Sea otters (Enhydra lutris) are unique among marine mammals in that they lack blubber and instead must rely on especially thick fur to keep warm in the marine environment. Despite a wealth of knowledge regarding the functional morphology of the adult pelage, almost nothing is known about the characteristics of lanugo (newborn pelage). To better understand the characteristics of sea otter fur across ontogeny, we investigated the morphology and thermal function of otter pelts (n=39) across six age classes: neonates, small pups, large pups, juveniles, subadults, adults. Guard hair length and hair circularity were collected for morphological analysis. Thermal conductivity and thermal resistance were measured to determine thermal function of pelts. Neonates and small pups had longer guard hairs compared to older age classes (p<0.001), consistent with the timing of the molt of the lanugo and growth of more adult-like pelage. Guard hair circularity showed no differences across age classes (p=0.86), suggesting the flatness of the guard hairs may be a relatively conserved characteristic across ontogeny. The neonatal pelage had a higher thermal conductivity in air (p<0.001) compared to the juvenile, subadult, and adult pelage, suggesting lanugo pelts are poorer insulators. However, thermal resistance did not differ across age classes (p=0.612), as the greater thickness of the lanugo pelage compensated for the higher thermal conductivity. Future work will determine fur density using histological methods and measure thermal function of pelts in water. This study is the first to investigate the functional morphology of sea otter lanugo, and will connect the fur structure to its thermal function.