Do cats meow only to humans?

[UEL]

The other day on the drive to work, the morning crew at the radio station had their usual "fun trivia". Most of their items are typically of no consequence, quite humourous in their oddness, and purely entertaining. Thursday's was one such one. It was on pets.

Their one claim that got me thinking was:

Cats only meow with humans. With all other creatures, including other cats, they use other forms of communication.

Now, I've been to Rome and the Colliseum. I've heard the noise of the feral cats there. I'm pretty sure that they weren't all meowing to me. So, I'm a bit leery of this claim.

Do any cat lovers have any ideas?

[A Turtle Named Mack]

You will notice that cats have only been observed meowing when there has been a person present. To observe the meowing, of course.

If a tree falls in a forest and no one is there, does it make a sound?

[Psihala]

The cat that makes it a point to come to the sliding glass door to pester *my* cat certainly isn't meowing to me.

~Psihala
(*The reverse is almost never true...)

[snapdragonfly]

It's been a while since I had the delight of a litter of kittens but IIRC those bebeh kittehs mew and they don't even know humans are there, they are mewling for they mama. Aren't they? Am I not remembering correctly?

snap*I WANT A BEHBEH KITTEH NOW*dragonfly

[Twankydillo]

We have two older stray males and their vast army of children (giving their biology, I assume not with each other) clogging up our backyard and they meow to each other. Proper meows too, not just the wail the two older stray males do to each other sometimes, when they realise they have to stop being friends and have a good fight.

My cat also meows at birds when she sees them through the window.

[Eddylizard]

I've heard cats meowing in the dead of night, and I'm pretty sure there were no humans around. Except for me but they cannot have been aware of my presence - different garden, tall fence.

I've also seen two cats meowing at each other (the aggressive territorial meow) with no knowledge that a human was there.