Chapter 30: Water, Salts, and Excretion: Mammals of Deserts and Dry Savannas

Welcome to Last Minute Lecture.

This free chapter overview is designed to help students review and understand key concepts.

These summaries supplement not replaced the original textbook and may not be redistributed or resold.

For complete coverage, always consult the official text.

Welcome back to The Deep Dive.

Today, we're embarking on a journey, really, to uncover the incredible world of how large mammals manage to, well, not just survive, but actually thrive in some pretty tough places.

Yeah, we're talking Earth's harshest environments,

the scorching hot deserts and those vast dry savannas.

Exactly.

And our Deep Dive today, it's built on some solid insights from a selected chapter of animal physiology, the fourth edition by Hill, Wise, and Anderson.

So we'll be pulling out the core physiological concepts,

the ingenious mechanisms, and the adaptive strategies these animals use.

And hopefully bringing it all to life with some really cool real world examples.

Definitely.

That's right.

Our mission, really, is to examine the incredible ways these big animals kind of orchestrate their water losses and gains, how they keep everything in balance.

It's a delicate balance, for sure.

Totally.

We'll look at their different strategies, compare them, and maybe touch on some of the clever experiments physiologists use to figure all this stuff out.

We're focusing mostly on the sizable mammals, you know, five kilograms or bigger.

And especially African species, just because there's such amazing diversity there.

And, frankly, a lot of the research has focused on them.

So before we meet the animals themselves, let's set the scene a bit.

What makes these deserts and savannas so punishing?

Well, deserts, fundamentally, it's all about water.

Water availability is the main thing controlling, well, everything biological.

Right.

Rains, infrequent, unpredictable,

often less than, say, 25 centimeters a year, though the definition's a bit more nuanced than just a number.

So one spot might get drenched while another stays bone dry.

Exactly.

One year could have five times the rain of the next.

So life there has to be incredibly opportunistic.

You've got to grab the chances when they come.

And dry savannas, they're often next door, right?

Yeah, often neighboring deserts.

They're characterized by really distinct rainy seasons, but then, crucially, long, rainless periods.

Like the Serengeti.

Perfect example.

The Serengeti plains in East Africa.

Every year, they get hit with the long dry season, that's four to six months, potentially, with basically zero rain.

Wow, that's profound drought.

It is.

Streams dry up, the grass just turns brittle and brown.

It's tough.

And what actually causes these places?

Is it just bad luck?

Hey, not exactly.

A big factor is global air patterns.

You get warm, moist air rising at the equator, it cools, drops its rain, then descends around 30 degrees north and south latitude.

And that descending air is dry.

Very dry.

That's how you get massive deserts like the Sahara.

Okay.

Then you also have what's called rain shadowing, more localized.

Like mountains blocking the rain.

Exactly.

Think of the Mojave Desert near LA.

The coastal mountains wring out the moisture so the land further east gets very little.

Gotcha.

And what's really fascinating, I think, is how dynamic these places are over long timescales, geologically speaking.

They shift.

They're not static.

Not at all.

The Sahara, the modern one anyway, it's actually less than 6 ,000 years old, though desert conditions have come and gone in North Africa for millions of years.

It's even expanding now, apparently.

So they're always somewhere, just maybe not always in the same place.

Pretty much.

Which presents both these immense challenges, but also unique opportunities for adaptation for the animals living there.

Which brings up a really key question.

Does size matter out there in these hot, dry conditions?

Ah, the big question.

Yeah.

If you compare a large mammal, like say an oryx, with the small guys we've talked about before, kangaroo rats, gerbils.

Right, the little burrowers.

Exactly.

Yeah.

The big guys seem to have immediate disadvantages, behaviorally speaking.

They can't just dig a hole to escape the heat.

Nope.

No easy burrows.

And they can't easily find those tiny cool spots, those microhabitats.

They're often stuck facing the heat head on.

But that's where their physiology gives them an edge.

A big edge, actually.

How so?

It comes down to surface area versus volume.

Larger animals have less body surface area relative to their weight.

That lower surface to volume ratio, it means less external heat getting in per pound, basically.

Oh, okay.

