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Abstract The evolution of organisms can be studied through the lens of developmental systems, as the timing of development of morphological features is an important aspect to consider when studying a phenotype. Such data can be challenging to obtain in fossil amniotes owing to the scarcity of their fossil record. However, the numerous remains of Rancho La Brea allow a detailed study of the postnatal changes in an extinct sabertoothed felid:Smilodon fatalis. Despite numerous previous studies on the ontogeny ofSmilodon, an important question remained open: how did the cubs ofSmilodonacquire and process food? By applying 3D geometric morphometrics and finite element analyses to 49 mandibles at various developmental stages (22 ofS. fatalis, 23 ofPanthera leo, and 4 of early diverging felids), we assess the changes in mandibular shape and performance during growth. Both lions and sabertooths exhibit a shift in mandibular shape, aligning with eruption of the lower carnassial. This marks the end of weaning in lions and suggests a prolonged weaning period inS. fatalisowing to its delayed eruption sequence. We also highlight distinct ontogenetic trajectories, withS. fatalisundergoing more postnatal mandibular shape changes. Finally, althoughS. fatalisappears more efficient thanP. leoat performing an anchor bite, this efficiency is acquired through ontogeny and at a quite late age. The delayed shape change compared withP. leoand the low biting efficiency during the growth inSmilodoncould indicate an extended duration of the parental care compared withP. leo.