Otaria, the Southern Sea Lion

I haven’t written much on pinnipeds (seal, sea lions and walruses) here at ver 4. Here’s a recycling of material originally published (here) at ver 2 way back in 2007, enjoy! I haven’t updated it…

Caption: captive Southern sea lion (at the Cornish Seal Sanctuary in Gweek, photographed in 2012). Note the massively built muzzle, the very dark pelt, and the stained, almost blackish hue to the teeth. This is common in sea lion teeth and I wonder what causes it. Various terrestrial mammals develop blackish teeth due to staining, but this is more of a herbivore thing than a carnivore one. What might cause this? Image: Darren Naish.

The Southern sea lion is a large pinniped that inhabits virtually the whole of the Pacific coast of South America, occurring as far west as the Juan Fernandez Islands. It also occurs on the south-eastern Atlantic coast as far north as Uruguay and southern Brazil, occurring also around the Falkland Islands. A dead one was found on the Galapagos in 1973 (Bonner 1994), and I think this is the most northerly record for the species. Adult males can reach 2.3 m and over 300 kg; females 1.8 m and c. 140 kg. The species is variable in colour, ranging from black to dark grey and reddish brown, with adult males being particularly dark. Females can be dull yellow on the head and neck. Males have an incredibly massive head and neck and a distinctive broad and up-turned snout.

Caption: a wild group of Southern sea lions at Peninsula Valdes, Argentina, showing adult males and females and numerous pups. Image: Reinhard Jahn Mannheim, CC BY-SA 2.0 (original here).

Taxonomy. Two scientific names are in use for the Southern sea lion: Otaria byronia (originally Phoca byronia de Blainville, 1820) and Otaria flavescens (originally Phoca flavescens Shaw, 1800). Because O. flavescens is older, many authors have used it in preference to O. byronia. However, Shaw’s “yellow seal” is not definitely a Southern sea lion, despite Rodriguez & Bastida’s (1993) argument that it was, as other otariids can sometimes have unusually pale coats.

In contrast, the type specimen for de Blainville’s Phoca byronia was undoubtedly a Southern sea lion as he referred to key cranial characters unique to this species. For this reason at least some authors have used O. byronia (e.g., King 1983, Berta & Sumich 1999, Brunner 2004, Brunner et al. 2004). I agree with them and it’s the technical name I prefer to use. For reasons that aren’t quite clear to me, however, O. flavescens currently appears more popular among specialist researchers.

Caption: a drawing of a male Otaria I created for the in-prep textbook (cough cough). Otariids are well able to hold the body high up off the ground when walking. The massive bulk of the head and habitually elevated head pose (where the snout is tilted upwards) are characteristic of this species. Image: Darren Naish.

A gnarly skull, a massive mandible. Adult male Southern sea lions have by far the most robust skull of any otariid, with a particularly prominent sagittal crest, a proportionally wide, short and robust rostrum, and an incredibly deep mandible. Large, shelf-like supraorbital processes are prominent, the palatal morphology is unique, with a palate that reaches all the way back to the level of the glenoid fossae. In the lower jaw, the masseteric fossa (the big concave area on the side of the jaw’s posterior section) of Otaria is immense, occupying about half the length of the dentary in some individuals.

Superficially, some of these details are similar to those of walruses and it’s interesting that, in her study of cranial variation among otariids, Brunner (2004) found Otaria to group separately from other otariids: a result that contrasts with previous classifications of this taxon alongside the New Zealand sea lion Phocarctos hookeri and the Australian sea lion Neophoca cinerea within a monophyletic Otariinae (Berta & Sumich 1999). There’s an awful lot that could be said about otariid phylogeny. Most molecular studies actually find Otaria to be close to Arctocephalus (the southern fur seals)… I’ll leave it at that for now.

Caption: an old and not good photo I took in the collections of the Natural History Museum, London, back in the early 2000s (when Stig Walsh and I were working on fossil pinnipeds). The insane gnarliness of this specimen (and others like it) never ceases to impress me. Look at the depth of that lower jaw! Image: Darren Naish.

With a skull morphology like this, it figures that male Southern sea lions are not limited to a diet of fish, squid and crustaceans (although these prey items do form the bulk of their diet). They will also catch and eat South American fur seals Arctocephalus australis, and not just the pups, but adult females too (Gentry & Johnson 1981, King 1983).

Penguins as prey. They also kill and eat penguins, including rockhoppers, gentoos and Magellanic penguins (Boswell 1972). Pinniped predation on penguins has now been reported in several species, and in some cases particular individuals develop a penguin-killing habit and can then have a significant impact on a colony. There is one case for example where, between 1997 and 1999, a Southern elephant seal Mirounga leonina single-handedly killed and ate 88 Magellanic penguins in one Argentinean colony (Clark & Boersma 2006). Predation on Yellow-eyed penguins Megadyptes antipodes by (apparently) a single female individual of Hooker’s sea lion Phocarctos hookeri on New Zealand is also posing a significant threat to the viability of the Otaga Peninsula Yellow-eyed penguin population (Lalas et al. 2007).

Caption: portraits of captive Southern sea lions, again at the at the Cornish Seal Sanctuary in Gweek. Features to note include the length of the longest whiskers, the short and scroll-like pinnae (external ears), and the slightly goofy, bulging eyeballs. Images: Darren Naish.

Sea lions vs fur seals. Intraspecific abduction, harassment and killing of pups is fairly widespread in pinnipeds and endemic in some species. In an Alaskan colony of Northern fur seals Callorhinus ursinus, Kiyota & Okamura (2005) described how each pup in the colony was harassed, attacked or abducted an average of 3.8 times (by adult Northern fur seals) during the breeding season. Interspecific aggression directed towards other pinnipeds is rarer, in part because species tend not to form colonies in the same place.

