My cat Cameo is now responding to both an external stimulus

the sound of a bag of treats, and an internal stimulus

her hunger, or at least her insatiable desire for treats.

Sometimes animal behavior can seem really far out,

but if you look closely enough,

you can see how all behavior serves a purpose

to help an animal mate, eat, avoid predators, and raise young.

And since behaviors can come with advantages like these,

natural selection acts on them just as it acts on physical traits

ensuring the success of animals who engage

in beneficial behaviors, while weeding out those that do stupid,

dangerous or otherwise unhelpful stuff.

The most beneficial behaviors are those that make an animal

better at doing the only two things in the world that matter:

eating and sex.

Still, that doesn't mean all behavior is about

just looking out for number one.

It turns out some advantageous behavior

is actually pretty selfless.

More on that in a minute.

But first, behavior is really just a product of a pair of factors:

Morphology, or the physical structure of an animal

and physiology, or the function of that morphology.

Now, an animal's behavior is obviously limited

by what its body is capable of doing

for example, Cameo does not have opposable thumbs,

so, much as she would like to get into the treat bag,

by herself, she cannot.

This limitation is strictly hereditary

no cats can open treat bags with their thumbs

because no cats have opposable thumbs.

Though some cats do have thumbs.

In the same way that a penguin can't fly to escape a predator;

or a gazelle can't reach the same leaves as a giraffe can.

Similarly, behavior is constrained by an animal's physiology.

Like, Cameo's built for chasing down little critters

and eating meat, not beds of lettuce.

This is because her physiology, everything from her teeth

to her digestive system, are geared for eating meat.

If she pounced on and ate every blade of grass she came across...

let's just say I would not want to be in charge of that litter box.

Now the traits that make up an animal's morphology

and physiology are often heritable,

so we generally talk about selection acting on those traits.

But as natural selection hones these traits,

it's really selecting their associated behaviors.

It's the USE of the trait,

using wings and feathers to escape predators,

or using a long neck to reach leaves,

that provides the evolutionary advantage.

Still, that doesn't mean all behavior is coded

in an animal's genes

some behaviors are learned.

And even for animals that learn how to do things,

natural selection has favored brain structures

that are capable of learning.

So one way or another,

most behaviors have some genetic underpinning,

and we call those behaviors adaptive.

Problem is, it's not always obvious

what the evolutionary advantages are for some of the nutty things

that animals do.

Like, why does a snapping turtle always stick out its tongue?

How does a tiny Siberian hamster find its mate,

miles across the unforgiving tundra?

Why does a bower bird collect piles of garbage?

To answer questions like those,

we have to figure out what stimulus causes these behaviors,

and what functions the behaviors serve.

To do this, I'm going to need the help of one of the first

animal behavior scientists ever, or ethologists, Niko Tinbergen.

Tinbergen developed a set of four questions

aimed at understanding animal behavior.

The questions focus on how a behavior occurs,

and why natural selection has favored this particular behavior.

Determining how a behavior occurs actually involves two questions:

One: what stimulus causes it?

And two: what does the animal's body do

in response to that stimulus?

These are the causes that are closest

to the specific behavior we're looking at,

so they're called the proximate causes.

In the case of the male Siberian hamster,

the stimulus is a delicious smelling pheromone

that the sexy female hamster releases when she's ready to mate.

The male hamster's response, of course, is to scuttle,

surprisingly quickly, over several miles if necessary

to find and mate with her.

So the proximate cause of this behavior

was that the girl hamster signaled that she was ready to knock boots,

and the male ran like crazy to get to the boot-knockin'.

Asking the more complex question of why natural selection

has favored this behavior requires asking two more questions:

One: what about this behavior helps this animal survive

and/or reproduce?

And two: what is the evolutionary history of this behavior?

These, as you can tell, are bigger-picture questions,

and they show us the ultimate causes of the behavior.

The answer to the first question, of course,

is that the ability of a male hamster to detect and respond

to the pheromones of an ovulating female is directly linked

to his reproductive success!

As for the second question, you can also see that male hamsters

with superior pheromone detectors will be able to find females

more successfully than other male hamsters,

and thereby produce more offspring.

So natural selection has honed this particular physical ability

and behavior over generations of hamsters.

So, who would have thought to ask these questions in the first place?

And where's my chair?

Niko Tinbergen was one third of a trifecta

of revolutionary ethologists in the 20th century.

Along with Austrians Karl von Frisch and Konrad Lorenz,

he provided a foundation for studying animal behavior

and applied these ideas to the study of specific behaviors

and for that all three shared the Nobel Prize in 1973.

You may have seen the famous photos of young graylag geese

following obediently in a line behind a man.

