Chapter 9: The Energetics of Aerobic Activity

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Welcome everyone to the Deep Dive.

Today we're diving into a really fascinating world.

It's all about the incredible unseen energy that drives life on earth.

Just picture this for a second.

Masked boobies, these amazing seabirds flying like over a hundred kilometers daily across the Pacific, making these spectacular plunge dives for fish.

And it's all just to fuel their lives, feed their young.

It's not just a pretty picture, is it?

It's pure energetics in action.

Exactly.

And the scale.

One colony, just one, on Clipperton Island.

They munched about 70 metric tons of fish every single day.

Seventy tons.

That's hard to even visualize.

Right.

And then globally,

all seabirds combined consume something like a hundred and ninety thousand metric tons daily.

Which is, you know, basically on par with what humans harvest.

It really puts the energy demands of the animal kingdom into perspective, doesn't it?

It absolutely does.

So yeah, this deep dive is all about that animal energetics, how animals generate energy, how they use it, manage it for everything from just, you know, basic upkeep to these mind -blowing defeats like migration.

We've got a bunch of sources here and we're going to try and explore the core physiology behind it all, the mechanisms they use,

compare strategies across different animals.

And how scientists even figure this stuff out, right?

The methods?

Exactly.

The experimental methods too.

It's quite clever stuff.

So our mission today really is to give you a shortcut.

A way to get up to speed on these hidden energy demands that shape the entire animal kingdom.

Yeah.

Hopefully you'll walk away seeing, you know, every bird flying or squirrel running in a slightly different light.

An energy light.

I like that.

An energy light.

Okay.

Let's get into it.

Where do we start?

Well, let's go back to those boobies flying for hours or think about a gazelle running or even just, you know, you going for a long walk.

Any activity that sustain, that lasts more than a quick burst, it boils down to one fundamental thing.

It has to be aerobic.

Aerobic,

meaning oxygen, right?

Exactly.

It means the vast majority of the ATP, that's the energy currency for every cell, has to be produced using aerobic catabolism.

And that process absolutely needs a steady continuous supply of oxygen delivered right to the working muscles.

Okay.

So this is different from, say, a really short, intense sprint where you feel that muscle burn.

That's anaerobic.

Precisely.

Anaerobic is great for quick, powerful bursts, but it gives you limited ATP and you fatigue fast.

Right.

For anything longer, a marathon run, a birds migration, those booby fishing trips you need that continuous sort of pay as you go energy from aerobic metabolism.

Pay as you go.

I like that analogy.

Yeah.

It operates in what scientists call a steady state.

It allows continuous activity without hitting a wall immediately.

Just think about us humans.

Resting, your metabolic rate is pretty low.

Uh -huh.

Start walking.

Your energy needs jump up significantly.

Start jogging or running.

Your ATP requirements just skyrocket.

It shows the basic energy costs tied to just moving around.

Okay.

That makes total sense.

So if we want to understand how animals manage these huge energy demands, how they pull off these feats,

we need to know how scientists measure it all.

Right.

Because studying active animals, I mean, in lab or especially out in the wild, that sounds really challenging.

You can't just give a fish a Fitbit, can you?

No, not quite.

But scientists are incredibly inventive.

In the lab, the big challenge is controlling the animal speed precisely while measuring its oxygen use at the same time.

So for running or walking animals, the classic tool is the motor driven treadmill.

Right.

I've seen those for dogs.

Yeah.

And they use them for everything.

Tiny cockroaches, land crabs, turkeys, even cheetahs.

They can tilt them for hills too.

And get this, they even have underwater treadmills for lobsters and crayfish.

Underwater treadmills.

Seriously.

That's amazing.

A lobster workout.

Yep.

And for flyers, birds, insects, bats, they use wind tunnels.

The animal flies into a controlled air current and researchers measure energy use at different speeds.

And similarly, water tunnels, sometimes called flumes, for fish.

Same principle, just underwater.

Controlled speed, measure oxygen.

So labs give you that controlled environment.

But what about the wild?

That seems way harder.

Oh, it is much harder.

Sometimes for larger animals, they can fit them with a mask and valve system.

The animal breathes through it and you can measure the oxygen consumed from a defined air stream.

Like for racehorses?

Exactly.

They've used it on racehorses, even on elite Kenyan runners to study their efficiency.

Wow.

I read something fascinating about Nepalese porters too.

Their incredible efficiency carrying loads using head straps.

Still a bit of a mystery, apparently.

That's right.

