Impact of Life History on Hippopotamus Skull Ontogeny.

The biological processes underlying the wide phenotypic mammal diversity are still not thoroughly understood. In this study, we examined how major stages in the life history of the common hippopotamus (Hippopotamus Amphibius) influence its craniomandibular morphology throughout ontogeny. Using geometric morphometrics and life-history meta-analysis correlations, we characterized skulls from 198 individuals spanning 20 developmental stages. The most significant morphological changes were observed during early infancy (0-3 years), coinciding with lactation and weaning, and during puberty (10-15 years), coinciding with reproductive maturation. These findings align with growth patterns typical of social mammals exhibiting high sexual dimorphism. Notably, we identified a pattern previously undocumented in any other vertebrate: cranial morphology stabilizes earlier than the mandibular one. Specifically, late-stage (20-25 years) shape modification in the mandibles indicates progressive reconfiguration of masticatory biomechanics as well as a continuous change of dental occlusion throughout life. This pattern is common in both male and female individuals and may be related to shifts in diet rather than sexual selection. This study provides the most comprehensive ontogenetic dataset for a semi-aquatic, large semigraviportal mammal with a polygynous social structure, offering a valuable foundation for future evolutionary studies based on comparative analyses.