Natural History
Sleepy Hawaiian monk seals
Reproduction
- Most pups are born between February and July, with a peak during April and May.
- Birth weight is 11 to 15 kg; normal weaning weight is 50 to 80 kg.
- Nursing time is 5 or 6 weeks, during which mothers do not eat.
- After weaning, pups learn to forage on their own.
- About 65% of all adult-sized females give birth each year, but birth rate can vary.
- Nursing pups are sometimes exchanged between females.
- Mating occurs in the water, and is rarely observed.
- Females give birth on beaches with adjoining shallow waters, which provide protection from sharks.
- Females give birth for the first time at 5 to 10 years of age.
Longevity
- Maximum age is believed to be 25-30 years, but few seals live this long.
- In the 1980s, pup and juvenile survival was high compared to other pinnipeds. Since that time, early survival rates have been lower.
Physical Description
- Newborn pups are black; adults are dark grey to brown dorsally, light grey to yellow-brown ventrally.
- Adults weigh 180 to 270 kg; on average, adult females are slightly larger than adult males.
- Annually, seals have a "catastrophic molt."
Feeding Patterns
- Prey items include reef fishes, eels, cephalopods, and crustaceans.
- Limited diving data indicate most feeding occurs at depths less than 75 to 90 m (but seals occasionally dive to depths exceeding 500 m).
Social System
- Monk seals are less gregarious than many species of seals; they do not aggregate in dense colonies.
- Males probably form a dominance hierarchy, with dominant males doing most of the breeding.
A Hawaiian monk seal watches Galapagos sharks swim dangerously
close to shore
Causes of Mortality
- Emaciation has been a major cause of death for juvenile seals, especially at French Frigate Shoals.
- Tiger sharks and Galapagos sharks are known to be predators of monk seals.
- Seals also become fatally entangled in marine debris.
- Diseases do not appear to be impeding population recovery, but our understanding of the role of disease is very limited. All wild seals carry internal parasites.
- Multiple male aggression, whereby groups of adult male seals kill adult females and immature seals of both sexes, primarily at breeding islands where the number of adult males is significantly greater than the number of adult females.
- Single males sometimes mount, injure, or drown pups.