So they absorb less environmental heat relative to their mass.

Precisely.

Which means they need less water for cooling.

Plus, larger mammals tend to have lower metabolic rates per unit of weight.

So they produce less internal heat too.

Exactly.

Less heat coming in from the sun, less heat being generated inside.

Both are huge advantages when it's hot and water scarce.

Puts them in a much better position physiologically.

Is there like actual data on this?

Numbers?

Yeah.

Classic stuff.

Back in the 1930s, D .B.

Dill did these famous walks in the desert.

Right.

I've heard of those.

He measured dehydration rates.

Himself, dogs, burrows, trekking under the desert sun.

And what did he find for, say, smaller animals hypothetically out there?

Well, if you plot it out evaporative water loss as a percentage of body weight per hour, the predictions for small animals are just brutal.

Like how brutal?

Like a hypothetical 100 gram rat out in the full sun actively moving, it lose almost 13 % of its body weight in water per hour.

13%.

That's fatal shortly.

Oh, absolutely.

It would likely die within that hour.

A tiny 25 gram mouse, maybe half an hour.

It just makes it crystal clear.

That a fully exposed active life during the desert day is impossible for those little guys.

Impossible.

Physiologically impossible.

Which is exactly why they have to rely on behavior.

Burrowing deep underground.

That's their only real option.

Whereas the large mammals.

They are physiologically much, much better equipped to face the heat directly.

Their size actually makes their exposed existence possible.

Okay.

So they can handle the heat better physiologically, but how do they handle the water situation?

Do they all do it the same way?

Not at all.

There's a really useful way to think about them.

A classification scientists use.

Drinking water independent versus drinking water dependent.

Yeah.

Break that down.

Dependent first.

Drinking water dependent species or DWDs, field beasts, zebras, the classic migration animals.

They need to drink pretty regularly, daily or maybe every other day.

Which must restrict where they can go.

Hugely.

It basically ties them to within about 25 kilometers, maybe 15 miles or so of standing water and you often see them actively seeking shade on hot days.

Makes sense, right?

They need to conserve.

Yeah, definitely.

So the independents.

Drinking water independent species, the DWIs.

Animals like Grants gazelle, the big common eland, even the tiny dik dik.

These guys can go for days, even weeks without drinking water.

Weeks.

Yeah.

They can remain healthy for extended periods.

And this ability obviously lets them use food resources much further away from water holes.

So they're less tied down.

Much less.

Grants gazelles, for instance, are often seen just out in the open, seemingly indifferent to shade.

Their physiology backs it up.

And the Serengeti migration really highlights this difference, doesn't it?

It's the most dramatic illustration, probably.

Those massive herds of wildebeest and zebras, the DWDs, making that huge trek.

Following the water.

Essentially, yes.

They move between the wetter northwest and the drier southeast based on where the water is seasonally available.

They hit the southeast when the rains come for the good grass.

But they have to leave when it dries up.

They have to.

They move back northwest to find the permanent water they need.

Makes sense why they leave the dry areas.

But why do they go there in the first place when the rains start?

Is that obvious?

You know, it's actually still debated.

The reason for leaving the southeast lack of water is clear.

But the initial trigger to move to the southeast at the start of the rains.

Still some hypotheses floating around.

Better grazing, maybe?

Avoiding predators?

It's complex.

Interesting.

And the DWIs.

The gazelles and elands.

They mostly stay put, relatively speaking.

They might shift their ranges a bit with the seasons, but no massive migration like the wildebeest.

And what about predators like lions?

Do they follow the water?

That's another fascinating wrinkle.

Lions are generally thought to get most of their water from their prey.

But some prides do follow the migration, while others seem perfectly content staying put and adapting locally.

It's not a simple picture.

Okay, so let's dig into how they actually get water.

You mentioned prey for lions, but the herbivores.

Right.

For the large herbivores, it's crucial to understand they all rely much more on preformed water than on metabolic water.

Preformed water being?

Water they get directly from drinking or from the moisture in their food.

Unlike, say, kangaroo rats, which get a lot from metabolizing seeds.

Exactly.

Kangaroo rats are masters of metabolic water production.

Large herbivores aren't nearly as reliant on it.

