Can Fish Count? Numberphile Films by Brady Haran
Brian Butterworth: It turns out
and there's now quite a history of this.
Relatively recently people have found that
there are a lot of species who can do
number tasks.
By number tasks I mean
tasks which depend upon understanding
the number of objects in the display,
or what we would call "numerosity".
So, you have to be very careful here,
because you can't talk to other species,
and they can't talk to you.
So you can't explain to them
what aspects of the stimulus you want them to respond to.
But you have to design the stimuli rather carefully,
so that the critical aspect of the stimuli
is the numerosity of objects.
Actually, before the second world war,
a brilliant German ethologist called Otto Koehler,
had shown that corvids, crows, jackdaws and so on,
had quite a good number sense,
that they could match to a sample numbers up to about 7.
So he showed them a sample, black blobs,
then they had to select
one of the two or three objects
which had the same number of blobs.
The blobs would be arranged differently
If they got it right they got rewarded,
if they got it wrong they didn't.
And they were pretty good at this, these corvids.
Since then, there was a... Since the 30's there's been
a kind of a hiatus in research of animal numerical cognition.
But since the 90's it really started to take off again.
And we now know that a whole range of different animals
can do this.
In the wild and in laboratories, particularly in laboratories.
We were interested in the counting abilities of fish.
Now, small fish often swim in shoals,
and if they are in a shoal there's a smaller risk of predation
than if they are by themselves.
And if they are in a bigger shoal, they have a smaller risk of predation
than if they are in a smaller shoal.
Brady Haran: Safety in numbers. Brian Butterworth: Safety in numbers, exactly.
So you can use this natural behavior of the small fish,
and test it the laboratory.
So you can put them in a transparent tank,
with a few fish on this side and a few more fish on the other side,
and you see which side they swim to.
It's been checked, not by me, but by my collaborators,
that they will swim to the shoal of the side of the tank with more fish in it.
What we were interested to do in a paper we published this year
was to see whether they had a difference between small number perception
and large number perception.
So in humans, what happens is that if you have to compare
4 versus 3, 4 versus 2,
4 versus 1, 3 versus 2,
this is all equally easy.
I doesn't depend on the ratio of one number to the other.
So 4 against 1 is just as easy as 4 against 3.
However, once you get larger numbers
let's say, 8 against 7, or 8 against 5,
then there's a ratio effect.
That is, the bigger the difference between those two numbers
the easier it is.
The question is, would this be true of guppies,
which are small fish about that big,
relatively small brain.
And the answer was - Yes,
that these small fish didn't show a ratio effect
when you asked them to compare 4 versus 1 as compared to 4 versus 3.
But for larger numbers, they did show a ratio effect,
which suggests that like us they have these two systems,
small number system and a larger number system.
And we thought this was really quite an intriguing result.
Brady Haran: I can imagine someone at home thinking:
"Well, surely the fish just looks at two different groups of fish
or shoals of fish, and can just measure which one is bigger,
which one takes up more volume and swims towards that.
And it hasn't used any number sense there, it just used an understanding of
how big something is."
How is the experiments that you have been involved with getting around that?
Brian Butterworth: Well, it is very difficult to control,
because not only there are more fish over here that go swimming around,
and you can have fewer fish over there, and they are swimming around.
So, it's really quite difficult for the subject fish to be able to make an estimate,
even though they do it actually rather well.
How do we know that they are enumerating the fish,
that side versus that side,
Well, there are ways in which you can manipulate
the amount of fishiness on this side
and the amount of fishiness on that side.
For example by putting slightly larger fish on this side
to balance the number of fish on that side.
That creates some problems, because of fish psychology
which I'm not an expert in.
Like, some fish prefer to swim in shoals with larger fish
and so, you know, that creates complications.
However, my collaborators have done a nice training experiment,
in which the fish doesn't have to choose between two shoals,
it has to select a gate into another tank, to join its friends.
And above the gate will be a number of blobs.
And the fish has to choose the right number of blobs.
It would have had a training session in which
a certain number of blobs will be rewarded and a different number of blobs will not.
Let's say, 3 is rewarded, 2 is not rewarded.
In the test there will be different blobs. There will be 3-ness and 2-ness,
but different arrangements of 3-ness and 2-tess.
And the fish can learn this.
Actually it can learn it quite well.
So, there's now quite good evidence that
the fish can respond to number.
But, of course, they might also in the wild be responding to other cues
like the amount of fishiness in the neighborhood,
and, of course, these are typically correlated with a number.
So, you want to try and tease them apart experimentally
but in real life, of course, they tend to get together.
Brady Haran: How is understanding the mathematical cognition
or the number cognition of guppies help anything?
Why do you have an interest in this?
Brian Butterworth: Well, we want to know what kinds of capacities
the human brain inherits.