Box 9 .1 in our source material.

Astonishingly efficient.

Like 50 -60 % less costly than Europeans with backpacks.

The biomechanics aren't fully understood.

It shows how much we can still learn even about human movement.

Definitely.

But for, say, a small bird migrating or a seal diving, you can't really use a mask.

How do you track their average energy use over days or weeks?

Ah, okay.

Now we get to a really key innovation for studying free -ranging, air -breathing animals.

The doubly labeled water method.

Doubly labeled water.

Sounds technical.

It is, but the concept is brilliant.

It lets researchers measure the average metabolic rate of an animal living its normal life in its natural habitat over days or even weeks.

No masks, no treadmills needed out there.

Okay.

Break it down for me.

How does it work?

Are they using tracers, like you mentioned?

Exactly like tracers.

Think of it like this.

You inject the animal with water that's labeled with two harmless stable isotopes.

One is a heavy hydrogen, deuterium, D or H, and the other is a heavy oxygen, oxygenin -18.

Okay, heavy water, basically.

Sort of, yeah.

These isotopes mix into the animal's body water.

Now, the key is how they leave the body.

The deuterium only leaves as water, think sweat, urine, breath vapor, but the oxygenin -18 leaves as both water and as carbon dioxide.

And co -uro is a direct waste product of aerobic metabolism, right?

Burning fuel produces co -uro.

Right.

So the heavy oxygen disappears faster because it's leaving through two routes, water and co -uro, while the heavy hydrogen only leaves as water.

Precisely.

So by taking a blood or body fluid sample later, maybe days or weeks after the injection, and measuring how much faster the uro disappeared compared to the deuterium, scientists can calculate exactly how much co -uro the animal produced on average over that period.

And from the co -uro production,

you can figure out its average metabolic rate, like its total energy burned over that whole time.

Yes, exactly.

It gives you the average daily metabolic rate, ADMR, or what's often called the field metabolic rate, FMR.

It captures everything sleeping, running, flying, digesting, staying warm, the total cost of living in the wild.

It's been revolutionary.

That really is ingenious.

Yeah.

Well, are there other ways to estimate energy use in the field?

Yeah, there's an older but still very useful approach called the time energy budget.

Here, researchers first figure out the energy cost of specific activities in the lab, like how much energy does it cost this bird to sleep, to stand, to fly at a certain speed?

Then they go out into the field and meticulously observe the animals, recording how much time they spend doing each of those categorized behaviors.

So you multiply the time spent doing something by its known energy cost and add it all up.

You got it.

Multiply time by cost for each activity, sum it all up, and you get an estimate of the total daily energy expenditure.

Like that example of the African penguins, where they broke down their day into hours spent on land, swimming, resting at sea.

Perfect example, yeah.

Table 9 .2 shows it clearly.

19 .5 hours maintenance on land, two hours swimming, 2 .5 hours resting on water, adds up to nearly 1 ,900 kilojoules a day.

It paints a really detailed picture.

And I guess technology helps now too, tiny trackers and stuff.

Oh, massively.

Miniaturize electronic monitors, telemetry devices, data loggers are transforming field studies.

They can record heart rates, acceleration, dive depths, location, all sorts of things in the wild.

And you can correlate heart rate with energy use, maybe.

Often, yeah.

You calibrate it in the lab, first measure oxygen use at different heart rates, then the logger records heart rate in the wild, and you can estimate metabolic rate or build incredibly detailed time energy budgets without having to watch the animal 2004 -7.

Amazing.

Okay, so we have ways to measure energy use.

Now let's talk about the actual cost of moving around the economics of locomotion.

Does it cost more energy to go faster?

Is it that simple?

It's not quite that simple, and it really depends on how the animal is moving.

Let's start with swimming fish.

Okay.

For most fish, as they swim faster, their metabolic rate, the energy cost per unit of time, increases, but not linearly.

It typically follows a J -shaped curve or a power function.

J -shaped.

So like, it doesn't cost much more initially, but then it really ramps up.

Exactly.

That's largely because the drag force from the water increases roughly with the square of the speed.

So doubling your speed means maybe four times the drag you have to overcome.

It gets disproportionately harder energetically to swim faster and faster.

Okay.

Water resistance is tough.

What about running on land?

Mammals?

Insects?

For running, it's generally different.

The metabolic rate usually increases as a pretty straightforward linear function of speed.

Ah, so a steadier increase.

Double the speed, roughly double the energy cost per minute.

