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.
I don't know, but I do know that if we all accepted that as a standard we would be in a much worse off place as a society since most times I hear the phrase "fully functioning member of society" the implied alternative is usually "penniless hippy".
If "participates" in society is sufficient for "fully functioning member of society", then at a minimum, trained dolphins who perform in shows are fully functioning members of society. They have complex interactions with their trainers, including fairly high level communication, and they perform work in exchange for goods (fish).
Dolphins cannot participate in our society by agreement. They don't understand what participating is. They're captured or born in captivity and then trained to perform a few tricks. They are beautiful and fascinating, but they have no contributions to make to the fiber of our society that can't be made by other animals, machines, or even simple objects.
They are able to do nothing further. Giving them a new status is just semantics. They are fundamentally not functioning members of our society. Not individually. Not as a species.
What I would like to see is a special legal status/recognition given to dolphins. My motivation for wanting to see this done is that I think it would provide a sort of precedence for extending traditionally human rights to things that are not traditionally seen as equals of humans. I think there is a possibility that doing so may become necessary in the future, when "machines" and intelligence begin to blur.
While theorizing tests of animal psyche and self-awareness might be helpful to the purpose, but what we humans forget is the preciseness of agent Smith's evaluation of us:
"I'd like to share a revelation that I've had during my time here. It came to me when I tried to classify your species and I realized that you're not actually mammals. Every mammal on this planet instinctively develops a natural equilibrium with the surrounding environment but you humans do not. You move to an area and you multiply and multiply until every natural resource is consumed and the only way you can survive is to spread to another area. There is another organism on this planet that follows the same pattern. Do you know what it is? A virus. Human beings are a disease, a cancer of this planet. You're a plague and we are the cure." 
There is no chance any animal/fish is going to survive the brunt of us 9 billion in next fifty years.
So tired of this meme. Humans are like a virus in the way that every species on the planet is like a virus. Look no further than invasive species to see that any form of life given the ability to expand it's population will do exactly that. The majority of species reach what we view as 'an equilibrium' because they run into the constraints of their environment on a scale small enough for us to make note of, but humans do the exact same thing; only our scale is the entire planet and we haven't run into the constraints yet. The moment humans were intelligent enough to form agriculture is the moment humans inherited this planet.
I totally agree with you here. Just saying that probably treating dolphins as people might not help (or be enough), considering that the size of planet would remain constant but we'll double our density soon.
We can see a 100 ft. wave coming at our rowboat, and some of us are trying to paddle with our hands, and some of us are trying to design a better rowboat (predicting that there is enough time to design/build this before the wave hits), while others are arguing that the wave doesn't exist or will dissipate before it reaches us. I don't have a whole lot of faith.
For one picture I see in future is us inhabiting an exo-planet; and Elon Musk et al showing us that direction and so on...
In this dynamic, we as a virus race will choose the easiest way out even at the cost of all the species of this planet. 9 billion of us will, mark my words, only kill, eat or ridicule whatever that is left of the ecosystem today.
I am actually afraid about cannibalism picking up too in distant hungry countries which are already off balance today. It's pretty grim.
Dolphins will continue to be enslaved, slaughtered or even ridiculed for any "equal treatment" justified exactly the way this thread shows. Ground reality will of course be worse.
To which I do agree with your statement that "we are aware" and that there is an exit for us even though some will definitely try to mitigate, protect and stay behind.
She has rights because she is human. Her parents and other members of society vouch for her right to a position in society and accept the responsibility of her membership. The practice is so ingrained and a part of the fabric of our society that it's just accepted implicitly.
It has nothing to do with how capable she is of accepting responsibilities in her current state.
Non-humans have no such implicit acceptance of responsibilities and inclusion within the framework of human society by humans.