HUNTING BETWEEN TWO WORLDS

TRANSCRIPT

Welcome to the food desert of Canada’s wilderness in mid-winter. Any animals choosing to stick it out here rather than flee south or tap out and hibernate have to devote a huge chunk of their day - every day - picking across it for minuscule scraps.

It’s a tough life. Unless, of course, you had a way to cheat. 

Like what if you had access to a hidden food source unavailable to most of your competition?

Canadian winters also put a premium on a very particular kind of real estate.

As the temperature settles below freezing, ice encroaches and fresh water freezes over - from puddles to pools to ponds to inlets to lakes.

But, there are certain spots that hold off the deep freeze and stay accessible. Often they’re bottlenecks between bodies of water where the current naturally flows faster. 

And these little oases can crowd in with waterfowl living in a shaky truce to share access to a secret smorgasbord.

Because hidden below the ice, the bottoms of these bodies of water can still provide life-sustaining nourishment in the bleak season.

Trumpeter and mute swans forage constantly, dipping as deep as their necks will allow into the unfrozen substrate for plant matter.

Same with overwintering mallards. They stick to shores and shallow water where they can probe any mud that hasn’t frozen.

Already they have an advantage over animals who are stuck scouring the land. But they’re also sharing the pool with a group of waterfowl who take things further, or, deeper: diving ducks, who have adapted to fully exploit the underwater buffet, not limited by water depth or neck length.

Like goldeneyes, who announce themselves as diving ducks every time they take wing: their distinctive in-flight whistle isn’t a vocalization, it’s produced by their extra-stiff primary feathers beating nine times per second - feathers that have evolved this way to prioritize underwater speed and maneuverability.

Or long-tailed ducks. They’re world-champion divers, descending down to 60m in search of food, and spending the most relative time underwater of all diving duck species - three to four times longer than at the surface.

They also get way more sociable in the winter - you kinda have to be when you’re packed into these little pools together. They’re the most vocal in the crowd, filling the air with their signature tri-tone calls to each other.

And they don’t just group together at the surface. They naturally synchronize their dives so whole flocks can just disappear at once. This group diving behaviour coordinates their search for food on the bottom and makes them collectively much more efficient.

Finally, we have mergansers. While many are migrators, climate change is encouraging more of them to gamble on staying put every winter - so long as they can find open water. 

There are three species in Canada - common, red-breasted and hooded. And they’re an example of one of the weirdest adaptations to life between two worlds.

This adaptation has only been closely studied in hooded mergansers, but it’s suspected that it’s shared by all three species since they’re so closely genetically related.

Mergansers are omnivorous, and their diet includes a lot of hunting, not just foraging. They’ll go after small fish and crayfish who stay semi-active through the winter.

So why does that matter? Because of the old adage, ‘there’s no such thing as a free lunch’ - literally. 

So far this all sounds great, right? The ground freezes over and gets buried in snow, you’ve got exclusive access to this reliable foraging and hunting ground of relative abundance.

So what’s the catch? It’s those pesky immmutable laws of physics. Specifically? We’re talking refraction.

Light, as it moves through a medium, is subject to a ‘refractive index’ - how much the speed of that light slows vs moving through a vacuum. The higher the refractive index of the medium, the slower the light.

Water has a higher refractive index than air. When light moves from air to water it slows down, and, it bends.

Why? Light is a wave - you know, when it’s not a particle - anyway, oversimplified, it’s a wave, if you think about that wave hitting the edge of a different medium at anything but a perfectly straight angle… one side is going to slow down before the other, which changes the angle of the wave - like one tire of a car braking would rotate the car.

And why does it ‘slow down’? Even more simplified, the incoming wave causes electrons in the medium to oscillate, re-radiate their own waves, which interfere with the original wave through a phase shift, the sum total of which is a slower propagation.

But the point is: slower, and bent. And there’s lots of fun simple ways to demonstrate this, like putting a straw in a glass of water and seeing it appear to bend or break below the surface. Or how an arrow can be made to flip directions by holding it behind a glass of water.

But OK, why does all this refractive index nonsense matter to a merganser? 

Because ever since they - we - terrestrial animals - hauled out of the primordial ocean hundreds of millions of years ago, our eyes have been evolving to deal with the very specific refractive index of air. 1.0003, by the way.

If you open your eyes underwater without a mask or goggles, you’ll immediately see - you can’t really see. That’s because your eye needs light input at the refractive index of air to properly focus it through the cornea and onto the retina.

Our eyes are physically built for this distance. So when they get input at a higher refractive index, like 1.33 in the case of water, their precise structure and distance… don’t work anymore. The cornea itself has a refractive index of 1.37 - almost the same as water. So instead of correctly bending the light, there’s almost no bend at all, which means almost no focusing effect.

If we wanted to focus light underwater with our corneas, we’d need to shift our retina back roughly three to four times its distance from the front of the eye.

That underwater blurriness is a mild inconvenience if you’re just out for a swim or can slap on a pair of goggles, and a layer of air, to restore the proper refractive index. But it’s a real problem if you’re a merganser.

Because remember, they’re not just foragers - when you’re digging through mud for food, who cares if it’s a little blurry. Mergansers are also hunters, going after darting minnows and crayfish. The kind of prey you definitely need clarity to catch.

But then, they still spend a lot of their lives at the surface. Most! So it wouldn’t help for their eyes to evolve for the refractive index of water - they’d just be trading one problem for another: terrible blurry vision whenever they’re not underwater.

So what’s a diving duck to do? Stuck between two worlds, mergansers declare ‘porque no los dos?’ Why not just alter the shape of your eye depending on the medium?

They use specifically developed, extra-powerful eye muscles totally unavailable to suckers like you and me, to push the lens of their eye forward and alter the curvature, angling them to more powerfully catch and bend light in that 1.33 refractive index.

Which in turn lets them effectively hunt fast-moving fish and crayfish through winter scarcity as other birds are stuck digging in the mud or, worse, picking their way across the barren landscape.

Other semi-aquatic species have all kinds of tricks to deal with this ‘underwater clarity’ issue but mergansers - and again, only fully studied in hooded mergansers - are pretty rare in using this ‘eye-reshaping’ method. And it kind of co-signs their whole strategy.

Because evolution doesn’t give you powerful eye-squeezing muscles on a whim to see if they work. The existence of such a specific adaptation means that their underwater hunting style must have been successful enough to select for it over countless generations. And mergansers crowding open pools in frigid months tells us that ‘winter oasis’ cheat is clearly a winner.