[TheOtherHobbes]

Traditionally, no human considers an octopus intelligent unless it can predict soccer results.

https://en.wikipedia.org/wiki/Paul_the_Octopus

Paul aside, we've been ignoring evidence that animals are sentient for centuries now.

So yes - as a general principle, there's a term missing from the Drake equation to cover recognisable similarity, technological equivalence, and mutual interest.

Two civs probably need to be within half a millennium or so of technological development (human time) to have any possibility of communicating.

Considering how old the universe is, it's quite likely civs pass each other by all the time, because larger development differentials aren't visible - literally in one direction, and because of perceived triviality in the other.

Imagine an ant colony in a city, looking for other ant nests, while the city, all the other cities, and the rest of the civ that built the cities can't be imagined by the ants. So even though they're in the middle of a busy civilisation, it's invisible to them.

[trentlott]

Ant 1: "Man, how do we get all these strange food deposits without plants?"

Ant 2: "Who knows? That's just the way the universe is."

Man 1: "Man, how do we have all these strange gravity deposits without matter?"

[orly_bookz]

Creepy.

[uxcn]

Two civs probably need to be within half a millennium or so of technological development (human time) to have any possibility of communicating.

Intelligence seems to expand polynomially or exponentially by a lot of measures though (e.g. technology). The closest analogy for us compared to another civilization that could actually receive our radio communication I think would be chimpanzees using sticks to eat termites.

If I had to guess, I'd guess there's probably some kind of technological asymptote, but considering the scale of the universe (more importantly its complexity) I think we are probably nowhere near it.