For nearly 2 decades, SETI, the Institute for the Search for Extra Terrestrial Intelligence has been monitoring audio transmissions from space, hoping to identify a signal from an alien species. But SETI physicist Laurance Doyle was concerned that if and when a signal arrived, it might be too alien for us to decode. So he began an effort at inter-species communication a little closer to home.

Bottle-nosed dolphins make more than a hundred different whistle calls. They range from a high-pitched screech to a short click. Some sound like the creaky door in a haunted house, others, like drumming. Over the past 10 years, Laurance Doyle and his colleges have recorded hundreds of hours of dolphin calls, analyzing the pitch of each whistle and the frequency with which it occurs.

"We found that the complexity of the dolphin whistles as far as we could measure was compatible with the complexity of human languages," he says.

That's because the dolphins' vocabulary, like all human languages, follows a distinct pattern of sound distribution. Linguists call it Zipf's law. Ziph's law is a linguistic study that shows that virtually all languages, from Chinese characters to Arabic characters, and human words and alphabetic symbols and so on, all produce this balance between diversity and repetition," he says.

Simply put, Zipf's law means that the most common sound in a language occurs 10 percent of the time. In English, that would be the sound made by the letter E. The second most common sound, 'TUH', occurs 8 percent of the time. Once you get to number 25, 'KWUH,' made by the letter Q, it's rarely used in speech. Although the specific sounds may vary from language to language, the patterns of sound distribution remain the same.

To Mr. Doyle's surprise, this same distribution of sound seems to occur with dolphin whistles. "The most frequent whistle, [what we call] number 2, is used about 10 percent of the time and on down. So dolphins show the same linguistic distribution that is argued to be uniquely human, and it isn't. And that's news because recently a lot of studies have been going on in human linguistics that show that one of the reasons that human linguistics is unique is because it has this distribution," he says.

Because the distribution of dolphin whistles follows human speech patterns, Laurence Doyle speculates that dolphins can communicate complicated thoughts and ideas. But his theory is controversial among linguists and communication scholars.

"I suppose the answer to your question, 'does this demonstrate a language?' obviously depends on how restrictive your definition of language is," says Leonard Hawes of the University of Utah. He doesn't believe that sound patterns necessarily constitute content or meaning. He says that the dolphins may be communicating, but that doesn't mean they have a complex language.

The question isn't, 'can two animals or two humans emit patterns of sound that produce particular responses in others of their kind, whether it's human or gorilla or dolphin?' The answer is yes, of course they do, we see it," he says. "Does it mean they have an elaborated syntax and vocabulary and grammar? How you would ever ultimately validate that is more problematic."

That's because dolphin communication may be so different from our own, it might not even be translatable. As Professor Hawes points out, there are translation problems just going from one human language to another. "But at least you can approximate German words for English words or French words," he says. "What we are less certain of is can we translate dolphin sounds or chimpanzee sounds or bird sounds into something that looks like human language. Can we put it into [sounds that mean] 'don't come any closer,' or 'get away'? Can that be translated into words? At best only approximately, and the problem is that we can't ask the dolphin if we got it right."

To make matters more complicated, there may not be a single whistle that means things like 'don't come any closer' or 'get away.' It's more likely that communication occurs through complex vocalizations.

Laurence Doyle says, after 30 years of research, scientists have only been able to conclusively identify the meaning of just a couple of whistle sequences, like the thunk call.

"When the mother thunks the baby comes right to her side. It means 'danger' or 'come here' or something, but we have seen baby dolphins drop everything and go immediately to the mom's side when she produces a thunk call. You have to keep in mind, they are probably talking about things that aren't present," he says. "Consequently trying to correlate objects with what they say hasn't been very successful. When you are talking about a symbolic representation, trying to get at the meaning of something is much more difficult for another species."

But despite those difficulties, Laurence Doyle is partnering with scientists at the University of California at Davis to search for similar complexity in bird, primate and whale calls.