...about 1,000 kilometres west of South America on the equator.

Martin Wikelski is heading or his research site.

It's an island called Santa Fe, part of the Galápagos Archipelago.

Santa Fe, like all the Galápagos islands,

is the tip of a volcano that became land

only a few million years ago.

Many of the animals and plants that now live there

are found nowhere else on Earth.

These island species have long fascinated biologists

interested in evolution.

But this is also a good place for animal physiologists to study.

Like all animals found in isolated, oceanic island groups,

the species found in Galápagos are astonishingly unafraid of people...

...because of the absence of predators.

And even on an inhabited island, on a hotel patio,

marine iguanas, a Galápagos species, lounge in the shade of the chairs.

With few natural predators, they don't see people as a threat.

They're easy to observe and study.

And a source of fascination.

Martin's work is on the marine iguana.

He's going to Santa Fe because it's home

to more than 10% of the world's population of these animals.

And here they're undisturbed by humans.

If you study animals here,

you're as close to understanding them as you can get.

This is the nitty-gritty of research.

What we can read in textbooks is knowledge,

one from situations like this.

There's actually an easier way onto the island.

There's a beach but it's four kilometres away

and that would mean carrying all the equipment to the research site.

So the researchers prefer the quick route.

Most of the islands in the Galápagos Archipelago are uninhabited.

In fact, you need permission to land on them.

Everywhere the human species goes, it affects the environment.

This is one of the few remaining places on the planet

where human influence and access is being tightly controlled.

It took Martin months to secure permission to work here.

More people have been up Mount Everest

than been allowed to come to this place.

This will be the group's laboratory, eating and sleeping quarters.

Everything needed for the three months' stay on the island

has to be brought with them.

It's a condition of their permission

that everything is taken away at the end.

So now they're here, what are they going to do?

First, catch your animal.

The iguanas don't fear him but are slightly wary,

staying beyond arm's length.

Again, this remarkable lack of fear of humans.

It doesn't like being handled but it is astonishingly passive.

The basic data first.

Body temperature, obtained by putting a thermometer

into the cloaca pouch,

which all reptile species, males and females, have.

It's 30.0.

For a cold-blooded animal, the temperature may seem high.

...

But it has spent most of the day soaking up the sun.

Next, length and weight.

These are basic pieces of information

but are crucial in understanding how this creature lives.

That's 2kg and 100g.

And for the next three months, this animal is one of the sample

that are going to be intensively scrutinised.

So it needs to be picked out from the crowd on the rocks.

Each animal is given a distinctive mark and a number.

The range of species found here, and here alone,

makes these islands an endless source of fascination for biologists.

Santa Cruz, the main inhabited in the archipelago,

is home to most of the scientific work.

At the Charles Darwin Research Station,

a study is underway, which relies as much on the geographical position

of the Galápagos, on the equator, as the species itself.

The giant tortoise.

Beatrix Schramm collects environmental information.

First thing every morning, she measures temperature, humidity

and takes a reading of light intensity.

She's interested in the mating behaviour of the tortoises

and is trying to find out what triggers it.

As the Galápagos are on the equator,

the length of day doesn't change all year round.

So what is triggering the mating behaviour?

Beatrix is looking for measurable signs of sexual activity.

For the physiologist, the starting point is sex hormones,

which can be found in the urine.

Collecting urine from a 70kg tortoise is a bit of a problem.

So she gets them from the next best thing.

Fresh faeces.

This faecal samples... I'm not sure from where.

This other one, there are two, I am not sure from whom

because, as you see, there are tortoises just running away now.

Because they are so fast, it could be from another one.

I don't know who else stayed here.

So this will be a little bit older. It's not so humid any more.

This one, perhaps, a little older. Like one hour or something like that.

You can check this. If there is lots of humidity, they just did it.

That's why I have to run.

If I see a tortoise and she's sitting somewhere

and she sees me, she runs.

I have to run, too, to look from which direction she was coming

and when I see she's running away from this faecal sample,

which I can see now -

she's half a metre away, I know this is from her.

But the best way is always that you can see the animals sitting

exactly on top of the faecal sample.

You can see where the tail is,

there is some faeces and you can collect it.

That's the best.

The animals produce the faeces first thing in the morning

and she needs it fresh.

They know she's around so it's a bit of a game of hide and seek.

Even when the animals are secure, in a rock-walled compound,

finding them can be tricky.

Tortoises can be extremely quick and are very shy.