More or less, yeah.

It's a more direct relationship compared to swimming.

Think of it like a steady burn rate that just increases with pace.

And then there's flying.

Birds, bats, insects.

You mentioned a U -shaped curve earlier.

Right.

Aerodynamic theory predicts that for flapping flight, the relationship between metabolic rate and speed should be U -shaped.

Why U -shaped?

Well, think about it.

Flying very slowly requires a lot of effort just to stay airborne, to generate enough lift.

Flying very fast requires a lot of effort to overcome air resistance or drag.

Ah, so there's a sweet spot in the middle.

A speed where it's most energetically efficient per unit of time.

Exactly.

That's the bottom of the U.

Now, actually measuring this accurately in wind tunnels is tricky.

And the data isn't perfect for all species.

But studies on birds like magpies and budgerigars do show support for this U -shaped relationship.

Interesting.

So swimming, J, running linear, flying U.

Yeah.

The way you move really dictates the energy cost curve.

Absolutely.

But there's another really important factor that applies across all these modes of What's that?

Body size.

Ah, okay.

How does size play into it?

Here's a key principle.

For any given speed,

smaller -bodied animals require higher weight -specific metabolic rates to move compared to larger animals doing the same thing.

Weight -specific.

So like energy cost per gram of body weight.

Exactly.

Think of a tiny mouse and a big deer running at the same speed, say, five kilometers per hour.

Each gram of the mouse's body is working much harder, burning more energy, than each gram of the deer's body to maintain that speed.

Same for a tiny hummingbird versus a big eagle flying.

So smaller animals are less efficient per gram when moving.

In terms of energy cost per gram at a specific speed, yes, it's a fundamental scaling relationship.

This must have huge implications for how animals live, right?

This leads into that idea of purpose driven speed, doesn't it?

It does.

We need to distinguish between two ways of looking at energy cost.

There's the metabolic rate energy cost per unit of time.

Okay, like joules per second.

Right.

And then there's the cost of transport, the energy cost to cover a unit of distance.

Like joules per meter or per kilometer.

Exactly.

And the two are related by speed.

The equation is simple.

Energy per distance equals metabolic rate divided by speed.

Okay.

So why is this distinction important?

Because the speed that minimizes your energy cost per minute might be very different from the speed that minimizes your energy cost per kilometer.

And which speed an animal chooses depends on its goal.

Ah, give me an example.

Okay.

Imagine a bird.

If its goal is just to stay airborne for the longest possible time on a limited amount of fuel,

maybe it's searching for something.

It should fly at the speed where its metabolic rate energy per time is lowest.

That's the bottom of that U -shaped curve we talked about.

Makes sense.

Minimum fuel burn per minute.

But if that same bird is migrating and its goal is to cover the greatest possible distance on its fuel reserves, it should fly at the speed that minimizes its energy cost per unit of distance, the minimum cost of transport.

And that's a different speed.

Likely faster.

Usually, yes.

It's a faster speed than the minimum power speed.

It's the speed that gets you the most kilometers per unit of energy burned.

Wow.

And do animals actually do this?

Do they switch speeds depending on the goal?

They do.

It's amazing.

Skylarks are a classic example.

When male skylarks are doing their song displays to attract females, they fly at a relatively slow speed, the one that minimizes energy cost per unit of time, allowing them to sing for longer.

But when those same skylarks undertake long migratory flights, their average speed more than doubles.

And this faster speed closely matches the predicted speed that minimizes their energy cost per kilometer traveled.

That is so cool.

They're literally optimizing their speed based on the objective time in the air versus distance covered.

Exactly.

It's a beautiful demonstration of natural selection shaping behavior for energetic efficiency.

Of course.

There's always a but.

Well, it's not always just about maximum efficiency.

Sometimes an animal might need to maximize its sustained speed, maybe to escape a predator, even if that's not the most efficient speed.

Survival trumps efficiency sometimes.

Right.

Or think about migrants that feed along the way.

Their optimal travel speed might actually be slightly faster than the minimum cost of transport speed.

Why?

Because getting to the next rich feeding ground sooner might provide more energy overall than they'd save by flying slightly slower.

It's always a cost benefit calculation.

Fascinating.

So many factors.

Yeah.

This seems like a good point to zoom out again.

You mentioned universal patterns in the cost of transport.

Yes.

This is one of the really striding findings in all of exercise physiology.

If you plot the minimum weight specific cost of transport, that's energy per unit, distance per unit body weight against body weight for lots of different animals on special log -log axes, you see an incredibly clear pattern.

