On Chiswell Island in the Gulf of Alaska, an endangered population of Steller sea lions returns year after year between May and July to breed and give birth. Since 1998, researchers at the Alaska SeaLife Center have utilized a remote-control live video monitoring system to study the behavior and population dynamics of these Steller sea lions, tracking territorial behavior, copulations, births, and maternal attendance. This long-term study has allowed them to collect detailed recorded histories of these sea lions spanning generations. In the process, they have identified returning individuals, using recognizable marks, such as scars, fungal patches, wounds, and distinct flipper features.
By collecting data on the presence of individuals, their location on the island, and their behaviors, researchers identified 11 key territorial areas that dominant males defend during the breeding season. The territories were defined based on natural rock features and topographical lines where aggression between males was seen. Holding territory is key to a male’s ability to reproduce, as a male will typically have access to breed with the females who enter estrus in his territory.
This summer, one of Chiswell Island’s territorial males, “Smoky,” returned to the island in mid-May to claim territory. Smoky was first identified by researchers in 2017, and since then, he has been observed claiming and defending his territory each year. Researchers believe that when a male holds territory over multiple years, it could increase his chances of copulation. Nearly two-thirds of males holding territory for the first time failed to mate. For Smoky, he followed this trend with no observed copulations in his first two years on the island. However, in his third year, he began seeing some reproductive success with two observed copulations. Then, by 2021, he had his highest number of observed copulations. With eight observed copulations, he was the most successful male on Chiswell Island that year, having more copulations than any other male.
Each year, the Steller sea lion breeding season follows a similar schedule on Chiswell Island. It begins with the arrival of the territorial males to the rookery, typically weeks before peak pupping. During this time, males like Smoky claim territory, hoping to secure access to receptive mates. Females arrive next, beginning in late May. Pregnant females typically give birth within one week of arriving, with a peak in births in mid-June. During this time, males continue defending their territory and stay on the island fasting, not leaving to forage for several weeks. After giving birth, females enter their perinatal period, when they fast while nursing and resting. After spending 3 to 13 days with their pups on shore, females begin to leave the island on foraging trips. At this point, females are also entering estrus, a period where they are sexually receptive. This is usually when researchers see a peak in copulations, typically in late-June when most females on the island have given birth and are in estrus.
This is when a male’s ability to hold territory becomes important, and not just any territory will do. If their goal is to copulate with the most females, a territorial male wants to hold territory that meets the needs of females.
At Chiswell Island, just over 50% of females move from the territory where they gave birth before copulation, which tells us that females may be exhibiting some level of choice with whom or where they prefer to mate. That is, a female may choose a preferred territory for copulation with access to resources or a bull with potential genetic benefits. At Chiswell Island, one of the desirable resources is believed to be clear pathways to the sea for thermoregulation and foraging trips. So, researchers believe that a primary breeding strategy for males is resource-defense polygyny, which means that males are competing for the territories females prefer to achieve mating priority. Researchers found that male copulation success varied by the distribution of females across breeding territories. Further, research found that only about 60% of males copulate in any given year, and a small number of bulls get the majority of copulations, while the majority see little or no success.
This year, Smoky lost his territory, and as a result, he joined the majority of males on Chiswell Island having no observed reproductive success. Smoky was first seen on the island on May 16th, as he established and held his territory like usual. However, on May 27th, another large male engaged Smoky in an intense territory battle.
Smoky fought hard to defend his territory, emerging with new wounds. However, he was defeated and retreated to the outskirts of the island. After an hour and a half, Smoky returned to fight to get his territory back. He engaged in another intense battle with the other bull. The battle was brutal, with additional bulls entering the fight for their chance to take over the territory. After a long, hard-fought battle, Smoky was defeated again, and his territory was held by other bulls for the remainder of the breeding season.
This is what is known as a turnover event, where an aggressive interaction between males leads to a male being driven out of his territory for over 24 hours as another one or more take over.
Since his loss, Smoky has been seen on the outskirts of the island, perhaps waiting for another opportunity to try to reclaim his territory. Researchers rarely observe copulations outside the key breeding territories, so it is unlikely that Smoky has had reproductive success this summer.
So, what will Smoky do now that he lost territory? By the third week in July, after the peak in copulations, territorial males began allowing subadult males and juveniles on the rookery, marking the break-up of the territory structure for that season. At this point, we see a spike in turnover events, as males from the outskirts begin to move into territories, while territorial males begin to break their fasts on foraging trips. These males are hoping to copulate with any females still in estrus. Smoky may seize the opportunity of the territorial breakup to move back into his territory to try to copulate, but researchers have not yet observed him doing so.
If Smoky does not return to the main part of the rookery this season, he may try to return before other bulls next season to reclaim his territory. In one study by ASLC researchers Pam Parker and John Maniscalco, at least 73% of territorial males returned to Chiswell Island in any given year. However, researchers have noted in the past that some territorial males do not return to the island after losing their territory.
So, what happens when a bull loses its territory for good? Researchers are unsure where these bulls go, but it is possible that Smoky may never return to Chiswell Island after this season, and his tenure may be over for good. In the past, researchers have observed “reproductive death” in cases like Smoky’s, where a new male beats out a resident male for his territory prior to peak pupping and marks the end of that bull’s reproductive years.
Researchers will continue to watch for Smoky in the years to come, however, even if Smoky does not return to claim territory again, his tenure has been impressive. Smoky, having established territory in 2018, would have a six-year tenure. In one study, researchers found that tenure ranged from two to eight years, putting Smoky on the high end of the range. This knowledge on his life-history, made possible by the long-term Chiswell Island remote video monitoring project, will contribute to the further understanding of this endangered Western population of Steller sea lions. By following the same individuals over many years, researchers can see patterns that short-term studies miss: how males adapt their tactics to changing environmental conditions, how territory turnover shapes the breeding hierarchy, and how reproductive success is distributed across a population. Smoky’s story is more than the tale of one sea lion’s struggle for dominance; it’s a small but important piece of a much bigger picture of survival, adaptation, and change in Alaska’s marine ecosystem.
Want to dive deeper? Click here to read the full study: A long-term study reveals multiple reproductive behavior strategies among territorial adult male Steller sea lions (Eumetopias jubatus).
Check out the ASLC on Facebook, Instagram, and YouTube to see our weekly updates from the Chiswell Island Steller Sea Lion Monitoring Team from this summer.
Photographs taken by Chiswell Island Steller Sea Lion Remote Video Monitoring Research team permitted by
USFWS Permit M-334-AM NOAA/NMFS #27408
This research was conducted under federal permits to the Alaska SeaLife Center: NMFS Permit No. 22293 and earlier versions for research on Endangered Steller sea lions, and USFWS Special Use and Right of Way Permits to conduct research on islands of the Alaska Maritime National Wildlife Refuge. Sponsorship was provided by the Federal Government with a grant from the National Oceanic and Atmospheric Administration – Award ID No. NA22NMF4370355.
