Building Cetacean Locomotor Muscles throughout Ontogeny to Support High-Performance Swimming into Adulthood.

This article focuses on the postnatal development of locomotor muscles in cetaceans, highlighting differences from terrestrial mammals and pinnipeds due to their aquatic environment and immediate need for breath-hold swimming at birth. Neonatal cetaceans have lower muscle mass, mitochondrial density, myoglobin (Mb) content, and buffering capacity compared to adults, with maturation of these features occurring over months to years depending on species. Unlike pinnipeds and terrestrial mammals, cetaceans generally show little ontogenetic change in muscle fiber type, though some deep-diving species exhibit a shift toward more fatigue-resistant fibers. The underdeveloped musculature of calves limits their swim performance, which they partially compensate for by swimming in "echelon position" alongside their mothers to reduce effort, but this limitation poses risks during anthropogenic disturbances that require rapid escape. Understanding these developmental constraints is important for assessing the impacts of human activities on cetacean populations.