Among the most amazing of pinnipeds are the elephant seals. There are two species: the Northern elephant seal Mirounga angustirostris of the north-east Pacific and the Southern elephant seal M. leonina of the southern oceans. These seals are famous for being big. They are the biggest of pinnipeds – bigger even than walruses – there being a record of a giant male Southern elephant seal that was between 6.5 and 6.8 m long and weighed over 4000 kg (Carwardine 1995).
After size, the best known features of these animals are the modified snout and enlarged, inflatable proboscis, both of which are unique to males. The proboscis is used in display during the breeding season when it’s elevated, pendulous and engorged; males roar and make deep gargling, ‘clapping’ and engine-like noises when doing these displays and a resonating role for the proboscis has been considered. It seems, however, that any such role is incidental and that their noises are mostly made in the mouth (Sanvito et al. 2007a, b). The skull is massive and broad overall and the incisor count is reduced to two uppers and one lower per side. Small, simple and apparently non-functional post-canine teeth are present and the bony palate is short. The canines are sexually dimorphic, being larger in males.
Elephant seals undergo what’s known as annual catastrophic moult; the short pelt they grow is shed in large sheets (sometime between, typically, November and March depending on the age and size of the animal) to reveal the naked skin beneath. This is often dry, cracked and scaly in old animals and a giant, thick, horny epidermal shield covers the throat region of adult males. The skin here can be 5 cm thick (Ling & Bryden 1992).
Dimorphism, beachmasters and runaway selection. As is obvious, elephant seals are what’s technically known as crazy dimorphic (kidding: not a technical term). The sexual dimorphism in these animals is amazingly profound, the skulls of males often being more than twice as long and wide as those of females, the males being two or three or more times the weight of females. Males also differ conspicuously from females in snout anatomy, as we’ve seen. Given that females look ‘more typical’ when compared to other phocids, I think it’s reasonable to state that it’s males that have been under the most intense selection pressure.
A reproductive system where males compete to control large groups of females and dominate favoured stretches of coastline as giant, battle-hardened, aggressive beachmasters has resulted in runaway selection for ever-larger size (Lindenfors et al. 2002) and increased armament. The costs are high – fights are vicious and stressful and many males do not get to breed at all – but the benefits are great to those that succeed. It should be added that the evolution of giant size in elephant seals has not solely been driven by the forces acting on males: females are under pressure to be large too, though apparently not because their size is genetically correlated with the condition in males; females in Southern elephant seals below a certain body size are apparently unable to produce male pups (which are larger than females) (Arnbom et al. 1994) and are thus not able to produce the ‘sexy sons’ most likely to be reproductively prodigious.
This reliance on terrestrial combat among males and the gathering of female harems on beaches makes elephant seals something of a paradox since their anatomy combines strong specialisation for pelagic life with a committed ability to move their enormous bulk on land, at speed. Gigantic overall size, massively enlarged hindflippers, low bone density, enlarged blood volume and eyes with particularly big lenses are (among other things) obvious specialisations for pelagic life while various features of forelimb, pelvic, hindlimb and vertebral anatomy are adaptations that – presumably – only persist due to their role in terrestrial behaviour. At some point I’d like to see a study that describes those terrestrial adaptations (so far as I can tell, one does not exist; it’s something I plan to do): it would be useful from a palaeontological perspective because the obvious terrestrial signal present in even the biggest of pinnipeds contrasts strongly with the condition in other marine tetrapod groups – I’m thinking plesiosaurs – where such features are absent.
A commitment to terrestrial breeding behaviour presumably explains why pinnipeds have never yet become fully aquatic (outside of the cryptozoological literature; see Conway et al. 2013). It has also been suggested that their moulting behaviour constrains them in the same way.
To dive and to surface but mostly to dive. Elephant seals are champion divers. But they don’t just dive deep (we’ll return to this particular issue in part 2), they also dive repeatedly and spend an extraordinary amount of time – as much as circa 88% of their time at sea (Le Boeuf et al. 1996) – submerged. They surface for just 2-3 minutes before diving again. In fact, elephant seals spend so much time diving and so little time at the water’s surface that some researchers propose (tongue implanted into medial wall of buccal cavity) that they might better be regarded as ‘surfacers’ rather than ‘divers’.
Part of the reason for spending so little time at the surface comes from predation: both white sharks and killer whales attack elephant seals by rushing upwards from the cover of deep, dark water. It seems that the seals are most at risk from predation when ascending, and consequently are slower at ascending than descending, apparently stopping to look around while doing it (Le Boeuf & Crocker 1996).
The size, formidable and weird appearance and brief surfacing time of elephant seals means that encounters with them at the sea surface must surely be memorable events. Consider that a giant male could, when surfacing, have the top of its head as much as a metre or so above the water’s surface, and now imagine being in a small boat or even swimming when such a creature emerges nearby.
I now think that at least some of the ‘Cadborosaurus’ sightings made in the north-east Pacific represent encounters with elephant seals (Naish 2017). It explains those descriptions of ‘camel-like’ snouts, giant black eyes and weird wrinkled skin referred to by some witnesses. A caveat is that full exposure of the head and much of the neck is apparently relatively rare, since attached cameras show that the seals surface in a near-vertical pose and typically only expose the snout and front portion of the head (Le Boeuf & Crocker 1996).
And that’s not all. More on elephant seals in the next article: on diving, on foraging, on evolution and fossils.
For previous Tet Zoo articles on pinnipeds, see...
- Sea lions really are quite impressive
- The Long-necked seal, described 1751
- England ‘does a Montauk’ (mostly on the Grey seal)
- Harbour seal kills and eats duck
- Statistics, seals and sea monsters in the technical literature
- The most inconvenient seal
- Pinnipeds Descended from One Ancestral Line, Not Two (was: Seals, the Early Years)
Refs - -
Arnbom, T., Fedak, M. A. & Rothery, P. 1993. Offspring sex ratio in relation to female size in southern elephant seals, Mirounga leonina. Behavioral Ecology and Sociobiology 35, 373-378.
Carwardine, M. 1995. The Guinness Book of Animal Records. Guinness Publishing, Enfield, Middlesex.
Le Boeuf, B. J. & Crocker, D. E. 1996. Diving behavior of elephant seals: implications for predator avoidance. In Klimley, A. P. & Ainley, D. G. (eds) Great White Sharks – the Biology of Carcharodon carcharias. Academic Press (London), pp. 193-205.
Le Boeuf, B. J., Morris, P. A., Blackwell, S. B., Crocker, D. E. & Costa, D. P. 1996. Diving behavior of juvenile northern elephant seals. Canadian Journal of Zoology 74, 1632-1644.
Lindenfors, P. & Tullberg, B. S. & Biuw, M. 2002. Phylogenetic analyses of sexual selection and sexual size dimorphism in pinnipeds. Behavior, Ecology and Sociobiology 52, 188-193.
Ling, J. K. & Bryden, M. M. 1992. Mirounga leonina. Mammalian Species 391, 1-8.
Sanvito, S., Galimberti, F. & Miller, E. H. 2007a. Vocal signalling of male southern elephant seals is honest but imprecise. Animal Behaviour 73, 287-299.
Sanvito, S., Galimberti, F. & Miller, E. H. 2007b. Having a big nose: structure, ontogeny, and function of the elephant seal proboscis. Canadian Journal of Zoology 85, 207-220.
Woodley, M. A., McCormick, C. A. & Naish, D. 2012. Response to Bousfield & LeBlond: Shooting pipefish in a barrel; or sauropterygian “mega-serpents” and Occam’s razor. Journal of Scientific Exploration 26, 151-154.