They need that preformed water.

So the DWD species, the wildebeest and zebra,

they just can't get enough preformed water from food alone.

Correct.

They absolutely must drink standing water regularly.

Whereas the DWIs, the gazelles and elands, can get enough from food, at least for a while.

Yes, for extended periods.

Though most will still drink occasionally if water happens to be available.

It's easier, right?

Sure.

But then you have the true specialists like the oryx and the eland.

It's believed they might never need to drink free standing water in the wild.

Never.

Just from food.

Relying entirely on the moisture locked up in the plants they eat.

That's pretty incredible.

But finding water, even for those that drink, isn't always straightforward, is it?

You mentioned salty water.

Yeah, that's a major complexity.

Yeah.

Standing water in deserts and savannas can be surprisingly salty.

Like in the Serengeti, measurements show salinity from 5 up to 30 grams per kilogram.

Which is getting close to seawater levels.

It is.

Seawater is around 35.

It happens because arid soils are often naturally salty, and the infrequent rains don't really flush the salts away.

They just dissolve into any water that pools up.

So drinking that could actually dehydrate an animal if they aren't adapted.

Precisely.

To actually gain net water from salty sources,

the animal's kidneys have to be exceptionally good at concentrating urine.

They need to pump out the excess salt in a smaller volume of water than they took in.

Wow.

Okay.

Another challenge.

What else?

Another fascinating complexity is how much the water content of plants can swing, even daily, especially in the dry season.

How does that work?

Takes seemingly dry grass, like Stipagrostis in the Namib Desert, looks parched.

But in the cool, humid pre -dawn hours, it actually absorbs moisture from the air.

Its water content can go way up.

So animals can exploit that.

Absolutely.

By preferentially grazing right before dawn, they can get significantly more water than they ate the same grass midday.

Clever timing.

They is clever.

And a third thing is how plant nutrition ties into water.

It's not just about growth cycles being irregular because of rain.

The actual quality of the plants changes dramatically.

Protein content, for example.

In some key grasses in the Kalahari, protein levels can swing threefold or more between the wet and dry seasons.

So during long droughts, the animals might not just be thirsty, but actually malnourished.

Exactly.

They can face serious nutritional deficiencies.

Lack of protein, even essential minerals like phosphorus or copper can become critical problems.

And one more source, you mentioned fog.

Right.

Especially in coastal deserts, like those near cold ocean currents.

Fog can actually deposit a significant amount of water, sometimes even more than rain.

Can big animals drink fog?

Not directly, like some insects can.

You see those desert beetles that tilt up to collect droplets.

Large mammals don't really do that.

For them, the benefit is indirect.

How so?

Fog boosts the water content of the plants they eat.

And crucially, it helps maintain the nutritional quality of those plants, keeping protein levels higher than they would be otherwise, even without rain.

So it helps on both fronts, water and nutrition.

Okay, this paints a picture of immense challenges.

Let's talk about the real champions, the animals that have mastered this.

You mentioned wildebeest are surprisingly successful despite needing water.

Yeah, it's a bit of a paradox.

They're DWD, totally dependent on drinking, yet in the Serendeti, they outnumber all other large mammals combined.

So their adaptation isn't just about conserving water.

It seems not primarily.

They must have evolved incredibly effective ways to find water.

Maybe advanced navigation, amazing sensory abilities to locate distant rain or water holes.

Their success is likely tied to water acquisition, not just conservation.

Interesting.

But some species do focus heavily on conservation, right?

They face that conflict between staying cool and saving water head on.

Absolutely.

Because they're large, they often can't hide from the heat.

So they've had to evolve these really sophisticated physiological ways to cope.

Like the oryx.

You mentioned them as maybe the ultimate desert survivors.

Yeah, oryxes are really at the pinnacle among wild large mammals.

Think of the Arabian oryx living in truly hyperarid deserts.

Their water conservation is just exceptional.

How good are we talking?

Their water turnover rates, how much water flows through their bodies, are maybe only a quarter to a half of what you'd expect for a mammal their size.

And when they get used to having less water available, they can actually lower their metabolic rate and reduce water loss through evaporation.