The data for runners, flyers, and swimmers each fall along distinct straight lines.

It shows a predictable alimetric relationship with body weight.

Alimetric, meaning it scales with body size in a predictable way.

Exactly.

And what's really mind blowing is this.

For animals using their main way of moving fish swimming, birds flying,

mammals running the minimum cost of transport depends mostly on the mode of locomotion, swimming, flying, running, and body size.

Not who the animal is, like not whether it's a mammal or a reptile or how its muscles work.

Largely no.

It's more about the physics of moving through water, air, or on land at a certain size.

It's like there's a universal energetic price tag for each mode of transport adjusted for size.

That's incredible.

So comparing those price tags,

which mode is cheapest and which is most expensive?

Okay, for any given body size, running is by far the most energetically expensive way to cover distance.

Really?

I guess gravity and friction take a toll.

They do.

To put numbers on it, let's take a hypothetical 100 -gram animal.

Running a kilometer costs about four times more energy than flying that same kilometer.

Four times?

Okay.

And running costs about 14 times more energy than swimming that kilometer, assuming it's a fish designed for swimming.

14 times.

Wow.

So swimming is the cheapest.

Swimming, especially for fish, is the most energetically efficient way to cover distance.

And within each mode, running, flying, swimming, larger animals are always more efficient.

Meaning they have a lower weight -specific cost of transport.

Correct.

A larger animal uses less energy per gram of its body to cover a kilometer compared to a smaller animal using the same mode of transport.

Being big helps you travel efficiently.

Now, you mentioned nuances in swimming.

Fish are super efficient, but what about other swimmers?

Right.

Fish mostly fall along one line.

They're masters of efficiency.

But other things that swim underwater, shrimps, sea turtles, even marine mammals like dolphins or seals, they generally have higher costs of transport than fish of the same size.

Why is that?

Less streamlined, maybe.

Different ways of propelling themselves.

Probably a combination of factors, yeah.

And then you have surface swimmers, ducks paddling, humans doing the crawl.

Yeah.

Their cost of transport is even higher.

There are fundamental hydrodynamic reasons why swimming at the surface is less efficient.

Plus, for ducks or humans, swimming isn't usually their primary, most evolved mode of locomotion.

So when I feel exhausted after swimming laps, I shouldn't assume a fish feels the same way.

Absolutely not.

Our strenuous human swimming experience gives us zero insight into how easy it is for a fish.

But interestingly, if you want a human comparison for efficiency, riding a bicycle actually gets pretty close to the cost of transport of a hypothetical human -sized fish swimming efficiently underwater.

Bicycles are pretty efficient then.

They are.

This whole cost of transport concept also helps explain migration patterns.

There's a sort of threshold, sometimes called the migratory line on these graphs.

Basically, long -distance migration is much more likely to evolve and be ecologically viable if the cost of covering distance is relatively low.

So you see lots of migratory birds flying as efficient, and fish swimming is very efficient.

But fewer mammals undertaking truly epic migrations purely by running because it's just so costly.

That makes perfect sense.

It's all about the energy budget.

Okay, let's shift gears slightly.

We've talked about the cost of moving at different speeds.

What about the absolute maximum rate an animal can work aerobically?

The performance ceiling.

Right.

That brings us to a really important concept, VOMax.

That little dot over the V means rate.

So it's the maximum rate of oxygen consumption.

VOMax.

I've heard that term used for athletes.

You have.

It's also called aerobic capacity or maximum aerobic power.

It represents the peak rate at which an animal's body can synthesize ATP using oxygen.

It sets the limit for the intensity of exercise that can be performed in that pay -as -you -go aerobic mode.

So a higher VOMax means you can work harder aerobically, sustain higher speeds.

Exactly.

Higher sustained speeds, more intense activity.

But VOMax is also crucial because it acts as a benchmark for effort.

How strenuous an activity feels and how long you can keep it up depends on what percentage of your VOMax it demands.

Okay, give me some numbers for humans.

Sure.

If an activity requires, say, only 35 % of your VOMax, you could probably keep doing it for 8 -10 hours straight.

Think steady walking or light jogging for some.

Right.

If it demands 75 % of your VOMax, you might last 1 -2 hours before exhaustion, like running a hard race.

Okay.

And if an activity demands 100 % of your VOMax, you're talking minutes maximum, full out effort.

This really connects to everyday life, doesn't it?

Especially aging.

Absolutely.

