Imagine that you’re hiking at high altitude. The air is thin, so you struggle to take a
breath. Not only that, you haven’t eaten in days so you feel like you’re on the brink of death. All of a sudden,
you get reception and notice that there’s a ramen spot less than a kilometer from you,
but it’s closing soon. If you run, you might be able to make it. But thanks to the lack of oxygen,
you can only run for only a few seconds before you’re gasping for air.
So what’re you supposed to do in that situation? If you keep running, then
you’ll pass out from the lack of oxygen, but if you stop running, then you’ll die of starvation.
This is what it’s like for the river otters of Yellowstone National Park — who not only live at
high elevation, but they can swim ridiculously fast underwater while holding their breath.
Their counterparts at sea level can swim similarly, but when they come up for air,
they have the luxury of topping off with a full tank of oxygen-rich air. So what is it about
the physiology of these Yellowstone otters that lets them not only live at high elevation, but thrive?
[intro]
If you’re new here, welcome, my name is Patrick and this channel is all about
anatomy and physiology. Usually I focus on the human body, but I’ve been thinking about these
otters for years now. Back in 2020, I was working on a video about otter hearts for SciShow, and I
came across an article published in 2012 titled “High altitude diving in river otters: coping
with combined hypoxic stresses” printed in the Journal of Experimental Biology. I read it and it
was interesting, but it didn’t quite fit into the SciShow script, so I saved the pdf and moved on.
But here I am in 2022 still thinking about how cool these otters are,
so I’ve got to tell you the story.
In 2005, American researchers from Wyoming and Indiana went to Yellowstone National Park which
is pretty high up, which is over 2300 meters in elevation. For reference, Denver Colorado is
only about 1,600 meters, so Yellowstone counts as high elevation — if you lived at sea level,
you’d probably have trouble breathing before you acclimatized to the elevation.
Now, with high elevation comes the risk of hypoxia. Hypoxia comes from hypo, meaning low,
and oxic or oxia which means oxygen. So altitude is one of the hypoxic stresses referenced in the
journal title. The other was how the otters manage to hold their breath while they hunt.
Otters mostly eat fish, which means they have to find and chase their prey underwater.
And while lots of diving animals undulate through the water like fish,
otters kick their limbs and wave their tails, so their muscles consume a bunch of energy
and oxygen. To make matters worse, their usual prey, the Cutthroat trout, is on the decline,
and the next best alternative, the lake trout, usually live much deeper — like
40 meters underwater. To the researchers, that raised the stakes a little bit. They wanted to
understand how these otters could live and hunt at this altitude, but they’d also be able to
calculate whether the otters could dive even deeper if their main source of food went away.
So the researchers captured five Yellowstone otters, 4 males, 1 female,
and compared them to a group of otters from the San Juan Islands off the coast of Washington.
These were their reference points for sea level otters.
They started by taking a small sample of blood from the otters’ veins and
paid particular interest to their blood chemistry.
They were especially interested in how oxygen behaved with a molecule called hemoglobin.
Right, while we say that there’s oxygen in your blood, it’s more
accurate to say that there’s oxygen bound to the hemoglobin in your blood.
And one way to adapt to low oxygen is just to just make more red blood cells and more hemoglobin.
This is exactly what human bodies do when we acclimate to elevation.
Our bodies make more red blood cells which we can pack with hemoglobin. This means that
a greater portion of our whole blood is made of red blood cells — what we call hematocrit.
The downside to having a higher hematocrit is that the blood is thicker and harder to pump,
which increases blood pressure and increases stress on the heart muscle.
So the blood of animals at sea level typically has the hematocrit as animals on land,
while animals at high elevation have a lower to moderate hematocrit.
Okay, so researchers could look at hematocrit as one of the otters’
adaptations. But they also wondered if hemoglobin itself might be a factor.
The scientists ran a bunch of simulations with the blood samples, and created this graph,
an oxygen dissociation curve. On the X axis is the partial pressure of oxygen in the oxygenated
arterial blood, and on the Y axis, how much oxygen is bound to hemoglobin. Basically,
it’s saying that the more oxygen you have in your blood, the more it binds to hemoglobin.
All kinds of things can affect this curve.
