Taxon of the Week: Phocidae

Still with the mammals. This week's highlighted subject is the Phocidae - the earless or 'true' seals. Phocidae are quite easily distinguishable from the Otariidae (sea lions and fur seals) by the absence of external ears (hence the name, duh) and the inability to turn their hind flippers under the body and use them in locomotion out of the water. This means that while otariids are still reasonably mobile on land, phocids are decidedly clumsy critters, reduced to a sort of humping, slug-like movement (Wikipedia claims that this is referred to as galumphing, which makes for the second time this week I've had to invoke Carroll [he left it dead, and with its head, he went galumphing back] and admit that I'd always imagined galumphing slightly differently - some of you may even recall the performance of The Jabberwocky on an episode of The Muppet Show, where Scooter rendered galumphing as a sort of jerky stomp with the word 'galumph' intoned at every step). Put them in the water, though, and phocids are magical.

The Phocidae are divided into two subfamilies, the 'Northern Hemisphere' Phocinae and the 'Southern Hemisphere' Monachinae. The inverted commas are there because the Monachinae do include a small number of Northern Hemisphere species, including the type genus Monachus (the monk seals). While it has been suggested in the past that Monachinae might be paraphyletic with regard to Phocinae (reported in Walsh & Naish, 2002), modern studies both molecular and morphological are fairly unanimous that the two subfamilies are monophyletic (Arnason et al., 2006; Davis et al., 2004).

Because I'm more familiar with them (being a Southern Hemispherean myself) I'm going to concentrate on the Monachinae. If you want to read about phocines, which are the classic white-pup-aren't-I-just-so-goddamn-cute-you want-to-hit-me-with-a-club seals, I recommend you go here and scroll down to Darren Naish's post on the Baikal seal. The Monachinae probably aren't as speciose as the Phocinae (it's a little difficult to tell, as the number of species in the Phocinae goes up and down from reviewer to reviewer) but are arguably more diverse in form. Living species are divided into three tribes - Monachini (Monachus), Miroungini (Mirounga, the elephant seals) and Lobodontini (Antarctic seals). The first two tribes only include one genus each in the modern fauna, but also include fossil genera.

Monachus (the monk seals) is the basalmost clade of monachines, and the only tropical/subtropical genus of Phocidae. There are three species with widely separated distributions - one each in the Mediterranean (Monachus monachus), Caribbean (M. tropicalis) and Hawaii (M. schauinslandi), though the Caribbean species is now extinct and the other two are endangered. Most references to monk seals I've come across mention their small size, but the Monachus Guardian website indicates that all are over two metres in length, demonstrating once again that size is a relative thing.

Mirounga are the two elephant seals - the Northern Pacific Mirounga angustirostris and the circum-Antarctic M. leonina. Elephant seals are so-called for two reasons - the males have a large inflatable proboscis that is used to make loud roaring noises, and because they are absolutely huge. Wikipedia cites a size of nearly 2.3 tonnes, with a recorded maximum of 5 tonnes. Elephant seals are also the most terrestrially capable of all Phocidae (Walsh & Naish, 2002). Back in New Zealand, elephant seals make the news every few years or so when one makes its way inland, including one dubbed 'Humphrey' that was present intermittently over a period of five years and was famed for his attachment to a herd of cows. Fantastic quote of the day about elephant seals inland - Although popular with locals and visitors, the impact of an amorous, two tonne male elephant seal on cars and other structures is significant (Harcourt, 2001).

The Lobodontini are the most diverse group of monachines, including the crabeater seal (Lobodon carcinophagus), Weddell seal (Leptonychotes weddellii), leopard seal (Hydrurga leptonyx) and Ross seal (Ommatophoca rossi). The Ross seal is a particularly obscure animal, little seen and restricted to the Antarctic. The leopard seal is the real star member of this group, though - a sleek super-predator that can be over 3.5m long and 400kg, able to eliminate about 2.5% of an Adelie penguin colony in a single season (Harcourt, 2001 - that's leopard seals as a whole, of course, not a single individual). Hydrurga is also known for its ability to extend its neck quite some distance from its body. Notably from this factoid, it is probably closely related to the fossil long-necked seal Acrophoca, inspiration of a whole swath of lake-monster theories.

And that's the Phocidae for this week. Now I really should get around to working on my conference poster....

REFERENCES

Arnason, U., A. Gullberg, A. Janke, M. Kullberg, N. Lehman, E. A. Petrov & R. Väinölä. 2006. Pinniped phylogeny and a new hypothesis for their origin and dispersal. Molecular Phylogenetics and Evolution 41 (2): 345-354.

Davis, C. S., I. Delisle, I. Stirling, D. B. Siniff & C. Strobeck. 2004. A phylogeny of the extant Phocidae inferred from complete mitochondrial DNA coding regions. Molecular Phylogenetics and Evolution 33 (2): 363-377.

Harcourt, R. G. 2001. Advances in New Zealand mammalogy 1990-2000: Pinnipeds . Journal of the Royal Society of New Zealand 31 (1): 135-160.

Walsh, S., & D. Naish. 2002. Fossil seals from Late Neogene deposits in South America: a new pinniped (Carnivora, Mammalia) assemblage from China. Palaeontology 45 (4): 821-842.