VOMax naturally declines as we age.

For sedentary folks, it's about 9 % per decade after peaking in early adulthood.

For active people, it's less, maybe under 5 % per decade, but it still declines.

And that makes normal activities feel harder.

Yes, because any given task, like climbing stairs or carrying groceries, now represents a higher percentage of your reduced maximum capacity.

Think about mountaineers at altitude.

The thin air drastically lowers their effective VOMax.

Suddenly, just walking slowly uphill might demand 100 % of their available aerobic capacity.

It shifts from being hard to being barely possible.

Wow.

So what actually limits VOMax inside the body?

Is it the lungs?

Yeah.

The heart?

The muscles?

That's a great question.

And honestly, scientists still debate the details.

One idea is the weak link hypothesis that one specific organ system sets the limit.

For mammals, many think it's the cardiovascular system's ability to deliver oxygenated blood.

Like the heart can only pump so fast or the blood carries so much oxygen.

Kind of, yeah.

The other main idea is called symorphosis.

This suggests that through evolution, all the systems involved, lungs, heart, blood vessels, muscle, mitochondria, are sort of co -adapted so that they all approach their functional limits more or less simultaneously.

No single system is drastically over or underbuilt compared to the others in the chain.

Symorphosis.

Interesting idea.

Like a balanced design.

Exactly.

It's an ongoing area of research.

We also talk about related concepts like aerobic scope, which is just the difference between VSHOMax and the resting metabolic rate.

How much you can increase your oxygen use.

Right.

And aerobic expansibility, which is the ratio.

VSHOMax divided by resting rate.

For most vertebrates, that ratio is around 10.

They can increase their aerobic energy production about tenfold from rest to maximum exhaustion.

Okay.

A tenfold scope.

Is that consistent across all vertebrates?

Ah, no.

And this is a huge point.

There's a massive difference between groups.

Get this.

Mammals and birds have VSHOMax values that are roughly 10 times higher than those of similar size fish, amphibians, and non -avian reptiles.

10 times higher.

Oh.

So a mouse compared to a lizard of the same size.

Yeah.

The mouse's aerobic capacity, its engine is about 10 times more powerful.

What does that allow mammals and birds to do?

Sustain much higher levels of activity.

This difference strongly supports the idea that being warm -blooded, homeothermy, evolved hand in hand with the ability for sustained high -speed locomotion.

A lizard might sprint quickly using anaerobic power, but it fatigues fast.

A mammal can keep going at a high aerobic pace for much, much longer.

That's a major adaptive advantage.

Huge.

Now, even within a group like mammals, there's variation.

Generally, smaller species have a higher VSHOMax per gram of body weight.

Like we saw with cost of transport, smaller animals live life at a higher intensity per gram.

Exactly.

But then you get these incredible specialists, like the pronghorn antelope in North America.

Ah, yes, the super runner.

The ultimate sustained runner.

Its VSHOMax is more than four times higher than you'd expect for a mammal its size.

Four times.

Adaptations across the board.

Huge lungs relative to its body size.

An incredibly powerful heart and circulatory system.

Muscles packed with more mitochondria of the cellular powerhouses.

It's built for aerobic endurance speed, unlike the cheetah, which relies more on anaerobic bursts.

So the pronghorn is the endurance champion.

Where do we see the absolute highest VSHOMax per gram in the animal kingdom?

That record goes to the real power flyers.

Certain insects, hummingbirds, and bats during flight.

Makes sense.

Flight is demanding.

Extremely.

In their flight muscles, mitochondria can take up an astonishing 35 % to maybe even 45 % of the entire muscle cell volume.

Wow.

Nearly half the cell is just power generators.

Yeah.

It seems like there's almost a physical limit there, a competition for space inside the cell between the mitochondria making ATP and the actual contractile proteins that make the muscle contract.

You can't just fill the whole cell with mitochondria or there's no room left the machinery of movement.

An ultimate ceiling perhaps.

What about variation in humans?

We're not all pronghorns or hummingbirds.

True.

Humans show huge individual variation in VSHOMax.

Between any two average people, one might easily have a VSHOMax twice as high as the other.

Twice as high.

Is that genetics or training?

It's a mix of both.

There's definitely a strong genetic component, but life experience and training play a big role too.

This variation explains why the same run or bike ride feels easy for one person and incredibly hard for another.

And it probably influences which sports people excel at.

Absolutely.

If you look at elite athletes, their VSHOMax values often reflect the demands of their sport.

