Fully functioning in society seems to be setting the bar too high; we don't consider that a prerequisite for rights in humans by any reasonable interpretation.
Now, will dolphins ever communicate their desire to achieve greater legal consideration? Not unless we put some more effort into researching communication with dolphins (and even then, I strongly suspect not). Not really a problem though I think.
we don't consider that a prerequisite for rights in humans by any reasonable interpretation.
You miss my point. It's not a requirement for any individual, but it IS a requirement for members of a species in general. Most humans are functioning members of society. The ones that aren't, whether it's temporary or permanent are protected implicitly and explicitly through social contract and the laws we've created.
No dolphin will ever be a functioning member of our society, thus dolphins are not part of society, thus dolphins do not take on the responsibilities of being in a relationship with humans, thus dolphins are not entitled to parallel status.
No, it is not a requirement, that is what the discussion is about. If it were a requirement, we wouldn't be trying to decide if it should be a requirement or not. Using a sample size of one to prove a point is absurd.
I'm not saying it's a proof by statistics. It's a requirement in order for the system to maintain logical consistency.
We can do whatever we want. We can give citizenship rights to gummy bears because they look like real bears and bears have two arms and two legs just like people. It would be illogical and counterproductive to any real advancement for society, though.
Reductio ad absurdum isn't helping your case. You haven't made an argument, just assertions. Why would giving some rights to a non-human animal break "logical consistency"? Just saying it does is meaningless, provide an argument.