Adults are silvery-grey on the back, fading to cream on the throat, chest and belly. Additional light patches may also be found on the body. Over time, the coat looks brown above and yellow below. Males, and some females, turn a dark brown or black with age. Some Hawaiian monk seals have a red or green tinge from algal growth.
Monk seals tend to be solitary, both on land and in the water.
The breeding season occurs between December and mid-August although most pups are born from March to June. Pups, weighing approximately 16-18 kg and measuring 1 m, are born with a long, woolly, black coat which is shed at about six weeks of age. At about this time, the pups are weaned. Females reach sexual maturity at 5 or 6 years of age, and may live to be 20-25 years old. Monk seals, like most monachines seals, walruses, fur seals and sea lions, have four mammae.