That was Lorenz, and his experiments

first conducted in the 1930s introduced the world to imprinting,

the formation of social bonds in infant animals,

and the behavior that includes both learned and innate components.

When he observed newly hatched ducklings and geese,

he discovered that waterfowl in particular

had no innate recognition of their mothers.

In the case of graylag geese, he found the imprinting stimulus

to be any nearby object moving away from the young!

So when incubator-hatched goslings spent

their first hours with Lorenz,

not only did they follow him,

but they showed no recognition of their real mother

or other adults in their species!

Unfortunately, Lorenz was also a member of the Nazi party

from 1938 to 1943.

And in response to some of his studies

on degenerative features that arose in hybrid geese,

Lorenz warned that it took only a small amount

of "tainted blood" to have an influence on a "pure-blooded" race.

Unsurprisingly, Nazi party leaders were quick to draw

some insane conclusions from Lorenz's behavioral studies

in the cause of what they called race hygiene.

Lorenz never denied his Nazi affiliation

but spent years trying to distance himself from the party

and apologizing for getting caught up in that evil.

Now how exactly does natural selection act

on behavior out there in the world?

That's where we turn to those two types of behavior

that are the only things in the world that matter:

eating and sex-having.

Behavior associated with finding and eating food

is known as foraging, which you've heard of,

and natural selection can act on behaviors that allow animals

to exploit food sources

while using the least amount of energy possible

this sweet spot is known as the optimal foraging model.

And the alligator snapping turtle has optimal foraging

all figured out.

Rather than running around hunting down its prey,

it simply sits in the water, and food comes to him.

See, the alligator snapping turtle has a long, pink tongue

divided into two segments, making it look like a tasty worm

to a passing fish.

In response to the stimulus of a passing fish,

it sticks out its tongue out and wiggles it.

Natural selection has, over many generations,

acted not only on turtles with pinker and more wiggly tongues

to catch more fish, it's also acted on those that best know how

and when to wiggle those tongues to get the most food.

So it's selecting both the physical trait

and the behavior that best exploits it.

And what could be sexier than a turtle's wiggly tongue dance?

Well, how about sex?

As we saw with our friend the horny Siberian hamster,

some behaviors and their associated physical features

are adapted to allow an animal to reproduce more,

simply by being better at finding mates.

But many times, animals of the same species live close together

or in groups, and determining who in what group gets to mate

creates some interesting behaviors and features.

This is what sexual selection, is all about.

Often, males of a species will find and defend a desirable habitat

to raise young in, and females will choose a male

based on their territory.

But what about those species, and there are many of them,

where the female picks a male not because of that,

but because of how he dances, or even weirder,

how much junk he's collected?

Take the male bower bird.

He builds an elaborate hut, or bower, out of twigs

and bits of grass, then spends an enormous amount of time

collecting stuff, sometimes piles of berries,

and sometimes piles of pretty, blue, plastic crap.

Ethologists believe that he's collecting the stuff

to attract the female to check out his elaborate house.

Once the female's been enticed to take a closer look,

the male starts to sing songs and dance around,

often mimicking other species, inside of his little house for her.

Females will inspect a number of these bowers

before choosing who to mate with.

Now, doing more complex dances and having more blue objects

in your bower scores bigger with females.

And ethologists have shown that a higher level of problem solving,

or intelligence, in males correlates

to both of these activities.

So yeah, it took some brawn to build that bower

and collect all that junk, but chicks also dig nerds

who can learn dances!

So the bowerbird's brain is evolving in response

to sexual selection by females.

This intelligence likely also translates

into other helpful behaviors like avoiding predators.

So thanks to the evolution of behavior,

we're really good at taking care of our nutritional and sexual needs.

But what's confused scientists for a long time

is why animals often look after others' needs.

For instance, vampire bats in South America will literally

regurgitate blood into the mouths of members of its clan

who didn't get a meal that night.

How do you explain animals who act altruistically like that?

We actually did a whole SciShow episode on this very subject

but basically, we can thank British scientist William Hamilton

for coming up with an equation to explain how natural selection

can simultaneously make animals fit and allow for

the evolution of altruism.

Hamilton found that the evolution of altruism

was best understood at the level of larger communities,

especially extended animal families.

Basically, altruism can evolve if the benefit of a behavior

is greater than its cost on an individual,

because it helped the individual's relatives enough

to make it worth it.

Hamilton called this inclusive fitness,

expanding Darwin's definition of fitness

basically, how many babies somebody's making

to include the offspring of relatives.

So I guess the only question left is,

if I forget to feed you two,

who is going to regurgitate blood into the other one's mouth?