Don't only humans have the ability to use language
and understand abstract concepts?
Even insects possess language, since
they communicate with each other using chemicals and body
language. All animals have a language, even though they do not
speak with a human voice.
However, even the ability to speak with a human voice is not unique to humans. There is an African gray parrot named Alex who has learned human speech, and can consistently give correct answers to questions he has never previously been asked, about objects which he has never previously seen. This parrot thereby demonstrates that he is not simply memorizing the answers, but is engaging in analytical reasoning, and demonstrating comprehension of abstract concepts such as shapes and numbers.
The language of many species is far more complex than we previously realized. For example, prairie dogs make various calls to each other which sound identical to the human ear. It is only when these calls were recorded, and analyzed on a sound spectrograph, that we learned that the prairie dog vocabulary was much larger than we had ever imagined. For example, although they all sound identical to the human ear, prairie dogs have different calls for warning each other about the approach of a human male and a human female. The calls are also different depending on what color clothing the person is wearing. And, they have different calls depending on whether or not the human is carrying a gun.
In the past, people have complained about racial bias in the construction of intelligence tests. We are now dealing with something even harder to eliminate in our intelligence tests - species bias. As one of the more obvious examples, we often judge the intelligence of animals such as dolphins based on how many human words they can learn and understand. However, dolphins have a very complex language which we have, so far, been completely unable to decipher. If dolphins were to apply this same test to humans, then the dolphins would have to conclude that humans are complete idiots, since despite our best efforts, we have failed to learn even a single word that the dolphins are saying.
Follow up questions:
Aren't only humans self aware?
What evidence is there that other animals can make moral decisions?
Don't only humans have the ability to pass on cultural information from one generation to the next?
Don't only humans understand the concept of death?
Don't only humans have the ability to enter social contracts?
Don't only humans have the ability to use tools?
How can some animal rights activists believe that other animals are our equals?