Like a more acidic environment can cause it to shift to the right,
where a less acidic environment shifts it to the left. Temperature, concentration of CO2, chloride,
nitric oxide, and a special chemical called 2,3 biphosphoglycerate all impact that curve.
One idea was that living at altitude might get them to hyperventilate,
causing them to lose CO2 through their breath, making their blood more alkaline,
shifting the curve to the left. This puts the body’s tissues on a kind of low power mode,
so they can reserve what little oxygen they have access to. On the other hand, breath holding makes
our bodies accumulate carbon dioxide, making blood more acidic and shifting the curve to the right.
A more acidic environment improves oxygen delivery to the tissues too. Plus, it gets
really cold in Yellowstone, which increases the animals’ base metabolism. And those are like, all
of the things that impact the dissociation curve — the researchers really had no idea what to expect.
Also animals that live at high elevation typically have hemoglobin that’s really
good at binding to oxygen, so their curve shifts to the left. They’ll get
more oxygen saturating their hemoglobin at the same partial pressure of oxygen,
which is perfect for taking advantage of the rare oxygen at high altitudes.
So the researchers wondered if these otters would have a left-shifted oxygen dissociation curve too.
Here’s what they found.
First off was the big letdown: the hemoglobin in the Yellowstone otters’ blood didn’t behave
any differently than sea level otters. Their oxygen dissociation curves and
most of substances dissolved in their blood were pretty much the same.
The biggest difference was a much bigger proportion of red blood cells to total
blood and thus, a higher total hemoglobin concentration than the sea level otters.They
also had lower serum albumin, sodium, chloride, and a higher bicarbonate and nitrogen
oxide concentration. I know that’s a lot, so let’s break down what these mean.
The increase of red blood cells and hemoglobin makes sense. This is typically what low elevation
species do to acclimatize to the thinner air at altitude. But to deal with the effects of thicker,
more viscous blood, these otters also had a reduction in albumin — the main protein in
your blood. Its chemistry makes it so that fluids don’t leak out of the bloodstream.
It also acts like a vehicle for other substance like hormones, enzymes, and certain drugs.
The Yellowstone otters had half the blood albumin concentration of sea level otters,
and with fewer dissolved proteins, the blood became thinner and offset
some of the increased thickness from having more red blood cells.
They also had an increase in nitrogen oxide, a chemical released by the inner
lining of the blood vessels that gets the entire blood vessel to widen, or dilate.
This is what you’d expect — if your blood all of a sudden had more blood cells in it,
your blood is pushing harder on the walls of the artery,
which means that blood pressure would go up. So for our otters, an increased
nitrogen oxide gets the blood vessels to widen and stabilize their blood pressure.
At the same time, past research has shown that nitrogen oxide
production can increase in response to really low levels of albumin too.
So the researchers weren’t quite sure which of these adaptations came first,
or which influenced the other. But at the end of the day, they answered their question:
how do these otters live at altitude and dive so well? Because of more red blood cells,
less albumin, and more nitrogen oxide. But in the background of all this,
the Yellowstone otters’ main prey, the cutthroat trout, was still on the decline. And their next
best option was a fish that lives much deeper. So the researchers were curious if this physiology
would be enough for the otters to swim even deeper, accessing a new food source to stay alive.
So they took all of their data, and built a mathematical model that allowed them to predict
the average dive limit for the otters. They came out with 53.6 seconds for the sea level otters,
and 55.1 seconds for the Yellowstone otters. Not much of a difference.
Past research had shown that when otters hunt for the trout,
they have a success rate of 38-40% per dive, which is pretty high. Unfortunately the
majority of these kills happened in shallow water, far from their physiologic limits.
The researchers noticed that even with a 22% increase in hematocrit and that severe reduction
of albumin, the otters only got another second and a half of dive time compared to the sea level
otters. So at the end of the day, they probably won’t switch to a new prey source, and if the
cutthroat trout population doesn’t increase soon, that’s a big problem for the Yellowstone otters.
A big hank you to my patrons on Patreon who make these videos possible. Special
thank you to Mike Wex and Dr Ron Blumenfeld for supporting me at the Oxytocin level.
I appreciate you guys. If you want to help support the channel you can do so
for as little as two dollars a month. Have fun, be good, thanks for watching.