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Dive Dry with Dr. Bill

#577: Go Your Own Way, I'll Go Mine

If you are interested in dieting, the northern elephant seal (Mirounga angustirostris) may have some great suggestions for you. These seals may lose up to half their body weight in one month. Although I'm rather fond of my Buddha belly, I'd be happy to lose 20 pounds in that time! And I may have the means to do that since the pinnipeds fast for the entire time they are mating and raising their pups, using stored fat as their energy source. I've got some "bioprene" in my body as extra insulation when I dive during winter. Well, on second thought, I doubt I have the will power to fast more than a few days (even given my cooking).

Elephant seals, which spend the majority of their lives at sea, come ashore beginning in December to give birth to their pups and mate. During this time they do not feed at all. Only the pups are nourished by the fat rich milk (up to 55-60%) from their mothers. Once the pups have reached a size where they can begin fending for themselves, the mothers abandon them and head back to the sea to munch. Of course once the girls are gone, the guys soon follow since their primary interest in coming ashore has been that other "M" word, mating. What else do guys have on their mind besides those two? They don't follow sports or drink beer.

Most people are unaware that at this time the northern elephant seal begins what may be the longest migration of any warm-blooded species. Yes, I know... you've read that the gray whales (Eschrichtius robustus) that travel past our island have the longest migration route of any mammal. I did a little research and found that the whale's migration route is 10,-12,000 miles. The elephant seal bests this by about 1,000 miles. Of course I think I actually hold the record for the longest migration since during the cold winter months in SoCal, I like to head to the Asian, South American or African tropics for some warm water diving (assuming my bank account allows that)! However, I don't have to swim, I can fly.

An interesting fact about the elephant seal's migration is that the boys and the girls travel separate paths. I guess after being ashore together for a month or so, they need a little distance. This is mostly due to the fact that the two genders have different dietary preferences. The males stay close to the continental shelf and feed near the bottom on fish, small sharks and rays. Their route takes them up to the cold waters of the Gulf of Alaska and Aleutian Islands. The girls feed primarily in midwater on squid, and head out into the open sea south of where the boys hang out (in other words, not "where the boys are"). They may head as far west as the Hawaiian Islands. The huge males are less threatened by predators like the great white shark and can remain closer to shore and dive to deeper depths, whereas the females seek safety out in deep, open water.

When a species (or several species) differ in their use of resources such as food, we biologists refer to it as resource partitioning. We humans do this too, with some preferring meat, others plant food and folks like me being rather omnivorous. Yes, I'll eat almost anything... I have to given my lack of culinary skills. The elephant seal diet may include at least 53 different species, although more than half of them are squid. Munchies include ratfish, swell sharks, spiny dogfish, eels, rockfish, skates, rays, octopus, crustaceans and Pacific hake or whiting. Now I'll admit I'd draw the line on some of those "delicacies." Most of the prey they consume is of low commercial value or only minimally harvested by Homo sapiens so they aren't major competitors with humans.

Feeding is facilitated by the fact that elephant seals are among the best divers of all pinnipeds. They dive deeper than any other species. The maximum authenticated depth recorded was over 5,000 feet although most dives are between 1,000 and 2,600 feet (300-800 m) for males with the females doing somewhat shallower dives. The maximum duration of a dive has been variously reported as a little over an hour to up to two hours. I can match that on a single aluminum 80 SCUBA tank, but only if I stay much, much shallower! Average dives for elephant seals are are about 17 minutes for the girls and 21 minutes for the boys. Sounds more like a newbie or a real "hoover."

Although some sources stated the seals only dive at night, researchers have found they actually may dive nearly continuously for long periods of time. One study observed a female for 34 days and reported she dove almost continuously during that time with only brief rest periods of about three (3) minutes at the surface between dives. I guess after a long period of fasting, munching is all you can think about!

So how do they accomplish such incredible dives? The first assumption is that they hold their breath, but this is not true. They actually expel the air from their lungs before submerging. With all that fat, a full lung would create even more buoyancy. Their heart rate drops from about 110 beats per minute to 20-40. The ocean is very cold at the depths these seals reach. To help counter this, their circulatory system in the extremities is adapted to recapture heat with a mixture of small veins surrounding the arteries. An additional adaptation for feeding at depth is the very large eyes that can detect bioluminescent critters like squid.

Elephant seals actually make two migrations per year. The second migration does not involve munching or mating. Instead it is for another "M" word not common to most marine critters, molting! They are mammals with hair (aka fur) just like dogs, cats and humans. Unlike dogs, cats and humans; they do not shed continuously throughout the year. If you could keep one as a house pet, you wouldn't need to vacuum as often... although you might need much stronger cleaning solutions!

Elephant seals undergo a "catastrophic" or "radical" molt. Once a year they shed all their fur and the upper layer of skin. They come ashore during the warmer months between April and August, a good choice when you're about to lose your fur coat! The females come ashore first, then the juveniles and lastly the males. It takes about 3-5 weeks to molt and grow back the skin and fur. Then they are off on another feeding migration until it is pupping and mating time again. So this species truly exemplifies the best of "Munching" and "Mating." Of course I prefer to avoid the battles over the ladies... I'm a lover, not a fighter.

© 2013 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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A girl... and the big boy; conserving energy on the beach and the different migration routes for the boys and girls.

This document maintained by Dr. Bill Bushing.
Material and images © 2014 Star Thrower Educational Multimedia