Elite cross -country skiers have some of the highest recorded VSHOMax values ever.

Endurance runners are also very high, weight lifters much lower.

It strongly suggests athletes tend to gravitate towards sports that match their inherent physiological capacities.

Can training significantly change your VSHOMax?

Yes, definitely.

For someone who's sedentary, starting a program of endurance training, running, cycling, swimming can increase their VSHOMax by maybe 10 % to 30 %?

That's a decent improvement.

What's changing in the body?

A whole suite of things.

Your muscles develop more mitochondria, increase the levels of aerobic enzymes, your heart gets stronger and pumps more blood per beat, and you grow more tiny blood vessels, capillaries within the muscles to improve oxygen delivery.

The whole system gets upgraded for aerobic performance.

And evolution can shape this too, right?

You mentioned selection experiments with rats.

Yes, those studies are remarkable.

Researchers selectively bred rats for high or low running endurance over many generations.

The results were striking.

They created lines of rats that differed hugely in their VSHOMax and endurance capacity, showing a strong genetic basis.

But here's a really sobering finding.

The rats bred for low endurance spontaneously developed symptoms of cardiovascular and metabolic diseases.

Things like obesity, high blood pressure, insulin resistance, kind of like metabolic syndrome in humans.

Wow.

So low aerobic capacity wasn't just about running ability.

It was linked to poor health.

Exactly.

It underscores how fundamental aerobic fitness is for overall physiological well -being, not just athletic performance.

That's a powerful connection.

Okay, so we've covered peak performance.

Let's bring it back to just

normal daily life for animals in the wild.

What do their average energy expenditures look like?

Right.

We touched on ADMR, the average daily metabolic rate measured often with doubly labeled water.

Just like with peak rates, ADMR scales with body size smaller animals expend more energy per gram per day, just living their lives.

Higher metabolic intensity again for the little guys.

Yep.

And one of the most profound insights from ADMR studies comes when you compare warm -blooded versus cold -blooded animals living in the same environment.

The homeotherms versus the poikilotherms.

Exactly.

Mammals and birds out in the wild typically have ADMRs that are 12 to 20 times higher than similar sized non -navian reptiles.

12 to 20 times.

That's a huge difference.

So that's the real cost of being warm -blooded day in, day out.

That's the energetic price tag.

Yeah.

Maintaining that high stable body temperature is incredibly expensive.

So for a typical wild mammal or bird, how high is their daily energy expenditure compared to resting?

Generally, their ADMR is about 2 .5 to 3 .5 times their basal metabolic rate, BMR, which is their resting rate in a comfortable temperature zone.

Two and a half to three and a half times resting.

What about humans?

For most people in developed countries, our ADMR is lower, maybe 1 .2 to 2 .5 times our BMR.

We're generally less active overall than wild animals.

Okay.

But what's the maximum rate that an animal can sustain, not just for minutes like Vishomax, but day after day for weeks or months, like peak sustainable energy expenditure?

That's a different question and a really interesting one.

We're talking about the highest average daily rate sustainable over long periods.

To get a handle on this, researchers looked at humans pushed to extremes, like Tour de France cyclists.

Right.

Those guys work incredibly hard for weeks.

They do.

Over the 22 days of the race, they averaged metabolic rates around 4 .5 times their ADMR.

That's incredibly high for a human sustained day after day.

Four and a half times resting.

What about other animals under high demand, like feeding young or living in extreme cold?

For other mammals and birds under maximum sustained load, like a mother nursing a large litter, or birds working hard to stay warm and feed young in harsh conditions, the peak sustainable ADMR seems to plateau around six to seven times their resting metabolic rate.

Six to seven times resting.

Okay.

Any exceptions?

Migrating birds.

Some species undertaking really long demanding flights can sustain average metabolic rates that are maybe eight to 10 times their resting rate during the migratory period.

They are the true masters of sustained high energy output.

Wow.

So there seems to be this general ceiling around six, seven times resting for most animals, maybe a bit higher for elite migrants.

Why?

What stops them from sustaining even higher rates, say 12 or 15 times resting?

That is a fantastic and still unanswered question in physiology.

What are the ultimate constraints?

Is it the rate at which they can digest and absorb food energy?

Is it limits in transporting fuels to muscles?

Is it waste heat dissipation or some internal metabolic processing limit?

We don't fully know what sets that six, seven X ceiling for most animals under maximum natural conditions.

It's a major puzzle.

A frontier of research.

Okay.

