|The Question: I am a frequent visitor to the harbor seal exhibit at the New England Aquarium, and I always notice the same thing: the seals swim almost exclusively upside-down! Is this a result of captivity, and their relatively small exhibit space? Do they swim this way in the wild? Are they just bored?
Submitted by: Marion, USA
The Not So Short Answer: Great question! I wish I had a more definitive answer, but it doesn’t appear that anyone has really studied this phenomenon. Jenny Montague, Assistant Curator of Marine Mammals at the New England Aquarium where you saw the harbor seals (Phoca vitulina), confirms your observation. She says seals at the Aquarium often swim upside down, including both wild seals kept temporarily at the aquarium as well as seals that have been in captivity for a long time. She estimated that they are upside down about 70% of the time. Jenny points out that “The eyes of many seal species are oriented toward the top of their head. Unless they are diving very deep, swimming upside down gives them a better view of their surroundings.”
Takashi Iwata, a researcher at The Graduate University for Advanced Studies, National Institute of Polar Research, in Tokyo, Japan, studies captive northern fur seals (Callorhinus ursinus) and wild Antarctic fur seals (Arctocephalus gazelle). He confirms that captive fur seals swim upside down about half the time. He thinks the simple answer is that upside down allows them to see the sides and bottom better in the enclosed space of a seal tank. When he observes wild Antarctic fur seals, Takashi says they are almost always right side up when diving or ascending. On the bottom and on the surface, however, they often swim with a rolling motion that puts them right side up part of the time and upside down part of the time.
Terrie M. Williams, who studies the energetics of seals and other large marine mammals with her research group at the University of California Santa Cruz, says there isn’t a lot of information about harbor seal swimming positions in the wild. She reports that in her research on Weddell seals in the Antarctic (Leptonychotes weddellii), they swim upside down frequently. “They do this right beneath the ice, on the bottom, and even will drift down the water column upside down. We find it curious – at times it seems as though they are looking for fish from that position due to the placement of their eyes. But it may also have to do with air in the body when they are submerged. Often if a Weddell is resting, it will drift down in the water column and turn upside down as they go. Makes me wonder if it has to do with where the air is positioned in the lungs when they are compressed at depth.”
So, to summarize what we know: It seems that at least four species of seals, all the ones for which I was able to get information, swim upside down at least some of the time. We’ve confirmed that Weddell seals and Antarctic fur seals do it in the wild. And that wild seals do it in captivity. Sometimes the best way to think about a question like this is to turn it around.
Why wouldn’t seals swim upside down?
In the water they live in an essentially weightless environment, so all orientations are probably about equal, energetically speaking, although as Dr. Williams points out, it may be that some positions are more comfortable than others.
It seems most likely to me that right side up or upside down orientation is probably related to vision. A wild seal mainly uses its vision for two things, to avoid predators and to find prey. Based on that, I would predict that seals would swim upside down at the surface of the water so they can see everything below them – prey and predators. Since there isn’t much of interest going on above them, there’s no need to look up. Predicting their behavior on the bottom is a little trickier. They would want to look up to avoid predators and see prey that is above them, but they might also want to look down to find prey that hugs the bottom. Dr. Iwata thinks that prey detection explains the rolling motion of his Antarctic fur seals. He thinks it’s a foraging strategy that allows them to search for prey above and below at the same time. It would also allow them to keep some watch for danger from above at the same time.
Back to the New England Aquarium … Jenny Montague reports that the outdoor harbor seal tank is eleven feet deep (3.3 m). This would constitute pretty shallow water for these seals. It’s possible that in shallow water, it’s normal to swim upside down because you can see prey below you and even when on the bottom, you don’t have to worry much about predators above. We know that, as Dr. Williams says, “Bottom line: upside down is darn normal in the wild for some seals.” It’s definitely something harbor seals are comfortable doing. Now imagine the life of a New England Aquarium seal. They’re smart enough to figure out that there are no predators around. Swimming around the tank requires tight turns, and it’s easier to do that if you’re looking at the sides and the bottom. It doesn’t hurt to bump into the surface of the water, so why bother looking up?
When they get fed, they get fed dead fish (fresh, of course), which promptly sinks to the bottom. The biggest challenge in their lives is probably competing with their fellow tank makes to get the choicest fish. And the best way to do that? Look down.
So that’s my speculation. Combine the slightly tight quarters with a lack of any danger from above and food bounty that is often below and the question becomes, “Why do they spend 30% of their time looking up?”
By the way, your question got Jenny Montague wondering about this issue, so she was thinking she might have someone observe the seals and record how much time they spend upside down. Maybe we’ll have a more definitive answer soon.