Why can harp seal pups lose more than half their body mass after weaning?

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harp seal A mother harp seal and a young “whitecoat.” Adult harp seals are gray with black spots. Young harp seals are called “whitecoats,” “bedlamers,” “beaters,” or “graybacks,” depending on their age.

Harp seal pups experience a significant reduction in body mass after weaning because they undergo a period of fasting that can last up to six weeks. During this time, the pups do not feed, forcing them to rely on their stored energy reserves until hunger eventually motivates them to enter the water and begin hunting for themselves.1

This post-weaning fast occurs because the nursing period is abruptly terminated when the females stop feeding their young to mate with males. While the pups gain substantial blubber during their 12-day nursing phase—reaching approximately 36 kg (79 pounds)—this transition leaves them without a food source for several weeks, resulting in the loss of more than 50 percent of their body mass.1