Clearly we haven't, given that there is very little agreement as to what intelligence is. This is just my definition, although there's a lot behind why I define it this way.
However, I do think that a meaningful intelligence comparison between humans and dolphins, etc, would conclude that we are more intelligent, especially based on our reasoning/planning (= multi-step prediction) abilities, which allows us not only to predict our environment but also to modify it to our desires in very complex ways.
>However, I do think that a meaningful intelligence comparison between humans and dolphins, etc, would conclude that we are more intelligent, especially based on our reasoning/planning (= multi-step prediction) abilities
I'm not sure how you would make meaningful comparisons here. We can't communicate to them as they communicate and we live in almost completely different environments. Any such comparison would be extremely biased to us.
>which allows us not only to predict our environment but also to modify it to our desires in very complex ways.
We modify our environment mostly through technology. Intelligence is a big part of technology sure but it's not the only part of it and without the other parts (hands with opposable thumbs, fire etc), technology as we know it wouldn't exist and our ability to modify the environment would seem crippled to any outside observer regardless of how intelligent we may be.
It's not enough to think that the earth revolves around the sun, we need to build the telescopes (with hands and materials melted down and forged with fire) to confirm it.
It's not enough to dream and devise of flight, we need the fire to create the materials that we dug with our hands and the hands to build them.
It's not enough to think that Oral communication is insufficient for transmitting information through generations. What else will you do without opposable thumbs or an equivalent ?
Fire is so important for so many reasons but one of the biggest is that it was an easy source of large amounts of energy that allowed us to bootstrap technology. Where's that easy source of energy underwater ?
Without all the other aspects necessary for technology, we are relegated to hunter/gatherer levels of influencing the environment at best. Even then, we still crafted tools that creatures without opposable thumbs would never be able to craft.
Another angle to look at intelligence is that not all species need it, or need it to same degree. If you are a cow, or a crocodile, then you are a 1-trick grass-munching or zebra-munching pony, and have no need for intelligence. A generalist species like humans, that lives in a hugely diverse set of environments, with a hugely diverse set of food sources, has evolved intelligence (which in turn supports further generalization) to cope with this variety.
At least to our own perception, and degree of understanding, it would appear that the ocean habitat(s) of dolphins are far less diverse and demanding. Evidentially complex enough to drive their intelligence though, so perhaps we just don't understand the complexity of what they've evolved to do.
Evolution is a blind, dumb optimizer. You can have a mutation that is over-kill and if it doesn't actively impede you in some way, it just stays. It's not like it goes, "Ok we need to reduce this to the point where it's just beneficial enough etc".
That said, i definitely would not say the Ocean is particularly less diverse or demanding.
Even with our limited understanding, there must be adaptations for Pressure, Salinity, light, Energy, Buoyancy, Underwater Current etc that all vary significantly by depth and location.
And the bottlenose dolphin for instance lives in every ocean of the world except the Arctic and the Antarctic oceans.
Intelligence is the ability to use experience to predict your environment and the outcomes of your own actions. It's a tool for survival.