By Rebecca Morelle
Science reporter, BBC News
The discovery that apes can "talk" using hand gestures may shed more light on language development.
Scientists are keen to look into language development
But these are not the only animals with communication skills - in the animal kingdom, it is all about getting your message out there.
And in a bid to understand how one of the most complex communication systems of all - human language - came about, scientists are also studying animals that, like us, use sound to communicate.
Surprisingly, they have discovered our closest relatives, despite their signing prowess, do not have much of a vocal repertoire.
Dr Klaus Zuberbuhler, an expert in primate communication from the University of St Andrews, UK, says: "Most of the non-human primate species only have a fairly limited number of sounds that they can generate."
But while primate "vocabulary" is restricted, scientists have found many species can attach meanings to some sounds to convey information. Vervet monkeys, for example, have three distinct alarm calls that trigger three distinct response calls.
Putty-nosed monkeys can combine their calls
And some species, says Dr Zuberbuhler, can do even more: they possess the ability to use and understand simple grammar. Putty-nosed monkeys, he explains, can combine their calls to create a sequence that carries a more complex meaning.
In the laboratory, some primates have also demonstrated an understanding of human language; Kanzi, a male bonobo, is said to be able to understand about 3,000 words, as well as simple sentences.
His trainer Dr Sue Savage-Rumbaugh has said he can even combine words to form simple sentences of his own.
Kanzi is obviously intelligent, Dr Zuberbuhler comments, "but [he] is tested on human linguistic systems, not on his own natural way of communicating, and this is an artificial system".
Rico the super-dog
As with non-human primates, most other mammals have the ability to understand some language but the inability to "talk" back.
Rico the border collie is a prime example, says Professor Julia Fischer, an animal behaviour expert at the German Primate Center.
"All he can do is bark and growl, but he knows the name of over 200 toys and he can understand simple syntax," she says.
Rico can understand the names of over 200 toys
"I thought it would be a great model to study how animals attach meaning to sounds."
However, she points out that like Kanzi, Rico is "enculturated" because he lives among humans. But, she says, he still shows us what such animals can do.
Scientists are trying to find out why most mammals have the ability to understand language but lack the capacity to articulate.
While there have been suggestions that it could be down to simple neurological limitations, such as lack of tongue control, some think the FoxP2 gene may be key.
Humans with a defective version of the gene have a great deal of trouble with speech.
Professor Fischer says: "When you look at the evolution of this gene, there are only three changes between mice and men - that's 70 million years - and two of them occurred after the split between chimps and humans.
"Maybe this constraint in articulating has had something to do with this gene."
Despite the fact that most animals are stumped when it comes to vocal communication, a limited number of species are able to perform the feat of hearing a sound, copying it, and then reproducing it - something scientists call vocal learning.
This trick of imitation plays a key part in human language. "One reason why we are looking at vocal learning," explains Vincent Janik, of the Gatty Marine Research Institute at the University of St Andrews, "is because it is so important to humans".
However, the small group of animals that can do this are rather dissimilar to humans - and to each other.
The vocal learners are an eclectic bunch
For example, on the list are dolphins that can copy signature whistles and humpback whales that learn and copy distinctive and complex songs.
And parrots are also famous for their mimicry: African grey parrots N'kisi and Alex have wowed with their vocabulary.
Seals, bats, elephants, songbirds and hummingbirds are other members of this eclectic bunch of vocal learners.
The key, says Dr Janik, is to find out why such diverse group of animals all possess this ability.
Let's talk about sex
Professor Erich Jarvis of the US-based Duke University Medical Center is studying brain pathways in the three types of birds that have a vocal learning ability - songbirds, hummingbirds and parrots - to try to answer this question.
"The question is: how can Mother Nature come up with this solution for this supposedly complex behaviour which gives rise to civilisation in humans," he explains.
He has found that humans and these three species of birds share a similar pathway that is involved with learned vocalisations.
Similar pathways were found in human and bird brains
And he believes they may have evolved out of a pre-existing pathway that controlled motor behaviours.
"It's like the evolution of wings. Wings evolved in bats, bird and ancient flying dinosaurs, and each time they did, they evolved from a substrate - the upper limbs. In the brain, there is something that this complex behaviour is evolving from."
But in terms of why this ability may have evolved, Professor Jarvis believes it all comes down to sex.
The clue, he says, is the more complicated the song, the more complicated the syntax, the more attractive that particular animal is to the opposite sex.
He explains: "Of the species that produce learned song, all of them will do it in mating interactions - including humans. You just think about singing and the Jennifer Lopezes and so forth - singing in humans is used for both abstract communication and to feel good to the opposite sex.
"And you find this in whales, and you find this in songbirds, parrots and hummingbirds. They all do that."
Dr Klaus Zuberbuhler explains that the key to animal communication studies is to try to elucidate why humans are so unusual in their speech and language capacities.
"Looking at other animals will hopefully give us some ideas of the building blocks of human language," he says.
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