Let's talk more about those ultimate endurance athletes, the long distance migrants.

These journeys are just mind boggling.

They really are.

Take the Osprey.

Satellite tracking has followed individual birds flying 7 ,000 kilometers.

That's over 4 ,000 miles between their breeding grounds in Sweden and wintering grounds in Ivory Coast.

And they do this year after year, usually with some stopovers to rest and refuel along the way.

An incredible commute.

But then there are the nonstop flyers.

Ah, yes.

The bar -tailed Godwits.

These shorebirds perform what seems almost physically impossible.

They fly nonstop from Alaska over the Pacific Ocean to New Zealand.

Nonstop.

How far is that?

About 10 ,000 kilometers, sometimes more.

Around 6 ,000 to 7 ,000 miles.

And they do it in six to nine days of continuous flight.

Six to nine days of nonstop flying.

That's insane.

How is that even possible?

Think of the challenges.

Exactly.

Complete starvation they can't eat while flying over the ocean.

No access to fresh water.

Presumably extreme sleep deprivation, although how they manage that is still debated.

Continuous, intense muscle work for days on end.

And on top of all that,

precise navigation over thousands of kilometers of open water, it truly pushes the limits of vertebrate endurance.

So fuel is critical.

What are they burning for that entire time?

Fat.

Almost exclusively fat.

Birds are masters of fat metabolism, much more so than mammals typically are.

Fat has the highest energy density of any biological fuel.

Way more than carbohydrates or protein.

So they pack on a lot of fat before they leave.

Huge amounts.

Some migratory birds nearly double their body weight, mostly by accumulating fat reserves.

And they have incredibly efficient physiological machinery to rapidly mobilize that fat from storage tissues, transport it in the blood as special lipoprotein complexes, because fat doesn't dissolve well in blood, and get it into the muscle cells to be burned.

They're optimized for fat burning during intense, long exercise in a way mammals aren't.

Pretty much, yeah.

Mammals rely more on carbs for a very high intensity, but birds seem specialized for high intensity, long duration fat oxidation, and some take it even further.

How so?

Adaptive organ shrinkage.

Birds like red -knot sandpipers, after they've stored up all that fat, their digestive organs, intestines, gizzard, liver, can actually shrink significantly right before or during migration.

They shrink their own organs.

Why?

To save weight.

Once they've loaded up on fuel, they don't need a large digestive system during the flight itself.

Reducing the weight of these non -essential during -flight organs lightens the load, making flight more efficient.

It's like jettisoning unnecessary cargo.

Then they regrow them quickly when they land and need to feed again.

That is an incredible adaptation.

Wow.

What about other migrants?

Navigation is key, too.

Yes.

Navigation is crucial.

We know sea turtles, for example, use the earth's magnetic field to navigate across vast oceans.

Birds use a combination of cues, sun compass, star compass, magnetic field, maybe even smell.

And then there's the eel.

Ah, the European eel mystery.

You mentioned that Box 9 .3.

They migrate thousands of kilometers to the Sargasso Sea to breed.

That's a prevailing theory, yes.

A journey of maybe 5 ,500 kilometers from Europe, and for centuries since Aristotle basically nobody knew where they went to reproduce.

Even now, finding adults or eggs in the Sargasso Sea is incredibly rare.

The big question was always, could they physically make that journey without feeding and still have enough energy left to reproduce?

Sounds almost as impossible as the Godwits.

How do they test it?

Ingeniously.

Scientists simulated the 5 ,500 kilometer journey in a specialized water tunnel, swimming eels continuously for almost six months, the estimated duration of the migration.

Wow.

What did they find?

Something truly remarkable.

The eels swam at a surprisingly low metabolic rate, only about twice their resting rate.

And their cost of transport was incredibly low, about one -fifth of what you'd predict for a fish of their size based on those general scaling laws we discussed.

One -fifth.

So they are hyper -efficient swimmers.

Extraordinarily efficient.

This ultra -low cost of transport meant they could complete the simulated 5 ,500 kilometer journey, losing only about 20 % of their initial body weight, mostly fat.

They arrived, energetically speaking, with plenty of reserves left for reproduction.

So the mystery isn't if they can do it, but perhaps exactly how they achieve such incredible efficiency.

Precisely.

It shows how evolution can sculpt an organism for extreme specialized performance, sometimes defying our general rules about energy costs.

It highlights that these limits are often way beyond what we might initially assume.

Absolutely mind -blowing.