But we now know that subadult male Southern sea lions grab, shake and bite the pups of South American fur seals – often with fatal results – and not for food, but apparently as a form of misplaced aggression: these individuals have yet been able to win mating battles with older males (Cassini 1998). In cases, the fur seal pups are shaken and thrown about for hours and, in about 40% of the cases recorded by Cassini (1998), the pups died as a result. Mothers attempted to rescue their pups in about a third of the attacks, but in only one case out of 31 was a mother successful.

Caption: I suppose you don’t often get to see images of swimming sea lions – Southern sea lions in particular – from under the water, but the enclosure at the Cornish Seal Sanctuary allows views like this. I like this image, since it has a mysterious aura that gives the animal a slightly amorphous, frightening aura. Image: Darren Naish.

Caption: pinnipeds of many sorts routinely swim upside down, and Otaria evidently does this too. Maybe they do this because they can, maybe because it’s fun, or maybe it’s because it allows them to better see what’s happening on the seafloor (or, in this case, the floor of the pool). Image: Darren Naish.

Female sea lions generally don’t get up to the same sort of nasty behaviour as males, but they still exhibit a fair amount of aggression towards each other, with biting and open-mouth threat displays being common in crowded colonies. Mostly this is due to defence of their own pups, and of their own little patch of the colony.

Aggression in females was studied by Esteban & Cassini (2007) who found that females were more aggressive to each other when the environment around the breeding colony limited their access to tide pools. Sea lions use tide pools in order to help control their temperature, so when these are in short supply, females are more likely to act territorially. If crowding and lack of access to a resource (tide pools) results in increased aggression, you might wonder why female sea lions (and other pinnipeds) bother to form colonies at all. Colony formation is a much investigated subject, with recent studies arguing that the main benefit that females derive from it is that their grouping together cuts down on the amount of harassment they’d otherwise get from males. And at the risk of spinning off at a tangent, I’ll stop there.

Caption: a male Southern seal lion with ‘his’ group of females, photographed at Chubut, Argentina. Image: Nestor Galina, CC BY 2.0 (original here).

Pinniped vs human. All in all, big otariids like Otaria are hefty and quite scary predators. One of my favourite mammals is the Leopard seal Hydrurga leptonyx, a species well known for its ability to kill other pinnipeds (including juvenile Otaria), large penguins, and even humans. Sea lions haven’t yet been reported to kill a person, but Californian sea lions Zalophus californianus in particular have bitten plenty, with the number of reported attacks increasing (Berkeley marina suffered a spate of attacks in 2006). In April 2007 a 13-year-old girl was pulled off her surfboard and apparently came close to death.

And that’s where we’ll end. Pinnipeds have been covered a few times at Tet Zoo before and I certainly plan to cover them more in the future. But it doesn’t look like there’s any material at ver 4. Here are links to other  Tet Zoo articles on pinnipeds.. Essentially all of them have been ruined thanks to the failures of ScienceBlogs and Sci Am, so I’ve had to dig around for intact versions at wayback machine…

If you enjoyed this article and want to see me do more, more often, please consider supporting me at patreon. The more funding I receive, the more time I’m able to devote to producing material for TetZoo and the more productive I can be on those long-overdue book projects. Thanks!

Refs – -

Berta, A. & Sumich, J. L. 1999. Marine Mammals: Evolutionary Biology. Academic Press, San Diego.

Bonner, N. 1994. Seals and Sea Lions of the World. Blandford, London.

Boswell, J. 1972. The South American sea lion Otaria byronia as a predator on penguins. Bulletin of the British Ornithologists Club 92, 129-132.

Brunner, S. 2004. Fur seals and sea lions (Otariidae): identification of species and taxonomic review. Systematics and Biodiversity 1, 339-439.

Brunner, S., Bryden, M. M. & Shaughnessy, P. D. 2004. Cranial ontogeny of otariid seals. Systematics and Biodiversity 2, 83-110.

Cassini, M. H. 1998. Inter-specific infanticide in South American otariids. Behaviour 135, 1005-1012.

Clark, J. A. & Boersma, P. D. 2006. Southern elephant seal, Mirounga leonina, kills Magellanic penguins, Spheniscus magellanicus, on land. Marine Mammal Science 22, 222-225.

Esteban, F.-J. & Cassini, M. H. 2007. Intra-sexual female agonistic behaviour of the South American sea lion (Otaria flavescens) in two colonies with different breeding substrates. Acta Ethologica 10, 23-28.

Gentry, R. L. & Johnson, J. H. 1981. Predation by sea lion on northern fur seal neonates. Mammalia 45, 423-430.

King, J. E. 1983. Seals of the World. British Museum (Natural History), London.

Kiyota, M. & Okamura, H. 2005. Harassment, abduction, and mortality of pups by nonterritorial male Northern fur seals. Journal of Mammalogy 86, 1227-1236.

Lalas, C., Ratz, H., McEwan, K. & McConkey, S. D. 2007. Predation by New Zealand sea lions (Phocarctos hookeri) as a threat to the viability of yellow-eyed penguins (Megadyptes antipodes) at Otago Peninsula, New Zealand. Biological Conservation 135, 235-246.

Rodríguez, D. H. & Bastida, R. O. 1993. The southern sea lion, Otaria byronia or Otaria flavescens? Marine Mammal Science 9, 372-381.