At birth, pups are roughly 60 to 70 cm and weigh approximately 6.5 to 8 kg. Their pelage is initially chocolate brown, but is replaced by adult-like fawn colred fur by the age of two months. Australian sea lions are sexually dimorphism, with a significant difference in the size of females and of males. As an adult, males grow to about 2.5 m and weigh 300 kg. Females grow to a length of about 1.8 m and weigh approximately 105 kg. Females reach sexual maturity at around 3 years of age, whereas males usually do not mature until they are 6 years of age or older. Males darken even further as they mature, and go through a transition phase where they have spots on their chests.
Australian sea lions have a large head with a long, narrow, and tapered muzzle. The skull has a sagittal crest approximately 30 mm in height. Males display a much larger head than females and juveniles, as well as significantly broader shoulders. Their pinnae, or ears, are incredibly small and lie close to the head. Juveniles are a dark brown with a pale crown and a dark facial mask. Subadult males and females display a coat that is fawn to silvery-gray on top and tan to pale yellow on the bottom. Mature males are dark brown with a cream-colored crown and nape, and a paler chest and throat area.
Range mass: 105 to 300 kg.
Range length: 1.8 to 2.5 m.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: male larger