And physiologically.

Kidneys.

Kidneys are powerhouses.

They can concentrate urine to about eight times the concentration of their blood plasma.

That UP ratio of eight is remarkable for an animal that size.

Wow.

But maybe the most famous adaptation is their body temperature.

Oryxese allow their body temperature to fluctuate dramatically over the day, especially in summer.

How much fluctuation?

An average daily swing of four to five degrees Celsius is common for the Arabian oryx.

Sometimes it can be almost eight degrees.

And how does letting yourself get hot save water?

Two main ways.

First, by letting their temperature rise during the hot day, they store heat instead of immediately spending water to evaporate it away.

Then at night, when it's cool, they can radiate that stored heat away passively without losing water.

Okay.

Passive cooling.

Right.

And second, if your body is hotter, closer to the temperature of the environment, the temperature gradient is smaller.

Less heat flows into your body from the outside, which again reduces the need for evaporative cooling.

That makes sense.

And where do they get their water?

You said maybe never drinking.

Mostly from preformed water in food.

Even acacia leaves that look dry can be 50 -60 % water.

And they do that dawn grazing trick on dead grasses too.

Yeah.

Diving.

Yeah.

This is amazing.

They have an almost uncanny ability to find and dig up underground plant storage organs, tubers, bulbs, stuff like that.

These can be 50 -70 % water and they can dig down a meter or more to get them.

Incredible.

So they can survive years without rain.

They can.

But here's a crucial point.

Field studies looking at oryxes that die during droughts often find the ultimate cause wasn't dehydration.

It was usually starvation, protein deficiency, or lack of other nutrients.

Ah.

So even for the water conservation champions, running out of food is the final limit.

Often, yes.

It really highlights that survival out there is an integrated problem.

It's not just about water.

It's about water and energy and nutrients all balanced against the heat.

Okay.

What about gazelles?

You mentioned Grants and Thompsons look similar but behave differently.

Right.

It's a neat little paradox.

In the lab, under controlled conditions, their water conservation abilities look pretty similar.

Thompsons might even seem slightly better on paper.

But in the wild?

In the wild, Thompsons gazelle tends to migrate with the wild beast and zebras sticking to the moisture savanna areas.

Grants gazelle is the one you find further out in the drier zones, seemingly more independent.

So why the difference if their basic conservation is similar?

It seems to come back to body temperature again.

Grants gazelle lets its body temperature climb much higher, sometimes up to an incredible 46 .5 degrees Celsius.

That's among the highest ever recorded for a vertebrate.

And like the oryx, that saves water.

Exactly.

By tolerating that higher temperature, it reduces the need for evaporative cooling panting, mostly.

Thompsons gazelle, in contrast, seems to regulate its temperature more tightly, keeping it lower.

Which means it has to pant more and use more water.

Right.

So in the real world, Grants' strategy seems to give it the edge in drier conditions.

And there's another gazelle, the sand gazelle that's even more extreme.

Yeah, a fantastic example of diversification within the same genus gazella.

This one lives in truly hyperarid deserts facing even tougher conditions.

It's about 20 kilos, so not tiny.

In its adaptations?

Recent studies show its evaporative water loss is incredibly low, only about 20 % of what you'd expect for an ungulate its size.

20%.

How?

A combination of things.

Body temperature cycling like the oryx, about 2 .6 degrees C swaying in summer.

And really significantly, it can reduce its metabolic rate by up to 45 % when it's acclimated to water restriction.

Just slows everything down to conserve resources.

Amazing.

Okay, we have to talk about the icon.

Yeah.

The dromedary camel.

The ship of the desert.

Ah, yes, the camel.

Lots of myths surrounding them.

Let's bust them.

They don't store water in the hump, right?

Absolutely not.

The hump is fat, pure energy storage.

And metabolizing that fat actually uses water because you need oxygen to burn fat, and breathing brings water loss.

And they don't store it in their stomachs either?

Nope.

Not in the rumen like some early theories suggested.

They just drink to replace water they've already lost.

They're incredibly good at rehydrating quickly, but they don't store water ahead of time.

So what are the real keys to their success?

Two main things.

