Interesting assertion. Tell me more - which ones? How have we discovered this?
Chimpanzees which have been taught to understand English and/or sign language are a perfect example.
There's a lot of nice documentaries about it that you could watch like I did. Or I am sure that there are lots of articles or books written about it.
I am also certain that dolphins, porpoises, killer whales can converse in both abstract and concrete concepts based upon watching their behavior.
I am also certain that lots of animals can think in abstract concepts but they lack the ability to communicate very well so most of their thoughts are trapped inside their brain.
Many animals can look at a problem and decide what tool they need and then they can go make that tool or find the tool and bring it to the problem and solve the problem with the tool. Thus proving that humans are not the only "intelligent" species.
Humans are not the only species that communicates with sound waves.
Humans are not the only species that uses tools.
Humans are not the only species that farms.
Humans are not the only species that engages in organized warfare.
Humans are not the only species that can navigate from point A to point B using measurements of distance and angles of travel.
Humans are not the only species with Emotions.
I have performed many experiments which show that various animals and plants are more intelligent than 50% of humans.
The main thing that gives humans their advantages is their hands. If Dolphins had hands then they would rule the world.
Various other animals do have nice hands but humans have larger brains than those other animals.
We also have an interesting mutation in our voicebox that gives us superior communication skills than any of the other animals with hands. Our hands give us superior writing skills too, which helps us to pass knowledge from one point to another which allows a civilization to develop.