Life and Stress at the Ice EdgePaul Nicklen
I KNOW LEOPARD SEALS. OR, AT LEAST I THOUGHT I DID.
After dozens of underwater encounters with one of Antarctica’s top predators I took the liberty of putting their range of behaviors into a box. On this clear, crisp morning, little did I know that my attitude would nearly cost me my life.
On the second day of my assignment to photograph Emperor penguins in the Ross Sea, I stood next to a series of open holes and ice leads to see where the penguins were entering the water when I spotted the unmistakable triangular head of a leopard seal breaking the surface. It was the first seal we had seen and I smiled with anticipation as I recalled the many interactive dives I had with these engaging and intelligent animals. I felt like I was seeing an old friend and could not wait to join this much-misunderstood animal in the water.
The next moment and without warning, the seal exploded from the water’s surface before I realized what was happening. Even though we were standing fifteen feet from the ice edge, the seal flew towards me putting his entire 600-pound mass at eye level. I only had time to raise my arm to protect my face and mutter some unintelligible expletive as his head smashed into mine. As the rest of his body hit me, I was violently thrown onto the ice. The impact knocked me down, hard.
He could have easily bitten me, but in the split second it took for him to realize that I was not a penguin, he closed his mouth. It was the most impressive display of ambush predation I had ever seen. What I had not realized is that comparing the hunting behavior of leopard seals gorging themselves on 8-pound penguin chicks in the Antarctic Peninsula where I had all of my experience, with those laboring to catch 70-pound adult Emperor penguins in the Ross Sea, is like comparing a golden retriever with a lion. I had just met the lion of Antarctica.
Later, while on a dive below the ice edge, I looked up and realized why this seal had mistaken me for an Emperor penguin. Through the clear window of water I could easily distinguish the dark silhouette shape of my assistant, standing, 10 feet away from the edge, just like the penguins do. It became evident that as leopard seals patrol the ice edge that Emperor penguins use to enter and exit the water, they look for shapes on the ice and listen for sound cues as they await to ambush the birds.
Even though they are striking, it is their behavior that interests me most, and the reason that I wanted to dive with them in the 28°F (–2°C) water of the Ross Sea. Most of what we know about these birds revolves around their extreme parenting behavior; the harsh isolation of their nesting colonies; the arduous trek to-and-from the sea to get food as they take turns caring, first for their egg and then for their chick.
What little we know about this resilient animal, from movies like ‘March of the Penguins,’ has left many of us yearning to find out what happens when the penguins reach the ice edge and enter the frigid waters off Antarctica. Knowing that the death of even one parent will cost the chick its life, I wanted to find out which survival strategies and physiological adaptations these “extreme parents” have developed to avoid predation.
I sat on the ice for hours observing how the penguins hesitated before entering the water, waiting for one bold bird to plunge in. When first approaching the ice edge, after the long march from the colony through the frozen landscape, the penguins would stand a good 30 feet from the water’s edge to avoid what had just happened to me.
Watching them exit the icy waters in leaps that defy gravity, however, is what intrigued me the most. Water is 800 times more dense than air, so I wanted to understand how these large birds manage to achieve sufficient underwater speed to avoid an ambush attack by a leopard seal, overcome the influence of gravity, and leap clear out of the water and onto the ice.
While fully aware of the potential for another accidental leopard seal attack, I had no choice but to get into my dry suit and slip into the icy water through a hole in the thinning sea ice. Snorkeling in this small hole, barely larger a hotel room, I peered through the water into the crystal clear depths watching hundreds of penguins race, execute sharp turns, and morph from the awkward lumbering mass we have come to recognize on the surface, into one of elegance, grace, power and speed. I quickly understood why out in the open water it is almost impossible for a leopard seal to outrun and outmaneuver an Emperor penguin.
Without realizing what was happening, all the penguins, who had clearly never seen a human in the water, had darted into the depths of the ocean, leaving me floating alone in the midst of a sea of bubbles; a “smoke screen” so effective, I could barely see my own hands. Being naturally curious and very intelligent, however, it took them only seconds to realize that I posed no danger. I smiled around my snorkel as the penguins soon relaxed and allowed me to remain in the ice hole with them while they went about their never-ending behaviors at the ice edge.
Over the next few days I observed how the penguins use bubbles not just as a “smoke screen” but also as a powerful means of propulsion. I was mesmerized by the beautiful bubble trails the penguins created. In the open ocean where they primarily feed, Emperor penguins dive up to 1,700 feet for 15 minutes at a time, unprecedented in diving birds.
On one occasion, while surrounded by hundreds of penguins at the surface, the biologist in me took over and I decided to pull my underwater housing away from my eye and pay closer attention to the entire mass exiting process. After spending weeks at sea, the emperors approach the ice edge with great caution because if they fail to get their belly full of fish to their hungry and waiting chick, it will die.
They are animated, nervous and cautious, constantly scanning the water below them, with occasional glances above. At first I thought they were preening and bathing themselves at the surface but soon it became apparent that in fact, they were filling their feathers full of air.
As they get ready to exit, they dive as a group to a depth of approximately fifty feet, milling, spinning and circling around one another all the time studying their overhead environment to spot any leopard seal that might be waiting to ambush them under the dark ice. They study one another waiting for that moment when one bird might panic. When one penguin bolts, they all move cohesively with lightning speed.
Once they decide on an exit path, one by one they race to the surface spewing off millions of bubbles from their beaks, chest, head and belly feathers, leaving a stream of bubbles that looks like a smoke trail at an airshow. They accelerate to speeds that are hard to comprehend. As they explode through the surface, they clear the ice edge by several feet and land with a funny squeak when the air gets knocked out of them as their 80 pound frames crash hard onto the sea ice. Then they lift themselves to their feet and start the long journey back to the colony.
This physiological adaptation, known as “air lubrication” was just recently described by Professor John Davenport of the University College Cork and his colleagues in a study published in the Marine Ecology Process Series. They studied hours of film and discovered that when on the surface, penguins raise their feathers to fill their plumage with air before diving underwater.
As they descend, the water pressure increases, compressing the volume of the air trapped in the feathers. At a depth of 80–100 feet the air volume has shrunk by up to 75 percent. When they are ready to surface, they depress their feathers, locking them around the new, reduced air volume and as they speed vertically towards the surface, the air trapped in the plumage expands and pours through the feathers. Individual birds can control when and how much air they release from their feathers. They can also control which feathers release the air.
The structure of the feathers is highly complex and the pores through which the air is pushed out are so small, that the bubbles are initially very tiny. So tiny, in fact, that they form a coat on the outer feather surface. This coat of small air bubbles is the key to the penguin’s strategy to avoid leopard seals. The bubbles act as a lubricant, drastically reducing drag and enabling the penguins to reach lift-off speeds.
The seemingly simple adaption of using air bubbles to reduce the friction of water, increase speed and explode out of the water, enables Emperor penguins to survive predators during their continuous feeding forays in the Southern sea. Witnessing firsthand the relationship between Emperor penguins and their predators, and the strategies they have developed to avoid predation, was the opportunity of a lifetime and one that could only happen in a place that has been as well protected as Antarctica.
Yet, as remote as this foreboding part of our planet is, the survival of Emperor penguins is intimately linked, not its predator the leopard seal, but to our human dependence on fossil fuels.
“As penguin populations around Antarctica continue to decline, warming temperatures and disappearing sea ice remain the number one threat to the future of this remarkable species.”