Body Building: Bulking up with male Steller Sea lions

Pilot reached an important milestone this year, hitting 2,000 pounds on May 14, 2022 and reaching his top weight of 2,030 pounds on May 25, 2022. Only two ASLC resident Steller sea lions have reached the 2,000-pound milestone: Woody, the famous first male Steller to call ASLC home, and now Pilot.

Pilot’s impressive weight change mimics that of mature male Steller sea lions in the wild that rapidly gain weight during the spring for the summer breeding season.

In the wild, male Steller sea lions bulk up in preparation to haul out on breeding grounds and guard their territory for the next few months. At the Alaska SeaLife Center, we see our adult males do the same thing, gaining 25 to 50 pounds a week.

Surprisingly, the massive weight gain does not come from an increase in food intake. In fact, male Stellers can go up to two months without eating during the breeding season. During the summer months, Pilot is still offered plenty of food each day but is likely to refuse or eat significantly less because he does not need it. Instead, the seasonal weight gain is completely driven by surging hormones.

In the wild, the males’ increased weight and testosterone levels allow them to stake out and defend territory from rival males on the rookery. This means only a small number of males mate with a large number of females on each rookery, which is why it is important for them to be one of the biggest and strongest males. Although Pilot does not face the same reproductive rivalries as he would see on a rookery, the natural behavior still kicks in each year thanks to the seasonal hormone levels.

The Science Connection

Rookeries are an ocean side land formation that a specific species returns to year after year to give birth and breed. For the population of Steller sea lions near Seward, AK their historic rookery is at Chiswell Island. ASLC Scientists have observed large wild male Steller sea lions on the Chiswell Island Rookery for over 20 years through the Chiswell Island Steller sea lion remote video monitoring research project.

Pilot was transferred to the ASLC in 2011 from a facility in the Netherlands with the hope that he could follow in Woody’s footsteps and be a part of the second generation of sea lion breeding. He has since sired three pups: Perl, Kuliak, and Mist.

Because of scientific research of this unique species, we know that Pilot may not have had the same early breeding success in the wild. While male Steller sea lions can reach sexual maturity as early as three years old, the age that Pilot arrived in Alaska, they usually are not large enough to compete with other males and hold territory until they are nine or ten. Although not in direct competition with dozens of other males like they would be in the wild, Pilot and Woody both followed this trend pretty closely, beginning to have noticeable weight fluctuations around seven or eight years and then hitting the 2,000-pound mark at twelve and thirteen.

Now that sea lion breeding season is coming to an end, Pilot’s weight is steadily and healthily going back down as his hormones level out and he returns to his “winter weight” of around 1,400-1,500 pounds. He weighed in at 1,700 pounds in August of 2022.

Based on his weights from past years and observations of Woody before him, he can be expected to begin to bulk back up around March and likely even exceed his 2,030 pound record next summer.

Written by ASLC Communications Intern Haley 

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