent after statistical correction. • Isometric growth prevails, reflecting strong fossorial constraints. • Both species exhibit synchronous reproduction and ant-brood diets. • Data reinforces morphological stasis with subtle ecological tuning.