>Because we're the only ones with language.

That theory is being steadily debunked. I cannot find the link, but I'm looking for it and as soon as I do, I'll post it - however, a very recent study involved showing dolphins in one aquarium a series of objects and recording the sounds they made when seeing them, then travelling to another aquarium hundreds of miles away, placing the objects in front of the dolphins, and playing back a recording from the earlier round. The dolphins, at a statistically-relevant exceedingly high percentage of the time, would then identify the object the recording was made in response to.

This link was posted in an HN comment maybe 3-4 months ago.

Other animals can communicate in front of each other (bees, elephants, etc.), but the ability to convey arbitrary messages to strangers is, I would say, the very hallmark of a language.

Edit:

Here we go:

* http://wakeup-world.com/2011/11/28/the-discovery-of-dolphin-...

In his bid to “speak dolphin” Jack Kassewitz of SpeakDolphin.com, [...] designed an experiment in which he recorded dolphin echolocation sounds as they reflected off a range of eight submersed objects, including a plastic cube, a toy duck and a flowerpot. He discovered that the reflected sounds actually contain sound pictures and when replayed to the dolphin in the form of a game, the dolphin was able to identify the objects with 86% accuracy, providing evidence that dolphins understand echolocation sounds as pictures. Kassewitz then drove to a different facility and replayed the sound pictures to a dolphin that had not previously experienced them. The second dolphin identified the objects with a similar high success rate, confirming that dolphins possess a sono-pictorial form of communication.

That's the link I was referring to, and as far as I'm concerned, that's enough to treat dolphins as sentient beings.

Another HN link:

* http://current.com/community/91825903_scientists-say-dolphin...

In one study, [...] showed that bottlenose dolphins could recognise themselves in a mirror and use it to inspect various parts of their bodies, an ability that had been thought limited to humans and great apes.

In another, she found that captive animals also had the ability to learn a rudimentary symbol-based language.

[...]

In one recent case, a dolphin rescued from the wild was taught to tail-walk while recuperating for three weeks in a dolphinarium in Australia.

After she was released, scientists were astonished to see the trick spreading among wild dolphins who had learnt it from the former captive.

There are many similar examples, such as the way dolphins living off Western Australia learnt to hold sponges over their snouts to protect themselves when searching for spiny fish on the ocean floor.

Such observations, along with others showing, for example, how dolphins could co-operate with military precision to round up shoals of fish to eat, have prompted questions about the brain structures that must underlie them.

What more do we need to stop being so arrogant as to think ourselves the only creatures on the planet capable of thinking, feeling, remembering, teaching, communicating, and being self-aware?