The Hidden Necks of Seals

Harbor Seal at Roger Williams Park Zoo

I’d bet that most casual observers of Harbor Seals (Phoca vitulina) are under the impression that the little pinnipeds are almost neckless. Of course, it’s not immediately obvious what the skeletal proportions are underneath that blubber, but judging by the forelimb origin, it would seem the neck is less than half the length of the head. Harbor Seals are hiding more neck than what they let on, as their cervical vertebrae are actually similar in length to the skull:

Harbor Seal skeleton from Museum of Comparative Zoology

Clearly the live animal above was not holding its neck in the same posture as this skeleton, and (not having X-Ray specs) I’d assume it was similar to the deep “S” curve in this Harp Seal. As for why seals would shorten their apparent necks so dramatically, King (1983) suggested phocids (“true” seals) require a spindle-like shape when swimming since their propulsion comes from oscillating the posterior end of their body. But why bother having a neck at all? A strongly retracted neck gives seals “slingshot potential” to capture prey (Rommel & Reynolds 2002), as alarmingly demonstrated by this Leopard Seal. It’s surprisingly difficult to find photographs of Harbor Seals showing off their full neck — presumably they only do it rarely and briefly — but blogger Kitty Kono has an amazing snapshot:

From Kitty Kono’s blog Back in the U.S.A. Used with permission.

I of course covered all this in my Weddell’s Long-Necked Seal article, but now, I’ll take things further by determining just how long seal necks are. The tails and sacral vertebrae varied much more than what I expected (plus one source often lumped the two), so rather than measure necks relative to the length of the entire spine, I did so relative to only the thoracic and lumbar vertebrae (T–L from here on). And yes, there is going to be an appendix to this blog post.

Black = Skull (gray for no data); Red = Cervical, light blue background equal to shortest neck, pink equal to longest neck; Blue = Thoracic; Green = Lumbar; Yellow = Sacral; Orange = Caudal.
Species are (top to bottom): Dog, Bearded Seal (Erignathus), Weddell Seal (Leptonychotes), Southern Elephant Seal (Mirounga leonina), Leopard Seal (Hydrurga), Hooded Seal (Cystophora), Harbor Seal (Phoca vitulina), Ringed Seal (Pusa hispida), Harp Seal (Pagophilus).

Phocid necks range from 21% T–L in Bearded Seals to 35% T–L in Harp Seals*, which falls short of dogs with necks 40.5% of their T–L. Weddell seals were initially described as “long-necked”, so it’s amazing to see they’re at the low end of the spectrum with necks only 23% T–L. Contemporary sources almost always describe Leopard Seals as “long necked”, however their necks are only 29% T–L, so perhaps “thin-necked” would be a more apt description. The extinct seal Acrophoca is also typically described as “long-necked”, although judging from this skeletal illustration and mounted specimen, the neck is around 30% T–L. The closest true seals have to long necks come from members of the “tribe” Phocini (Phoca, Pusa, Pagophilus), although I have no idea why that would be the case.

* Piérard (1971) cites a source giving 35% for Harbor Seal T–L, so my figure above is probably a shorter-necked young individual.

Northern Fur Seals (Callorhinus ursinus) at Mystic Aquarium.

As for otariids (“eared seals”) — sometimes described as having “snakelike” necks (e.g. Riedman 1990) — do they truly have long necks, or is this a deception from thin necks held out straight? Otariids require a considerable amount of mass before and after their foreflippers (their source of propulsion) for stability, and so they keep their necks out fairly straight (King 1983). Phocid and otariid necks have been described as “similar” in length (Rommel & Reynolds 2002), although just to be on the safe side, I’ll quantify this as well:

Otariid necks vary little, from 34.5% T–L in Australian Sealions (and most other being only slightly higher) to 41% T–L in Northern Fur Seals. So there is certainly quite a pronounced difference in neck length between Bearded Seals and Northern Fur Seals, it does not seem that the average phocid, otariid or walrus is not truly that different in neck length. It should be cautioned that proportional neck length can change considerably during maturity, so there is certainly some error in these figures. The Bearded Seal was reportedly an old female (Hayden 1880) so if anything, it may have had a longer neck than average.

One thing I haven’t mentioned so far is that pinnipeds have far longer tails than most observers would expect, ranging from 13.5% T–L in Northern Fur Seals to 41% T–L in Ringed Seals. I really have no idea why this would be the case.

References:

Allen, J. (1880) History of North American Pinnipeds. Available.

King, J. (1983) Seals of the World.

Piérard, J. (1971) Osteology and Myology of the Weddell Seal Leptonychotes weddelli (Lesson, 1826). Available. IN: Burt, W. (editor) Antarctic Pinnipedia.

Riedman, M. (1990) The Pinnipeds: Seals, Sea Lions, and Walruses.

Rommel, S. & Reynolds, J. (2002) Skeletal Anatomy IN: Perrin, W. et al. (eds.) Encyclopedia of Marine Mammals. Relevant Passage.

