Tetrapod Zoology

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Well done and many thanks to everyone who tried identifying the mystery skull published on the blog yesterday. And as several people correctly worked out…

…. it belonged to a pinniped, and more specificially to an otariid, and to a sea lion. Well done in particular to Andrés Rinderknecht and Rafael Tosi: it is indeed the skull of a Southern sea lion Otaria byronia (more appropriately termed the South American sea lion or Patagonian sea lion), and a big old gnarly male at that (like the one pictured in the adjacent image). The specimen was collected from the Falkland Islands in the 1930s and is in a museum collection. Obviously, no-one round these parts has looked much at my flickr site, because the answer is given away here. To people that know pinnipeds well, the skull of Otaria is unmistakable and instantly recognizable, but to the other 99.9% of the human population, it is understandable that few realize that sea lions have such awesome and scary-looking skulls.

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The Southern sea lion is a large species that inhabits virtually the whole of the Pacific coast of South America (it occurs as far west as the Juan Fernandez Islands), and 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 [adjacent image, showing captive individual with person, might give you some idea of their size]. The species is variable in colour, going through black, dark grey and reddish brown phases during growth, with adult males being particularly dark brown. 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 [see image below, which nicely illustrates sexual dimorphism].

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), and I agree with them.

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As was hopefully clear from the skull photo posted yesterday, 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. The large, shelf-like supraorbital processes identify the skull as an otariid, as these are absent in phocids and living walruses. The palatal morphology of the species is unique: the palate reaches back to the level of the glenoid fossae, and the posterior border of the bony palate is straight (rather than anteriorly concave). In the lower jaw, the masseteric fossa 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 Phocarctos and Neophoca within a monophyletic Otariinae (Berta & Sumich 1999).

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). 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).

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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 during the breeding season! Interspecific aggression against 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 [adjacent image shows captive male and female].

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 recently 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.

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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 that is well known for its ability to kill other pinnipeds (including juvenile Otaria), large penguins, and even (on at least one occasion) 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 in recent months (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: a news article on that is here [adjacent pic shows Otaria mandible. Scale bar = 10 mm].

Anyway, well done again to those who identified the skull – and maybe we should do this sort of thing more often?

If you like pinnipeds there is a ver 1 article on Acrophoca here, and one on Lake Baikal seals here.

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.

- ., 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.

Comments

  1. #1 TheBrummell
    June 21, 2007

    That sagittal crest is indeed quite large, and from the photos I see that adult males have a distinctive bulge of tissue behind the head. My limited understanding of cranial morphology is wispering to me that sagittal crests are attachment points for big jaw muscles.

    Is that bulge behind a male’s head muscle, blubber, or some other tissue?

  2. #2 Rafael Tosi
    June 21, 2007

    I must admit the credit for identifying the skull is all due to Andrés Rindercknecht who told me what it was. I just didn´t know he had posted a comment since he doesn´t speak much english, he seems to have done it without any problems though. Anyway, post more of these!!!

  3. #3 Ed Yong
    June 21, 2007

    “few realize that sea lions have such awesome and scary-looking skulls.”

    My own experience of the scariness of sea-lion skulls came while snorkelling in the Galapagos islands. I was happily swimming along looking down at beautiful stretches of coral and when I looked up, I was staring an absolutely *massive* bull sea lion, who was in turn staring me in the face. Through some minor miracle, I managed to avoid (a) soiling myself instantly and (b) getting bitten in the face.

    Anyway, the moral of that story is: respect pinnipeds. Just look at these insane photos of leopard seals…

  4. #4 johannes
    June 21, 2007

    > Anyway, the moral of that story is: respect pinnipeds. Just look at these insane photos of
    > leopard seals…

    Awesome. Has anybody ever tried to measure how intelligent Hydrurga is?
    Offering prey as a gift or bribe to an entirely unknown entity (I guess it has never seen a diver before) looks almost human…

  5. #5 Tengu
    June 21, 2007

    I once saw a boy stray too close to a californian sea lion at a safari park.

    CHOMP!

    They arent called sea LIONS for nothing.

  6. #6 Alan Kellogg
    June 22, 2007

    Johannes,

    The fellow in question had been interacting with that particular leopard seal for some time. Apparently she had come to see him as some sort of odd leopard seal pup who needed extra nourishment. Gifting prey to humans seems to be an oddly common behavior among carnivora. The most famous example being the domestic cat, though dogs, bears, and others have been known to do it.

  7. #7 johannes
    June 22, 2007

    > Johannes,

    > The fellow in question had been interacting with that particular
    > leopard seal for some time. Apparently she had come to see him as
    > some sort of odd leopard seal pup who needed extra nourishment.
    > Gifting prey to humans seems to be an oddly common behavior among
    > carnivora. The most famous example being the domestic cat, though
    > dogs, bears, and others have been known to do it.

    This sounds reasonable, if indeed there had been interaction between the diver and the leopard seal before. I have seen TV footage of this scene, and they presented it in a way that suggested that this was the first time the diver and the Hydrurga met, the gifting looked like a spontanous act of the sea leopard as a result.

    BTW, our wire-haired terrier used to swallow rodents and to reurgitate them in our kitchen, much to the disgust of my mother. But of course he considered us to be part of his family or pack.

  8. #8 Michael P. Taylor
    June 22, 2007

    That mandible really is absurdly over-engineered. It looks like it was drawn by a three-year-old. They have to be doing something more impressible with this than eating fur seals. I bet they chew rocks for fun.

  9. #9 Tuckeg
    June 22, 2007

    A woman snorkeling off La Paz in Baja, Mexico received a nasty bite from a California sea lion a few years ago.

  10. #10 Susan
    June 22, 2007

    I realize it’s unfair to attribute human qualities to animals, but I’ll have to admit, some species of seals and sea lions are some of my least favorite creatures. The sexual dimorphism aspect alone is rather disturbing. Male southern elephant seals are 5 times larger then the females! I guess I’ve seen too many nature films depicting male sea lions or elephant seals squashing females and pups as they battle it out with each other. There are also those scenes of females tossing around biting to death unfamilar pups who happen to wander into their territory. I’m guessing that they’re not terribly social creatures, but because they give birth to helpless young they’re forced to return to land until their pups are able to fend for themselves. I’ve occasionally wondered if that’s why whales and dolphins were able to become fully aqutic while seals were not. Whales are decended from undulates, who give birth to precocious young who can look after themselves right away. While seals are decended from carnivors who give birth to helpless babies who need to be tended to for awhile before they can be on their own.

  11. #11 Darren Naish
    June 23, 2007

    Susan: it has indeed been suggested that pinnipeds have never been able to evolve a truly aquatic lifestyle because their terrestrial moulting and rookery-forming behaviours force them to remain tied to land. I forget where I first read this, it might have been…

    Bonner, W. N. 1984. Seals of the Galapagos Islands. Biological Journal of the Linnean Society 21, 177-184.

  12. #12 amara
    March 4, 2008

    cool

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