Hooded seals have blue-gray pelage with black spots over the body. The front of the face is black and this coloration extends posteriorly to just behind the eyes. Their limbs are rather small in proportion to their body, but are powerful nonetheless, making these seals excellent swimmers and divers. Hooded seals exhibit marked sexual dimorphism. Males are slightly longer than females, and reach 2.5 m in length; females average 2.2 m. The more significant difference between the sexes is weight. Males weigh up to 300 kg while females only weigh up to 160 kg. Also unique to males is the inflatable hood and nasal septum.
Hooded seals get their name from the inflatable “hood” on the top of the heads of males. The hood is not present until males are about 4 years old. When the hood is deflated, it hangs down over the upper lip. Males inflate this red, balloon-like nasal septum until it protrudes out of one nostril. Males use this nasal sac for aggressive display and also to get the attention of females.
Hooded seals have many characteristics that differentiate them from other phocids. They have the largest nostrils in the family. The skull is short with a wide snout. They also have a palate that projects posteriorly further than any other seal. One-third of the nasal bone extends beyond the edge of the maxilla. Their incisor formula is unique, with two upper and one lower incisor. The teeth are small and the tooth row is narrow. The dental formula is I 2/1, C 1/1, PC 5/5. Hooded seals have light and dark bands of cementum in the canines that can be used to determine age.
At birth the coloring of young is silver on the dorsal side, without spots, and blue-gray on the ventral side, which accounts for their nickname ”bluebacks.” Young are 90 to 105 cm in length when born and average 20 kg. Around age 1, differences between males and females can be observed; males begin growing larger in weight and length.
Range mass: 160 to 300 kg.
Range length: 2.2 to 2.5 m.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: male larger; ornamentation