Truly extraordinary water conservation, even better than the Oryx in some ways, and an almost unbelievable tolerance for dehydration.

Okay, let's hear the conservation strategies.

Well, they do the body temperature fluctuation thing too, big time.

Up to a 6 degree C swing daily.

They have special mechanisms to keep the brain cooler than the core body, which is vital.

Brain cooling, right.

Extremely concentrated urine, very dry feces.

They wring out every possible drop of moisture before excreting waste.

They can also drastically cut down urine production when dehydrated.

Their thick fur isn't just for insulation against cold, it's a fantastic heat shield against the sun.

And behaviorally, they're smart.

They'll face the sun to minimize the body surface exposed, or even huddle together weirdly to shade each other.

Huddling in the heat seems counterintuitive.

It does, but it reduces the overall heat gain for the group compared to standing individually exposed.

Okay, and the dehydration tolerance.

You said it was profound.

It really is.

Most mammals are in serious trouble if they lose 10 -15 % of their body weight as water.

Camels.

They can lose 30%, even up to 40%, and still be alert and functional.

40%.

It's double or triple what others can handle.

Easily.

It's truly exceptional.

It allows them to endure long periods between water sources.

So summing up the camel,

conserve water incredibly well, tolerate massive dehydration, and get preformed water from eating tough desert plants.

That's the essence of it.

They combine top tier conservation with unmatched tolerance.

These adipations are just incredible.

It shows how perfectly tuned these animals can become to their specific environments.

What happens when humans throw a wrench in the works,

particularly concerning water?

Yeah, this is where it gets really challenging.

Water conflicts are almost inevitable when you have ecosystems where water is the absolute limiting factor and then human demands increase.

Like conflicts between ecosystems and human needs.

Exactly.

We see it all over.

In the US, the Colorado River is a prime example.

So much water is diverted for cities, for agriculture, that it often just stops, dries up before it even reaches the sea anymore.

And in Africa, you mentioned the Mara River.

Right.

The Mara is crucial for the Serengeti migration, especially during the dry seasons, but it's facing threats from deforestation in its catchment area and proposals for dams, irrigation projects.

Which could disrupt that whole migration cycle.

Potentially, yes.

It could have huge consequences for the entire ecosystem if that dry season water source is compromised.

And there's also conflict with traditional human ways of life, right?

Like nomadic herders.

Yes.

That's another really complex layer.

Think of cultures like the Maasai in East Africa.

Their traditional nomadic lifestyle was perfectly adapted to following dispersed water and grazing resources.

But modern pressures change that.

Absolutely.

The spread of private land ownership, borders, designated parks, it directly clashes with nomadism.

People are settled, land is fenced.

It fundamentally breaks that traditional relationship with the land and, critically, with water access.

It's a tough situation.

It really is.

And it leads to difficult questions.

Should nomadic ways of life be protected?

Should land be set aside?

How do you manage the fact that their livestock often compete directly with wild animals for the same limited water sources, especially inside protected areas?

No easy answers there at all.

None.

It's a major conservation and social challenge.

So, reflecting on everything we've discussed,

the harsh environments, the incredible adaptations, the human pressures, what does it all mean looking forward?

Well, we've certainly covered some amazing ground today.

From understanding the basic advantages of being big in a hot place to the different strategies for getting water or not needing it, and those incredible physiological tricks like goddy temperature changes and super -efficient kidneys.

Yeah, what really stands out is the sheer ingenuity of life, isn't it?

Faced with the fundamental challenges of heat and lack of water, evolution has come up with this incredible diversity of solutions, from massive migrations to subtle metabolic adjustments.

It really shows there's more than one way to solve the problem.

Definitely.

Different species, different toolkits, all aimed at surviving in these arid and semi -arid lands.

So, maybe a final thought for you, our listener.

As you think about ongoing global climate change, potentially making these environments even hotter and drier, how might that delicate balance be further challenged?

The balance between water availability and these animals' amazing adaptations?

Right.

What new physiological or behavioral strategies might become even more critical?

Or could some species reach their absolute limits?

It's something critical to consider for their long -term survival.

A sobering but important thought.

Thank you, as always, for being part of the Deep Dive family.