To use a sample of faeces in her research,

she has to be absolutely sure

that it has come from the tortoise she has spotted.

There is one.

And the sample really must be fresh.

Any delay and the hormones in the faeces

are degraded very quickly in the heat.

I know exactly where she's sitting.

So if she runs away when we're coming,

I know that this is the faeces of her.

And now.... A very fresh one.

I have to collect that.

Success.

The next job is to stabilise it before any changes take place.

In a nearby lab, she teases out a 4g sample of the material

and suspends it in alcohol.

The sample is now ready for analysis.

But although the station has sophisticated facilities,

compared to Martin Wikelski's iguana research setup on Santa Fe,

we're still over 1,000 kilometres away from the nearest equipment

capable of analysing for the testosterone and oestrogen

at the extremely low concentrations found in faecal samples.

So the samples are chilled. and stored for later analysis.

The telltale signs of the volcanic origins of Santa Fe

are littered all around.

For most of the year, the island is parched, desert-like.

Three months can seem a long time in this sort of environment.

It's the sort of research you need to pace yourself for.

The marine iguana is something of a curiosity.

It's the only iguana in the world that lives by and in the sea

and it's only found on the Galápagos Archipelago.

Iguanas probably arrived on these remote islands

on rafts of vegetation and adapted to the local conditions.

Some, like the land iguana on Santa Fe,

adapted to eating the cactus pads.

The marine iguana has adapted to eating the seaweed

on the rocks close to the shore.

But foraging for this food has caused them a number of problems.

As they are cold-blooded and the sea is cold,

they must warm up in the sun before going down to the sea

to graze on the algae.

The chilling effect of the sea means that they must rest up for the day,

warming up again.

Passively digesting their food...

...with the males indulging in occasional territorial skirmishing

or having ticks picked off them by a ground finch.

Until the next foraging excursion.

The chilling effect of the sea is apparent

in the effort they have to make to get back onto the land.

After a long excursion,

holding onto the rough lava blocks and climbing to safety

is visibly draining.

But how draining?

What is the measurable impact on the animal?

With a simple strain gauge, the animal is pulled off the rock.

Correlate this against its weight and body temperature,

at the time of the experiment,

and you have an index of stamina.

That's five kilos exactly.

But there's one problem with this experiment.

If all the iguanas he can catch are hot from basking in the sun,

how can he measure one as if it's just come out of the sea?

Improvise.

A nearby tidal pool, used as a playground by sea lions.

It's a convenient place to bring an iguana down to low temperature.

But you have to shift the residents for a while.

Another index is speed.

Time trials for iguanas.

They take significantly longer periods to run the course

when they're colder.

It's as good an index a performance as you need.

What's more, the experiment is conducted so close

to where the animals live.

The disruption to their lives -

always a problem in experiments on animals -

is minimised.

I will put it here if you can help us there.

OK.

Beatrix needs a check on whether the sex hormones

collected from the faeces are a reliable indicator.

She periodically takes a blood sample from her tortoises.

This week, it is the turn of a large male called Chico.

A procedure which I'm lucky enough to help with.

Sometimes we have to change the arm because it's the same with us -

some veins are good on one side, some are very bad.

So we try the other side.

So you can see, here, this part and these lines

going from there till this tendon.

Here is a tendon from the muscle.

And we are following this line, going in this small hole.

Here's the vein going up there and so we try to find this vein.

You can't just feel it so you have to follow these lines.

That's the best.

Sometimes it's difficult to find it. Sometimes not at all.

I got it. You see?

When you've got it, it's just a few seconds.

Then you have it.

Muchas gracias, Chico!

Now we have to be very fast.

This is a heparin tube.

Thank you very much.

So this is a heparin tube and afterwards I will centrifuge it.

I will centrifuge it for about five to ten minutes.

We only need the blood plasma inside

so we will throw the blood cells away.

In the blood plasma, there are the sexual hormones.

These we need.

At very low spring tides,

when rocks which are normally covered or exposed,

the iguanas graze the red algae.

Martin takes a chance to sample the weed

with a watchful eye on the waves.

The samples are analysed to identify the species.

Each has a particular energy content and the growth rate of each is known.

If you know what's in the animal's stomach,

you get a picture of the energy balance,

what and how much it has eaten.

To an expert, the chewed fragments from the stomach are identifiable.

The purple colouration comes from the seaweed itself.

For a complete picture, fresh faeces are collected from the rocks,

dried and analysed for energy content.