The Grand Dame of Monk Seals

March 16, 2017

Monk Seal of the Month: March 2017

The monk seal of the month for March is an adult female known as Y377.

People often ask how long monk seals can live, and despite that we've been studying this species for more than three decades, we still don't know the answer to this question. Since the early 1980s, the Hawaiian Monk Seal Research Program has been tagging newly weaned pups and following them throughout their lives. Y377 turned 32 years old in 2016, making her, to our knowledge, the oldest wild seal based on tagging data.

Y377 was born in 1984 at French Frigate Shoals in the Northwestern Hawaiian Islands (NWHI) and, as far as we know, she's lived there her whole life. Some seals move between the islands and atolls of the NWHI, and even between NWHI and the main Hawaiian Islands, but we've seen Y377 at French Frigate Shoals — and only there — every year since she was born. She may have sneaked off to vacation at other sites when our researchers weren't around to see her, but French Frigate is clearly her home.

Although Y377 is outstanding in terms of her longevity, her early life was relatively ordinary. She matured into an adult at age 6 and began having pups by at least age 10. She had a total of 8 pups that we're aware of, the last one when she was at the ripe old age of 24! Monk seal researchers are only in the field for a part of each year, so this pup count is really a minimum — she may have birthed and weaned more pups when researchers weren't around to see them. Considering this, Y377 surely did her part to sustain the seal population of French Frigate Shoals. 

 French Frigate Shoals is infamous for being a tough place to make a living as a monk seal. At the time Y377 was born, the area hosted the single largest population of monk seals in all of Hawai'i, but the population there began to drop precipitously just a few years later. Though lots of pups were being born, many starved once they were weaned and on their own. Then in the late 1990s, Galapagos sharks began killing and eating a large portion of the pups born every year, which prevented the population from recovering. Luckily, in recent years, survival of young seals has improved at French Frigate Shoals and the population is showing some signs of recovery.

In light of this checkered history, it might seem surprising that the oldest known wild seal lives at French Frigate Shoals. How is this possible?

For one thing, Y377's mother gave her a very good start in life. Monk seals nurse their pups for 5–7 weeks, then abruptly wean them without teaching them to find food. Ideally, a mothers' rich milk will allow her pups to build a large blubber reserve that sustains them while they learn to feed on their own; because of this, fatter weaned pups have a greater chance of surviving than skinnier ones. Y377 was an extremely fat baby. At the time she was tagged in October 1984, she had a very robust girth of 124 centimeters (49 inches). That's the upper end of the baby fat range! 

Also, Y377 was lucky to be born at a time when around 75% of weaned pups lived to become adults. Around a decade later, that number fell to below 10%. Interestingly, even in times when young seals struggle, adults still tend to do well — Y377 was already an experienced adult seal and knew how to weather the hard times when the conditions deteriorated at French Frigate Shoals.

The question of how long a monk seal can live remains unanswered, but because of Y377, we can definitively say it's at least 32 years. In a few months, field researchers will return to French Frigate Shoals to check on the seals, as they do every year. We hope and expect to find that Y377 has successfully taken another spin around the sun, breaking her own record.