By Rachael Orben, Assistant Professor (Senior Research), Seabird Oceanography Lab
There aren’t that many Steller sea lions that call the Pribilof Islands home. The way I learned to spot them, was to watch for excited groups of kittiwakes materializing out of nowhere, just off-shore. The kittiwakes circle, periodically dipping down to grab something from the water. Then a sea lion head emerges from the water and more often than not, the lion would have a flatfish. The sea lion whips the fish back and forth, splashing and causing pieces to break off. The kittiwakes drop down and pick up the little bits. The black-legged kittiwakes that we were tracking with GPS dataloggers often flew in laps around the island (Paredes et al. 2012, 2014); stopping at the outflow of the fish processing plant, and perhaps, also on the lookout for foraging Steller sea lions to pick up an extra snack.
Gape limitation
At first glance, one might assume that sea lions are gape-limited. What do I mean by this? Basically, gape limitation means that predators can’t consume anything that doesn’t fit into their mouths whole. This idea is typically considered in the context of fish but does come up in seabird and marine mammal ecology from time-to-time. Specifically, when a predator doesn’t have a method for pulling its prey apart so is required to consume it whole. For instance, seabirds that feed their chicks whole fish can encounter this problem (e.g. puffins, terns, murres). Small chicks can starve if parents are bringing back fish that are too large to fit into the gape of the chick.
Sea lions and their eclectic large prey
I don’t know if the flatfish consumed by the Steller sea lions are too large to be swallowed whole. But, I do know that they use a strategy known as ‘shake feeding’ (Kienle et al. 2017). This feeding style is important as it offers the behavioral mechanism that allows sea lions to consume prey that exceeds their gape limitations. When sea lions are observed eating large prey it often occurs in surprising circumstances, but I suspect this foraging tactic is fairly common (e.g. Hocking et al. 2016). I have compiled a few examples both from the scientific literature and the internet to see.
Observations
Galapagos sea lions and tuna. This example is amazing and features Galapagos sea lions working together to herd tuna into shallow lagoons. Compared to the sea lions, the tuna are large! (When you are done looking at the amazing photos please return and finish reading my blog.)
Besides the flatfish I observed Steller’s sea lions eating, there is an observation of a Steller catching a shark (the online photo account stops before the shark is consumed so I don’t know what happened) and catching and consuming northern fur seal pups (Gentry & Johnson, 1981).
Gentry & Johnson 1981, include a particularly gruesome description of the predation events: “Fur seal young most often were caught by the abdomen and eviscerated with a sideways shake of the sea lion’s head (in the same manner used to tear apart large fish). Sea lions most often dived with the prey still moving and surfaced father offshore, usually beyond the kelp beds, with the pup motionless. …Larger sea lions broke apart their prey under water, surfacing only to swallow large bits of tissue. Smaller sea lions vigorously shook the carcass at the surface using the same sideways snapping motion used to eviscerate the pup at capture.”
I found a fascinating series of photographs of a California Sea Lion and a Mola Mola. It is a little hard to tell what is going on, but the photographer has labeled his photos “A California Sea Lion kills, and eats, a Mola Mola”.
Southern sea lions consume large prey in the form of penguins and more surprisingly fur seals. Thus far these observations are limited to males.
Eating octopus
Sea lions also use shake feeding to consume octopus. Though an octopus might be smalled enough to be eaten in one gulp, they are a smart and agile prey whose tentacles make them harder to swallow. I have seen Southern sea lions flipping octopus at the surface using the ‘shake feeding’ mode. Once I watched a young juvenile bring one ashore to eat (photos below). Perhaps foraging on octopus offers some opportunities for learning how to eat large prey?
References
Gentry, R. L., & Johnson, J. H. (1981). Predation by sea lions on northern fur seal neonates. Mammalia, 45(4), 423–430. http://doi.org/10.1515/mamm.1981.45.4.423
Hocking DP, Ladds MA, Slip DJ, Fitzgerald EMG, Evans AR (2016) Chew, shake, and tear: Prey processing in Australian sea lions (Neophoca cinerea). Marine Mammal Sci 33:541–557
Kienle SS, Law CJ, Costa DP, BERTA A, Mehta RS (2017) Revisiting the behavioural framework of feeding in predatory aquatic mammals. Proc Biol Sci 284:20171035–4
Paredes R, Harding AMA, Irons DB, Roby DD, Suryan RM, Orben RA, Renner HM, Young R, Kitaysky AS (2012) Proximity to multiple foraging habitats enhances seabirds’ resilience to local food shortages. Mar Ecol Prog Ser 471:253–269
Paredes R, Orben RA, Suryan RM, Irons DB, Roby DD, Harding AMA, Young RC, Benoit-Bird KJ, Ladd C, Renner H, Heppell S, Phillips RA, Kitaysky AS (2014) Foraging Responses of Black-Legged Kittiwakes to Prolonged Food-Shortages around Colonies on the Bering Sea Shelf. PLoS ONE 9:e92520