Skull Cerv. Thor. Lumb. Sac. Caud.
Dog (King) ? 17.00% 24.00% 18.00% 4.00% 37.00%
Bearded (Hayden) 230 250 800 390 175 350
Weddell (Piérard) 0 274 820 385 150 385
S. Elephant (Hayden) 480 570 1690 670 250 680
Leopard (King) ? 17.00% 37.00% 22.00% 3.00% 21.00%
Hooded (Hayden) 265 275 630 320 190 290
Harbor (Hayden) 220 210 445 216 120 230
Ringed (Hayden) 163 200 410 190 100 245
Harp (King) ? 19.00% 37.00% 17.00% 7.00% 21.00%
Walrus 390 400 1170 380 ? 550
N. Fur (Hayden) 275 430 770 270 160 140
N. Fur (Hayden) 245 360 680 245 145 145
N. Fur (Hayden) 200 200 520 185 105 160
N. Fur (Hayden) 185 172 470 173 95 120
Aus. Sealion (King) ? 20.00% 43.00% 15.00% 7.00% 15.00%
Aus. Fur (King) ? 21.00% 44.00% 15.00% 6.00% 13.00%
Cal. Sealion (Hayden) 236 320 640 230 ? 280
Steller’s (Hayden) 374 500 1050 340 ? 440
Steller’s (Hayden) 385 540 1090 400 ? 520

Data from King (1983) were in percentage of the entire spine with no data on skull length. Some Data from Hayden (1880) is in millimeters, and for instances where sacral and caudal vertebrae were lumped, I signified this with “?” in the area for sacral measurement.

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4 thoughts on “The Hidden Necks of Seals

  1. Allen Hazen

    Hi Cameron! Just discovered THIS website today. (BTW, I thought some of the posts on your first “Lord Geekington” site– those about maximum sizes of various species, for example– were of enduring interest: are they still access-able?)

    Two remarks on seal necks.
    (1)Cetaceans– at least such “modern” types as Dorudontids and up– share with Phocids the locomtory style of propulsion by hind-body oscillation, so are under the same sort of hydrodynamic constrains, so have convergently “adopted” a typically short-necked form. Difference: they have REALLY short– short in skeletal structure– necks. Evidently they have never “felt the need” for “slingshot potential” for prey acquisition!
    Question: there are a few Cetaceans that have mobile and visibly long necks: Beluga, and some of the freshwater dolphins. What do their cervical vertebrae look like? I would assume they aren’t as short and as totally fused as those of other Cetaceans, but I don’t remember ever looking at and noticing the necks regions of skeletons of these species.

    (2) I was describing your post to my wife, and she suggested another possible function for an extensible neck: suddenly extending it, and bending to one side, while swimming at high speed might help initiate a turn. (No I don’t know enough hydrodynamics to even begin to think about guessing how effective this would be.) So maybe in addition to being prey acquisition devices (analogous, maybe, to the “great appendages” of some Cambrian arthropods?) they are also deployable as front-end (like “canard” stabilizers in aircraft?) rudders.

    Reply
    1. Cameron Post author

      Yikes, sorry about the moderation, hopefully I’ve fixed that problem!

      The oldest Lord Geekington did go down for a while when my domain name expired, but it seems to be up and running again. I should probably port over some of those classics just to be sure.

      I wonder if the lack of “slingshot potential” in cetaceans has to do with early members of the group having ludicrously large heads. Oddly, Kutchicetus has a neck that seems moderate in length, so maybe not.

      I did some quick and dirty measurements of a Beluga and Inia and found their necks were about 5% of the total spine, versus about 3% for a Common Porpoise, Common Dolphin and White-Beaked Dolphin. Fusion-wise, the Beluga and river dolphin have cervical vertebrae that are all free, whereas the porpoise has six that are fused, but only the first two are fused in the dolphins. Things get a bit confusing since Snubfin Dolphins also have a fused atlas and axis vertebrae, but they have thin and flexible necks that seem to rival those of Belugas and river dolphins! I can’t find an illustrated skeleton to know if theirs are also unusually long or not. On the other hand, rorquals usually don’t have fused neck vertebrae, but it doesn’t appear their necks are very flexible (if they move at all). I should probably work on something about whale necks…

      Next time I watch seals, I’ll have to keep an eye on their necks! Sealions at least certainly seem to swing around their necks a lot when swimming.

      Reply
  2. Allen Hazen

    Thanks for reply. It has been a while since I looked at whale skeletons, so I’m not sure anymore what rorqual cervicals look like– SOME big whale I looked at had cervicals that may not have been fused, but were very thin disks, so the whole stack of cervicals was only a few inches long… which would surely prevent neck flexibility!

    About early cetaceans having big heads (and in many cases long snouts): yes. I though of that as an alternative to “slingshot” for prey acquisition. Perhaps there is also a scale factor: many cetaceans have been very big animals, and the mass of their heads would perhaps make slingshotting impractical.

    Reply
  3. Pingback: Morsels for the mind – 22/3/2013 | Six Incredible Things Before Breakfast

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