Okay, so let's try to tie all these amazing physiological details back to the bigger picture.

Ecological energetics.

Right.

This field studies energy needs, how animals get energy, and how they use it, all within the context of their actual environment and ecological interactions.

Like those seabird colonies we started with.

Perfect example.

By quantifying the total daily energy demand of a whole colony, accounting for the adults' foraging flights, their resting metabolism, the energy needed to produce eggs, and especially the huge energy cost of feeding and growing chicks, scientists can estimate the colony's total impact on its food resources.

And they found colonies can consume a really significant chunk of the local fish.

Yeah.

Studies suggest large seabird colonies might a quarter, maybe even a third of the total annual production of their main prey fish species within their foraging range.

Wow.

So understanding their energy needs helps explain population dynamics, competition, even interactions with human fisheries.

Exactly.

It links physiology directly to ecology.

Another classic example is the work done by Bern Heinrich on bumblebee foraging, what he called bumblebee economics.

Bumblebee economics.

I love it.

It sounds small scale, but I bet the principles are the same.

They are.

It's all about costs versus rewards.

For a bumblebee foraging for nectar, the energy costs are substantial.

Like flight.

Flight is hugely expensive, yeah.

It can boost their metabolic rate 20 to 100 times above resting.

But that's not all.

Temperature regulation is a major cost, especially in cool weather.

Temperature regulation.

For a bee.

Oh yes.

To fly, their big flight muscles on the thorax need to be warm around 30 degrees Celsius or even higher.

If the air is cool, say 10 or 5 degrees Celsius, the bee loses heat rapidly.

So what does it do?

It has to shiver.

It contracts its flight muscles isometrically without moving its wings just to generate heat.

When foraging in cool weather, a bee might land on a flower, and while it's sucking nectar, it might have to shiver intensely just to keep its muscles warm enough to take off again for the next flower.

So just staying warm enough to fly can be as costly as flying itself.

At low temperatures, like 5 degrees C, the cost of shivering thermoregulation can indeed be as high as the cost of flight itself.

So the total energy expenditure for foraging goes up dramatically as it gets colder.

Okay.

Those are the costs.

What about the rewards?

Nectar?

Exactly.

The rewards are the energy gained from the nectar, primarily sugars.

The key factors are the volume of nectar per flower, the sugar concentration, and how many flowers the bee can visit per minute.

So the bee has to make choices, right?

Which flowers are worth visiting?

Precisely.

It's an economic decision based on profitability.

Some flowers are energy bonanzas.

Rhododendrons, for example, offer a high volume of sugar -rich nectar.

Bees can tap maybe 20 Rhododendron flowers a minute.

Even at near freezing temperatures, 0 degrees C, the energy gained from Rhododendrons easily exceeds the high costs of flight and thermoregulation.

It's always profitable.

Okay.

Rhododendrons are good business.

What about less generous flowers?

Take wild cherry.

Its flowers offer very little sugar.

Heinrich calculated that at 0 degrees C, a bee would need to visit about 60 wild cherry flowers per minute just to break even energetically.

60 per minute?

Is that even possible?

No, it's physically impossible for the bee to move that fast.

And so what do you observe in nature?

Bees only forage on wild cherry flowers when the weather is warm, reducing their thermoregulatory costs enough to make the low nectar yield barely profitable.

Wow.

So the bee's decision, do I visit this flower patch or not,

is directly constrained by the energy balance sheet, especially temperature.

Absolutely.

It's a beautiful, simple illustration of how fundamental energetic considerations shape animal behavior, foraging choices, and ultimately their ecological niche.

Every action, every decision has an energetic consequence.

It really changes how you look at things.

As we wrap up this deep dive, it's just staggering to think about the diversity of strategies animals use to manage this constant energy balancing act.

Isn't it?

From that tiny bumblebee making a micro decision about a flower, weighing costs and benefits moment by moment.

To a godwit executing a seemingly impossible nonstop flight across an entire ocean, relying on massive fuel stores and incredible efficiency.

Their lives are absolutely governed by these fundamental principles of energy acquisition, storage, and expenditure.

Understanding these physiological limits, these adaptive choices, it really feels like it reveals some of the core design principles of life itself, doesn't it?

So maybe the question for you, our listener, is what aspect of these hidden energy demands, now that you've heard all this, will you look at differently next time you see an animal moving?

It's been great diving into this.

Thank you so much for joining us on this deep dive into the energetics of animal activity.

We're really glad you're part of the Last Minute Lecture family.