Human cultural traits such as language, dress, religion and values are generally said to be passed from one generation to the next by social learning. And in animal species which have language, the same is true; male song birds, for example, learn the songs with which they serenade potential mates from older male relatives.
A new study, published online in the journal Nature, shows that the songs of isolated zebra finches evolve over multiple generations to resemble those of birds in natural colonies. These findings show that song learning in birds is not purely the product of nurture, but has a strong genetic basis, and suggest that bird song has a universal grammar, or an intrinsic structure which is present at birth.
Birdsong shares similarities with, and is considered by some to be analagous to, human language. For example, both have grammar and syntax, and the songbird brain contains brain areas which are analogs of the speech centres in the human brain. Also, birdsong is passed down from one generation to the next, just like human speech. Male zebra finches learn their song by imitating an adult male relative - usually their father, or an uncle. The song is based on a template which consists of stereotyped syllables that are repeated in a fixed order. However, each individual bird introduces small variations into this template, and thus has its own unique song.
In the new study, Partha Mitra of Cold Spring Harbor Laboratory in New York and his colleagues investigated the songs of zebra finches which were raised in isolation, and thus not exposed to singing males during development. They placed juvenile finches in sound chambers, between 30 and 130 days after hatching, a period of development which is known to be critical for song learning. As expected, the isolated birds produced songs which were markedly different from those of the wild-types, or birds raised in natural colonies or with other birds in the laboratory. The songs had an irregular rhythm and were less structured, containing noisy broadband notes and high-pitched upsweeps. Some of the syllables were also prolonged, and often monotonous or stuttered.
Newly-hatched finches were then placed into the sound chambers with the isolated males. These "pupils" readily imitated the songs produced by their "tutors", producing accurate copies. However, closer examination showed that they changed certain of the songs' characteristics - copies of syllables which were longer than a given length (about one quarter of a second) were, on average, 30% shorter than the originals, and the relative frequency of highly abundant syllables was reduced.
When this first generation of pupils matured, a new batch of hatchlings was placed into sound chambers with them. This second generation of pupils also imitated their tutors' songs, but again, they introduced minor variations into their otherwise faithful copies. The variations thus accumulated over successive generations, such that, over the course of three to four generations, the songs had evolved to sound more like the songs of wild-types than those of the isolated colony founders.
The authors note that their findings resemble the well-known case of a large group of deaf children in the Nicaraguan capital Managua, who in the 1970s and 80s spontaneously developed a unique form of sign language called Idioma de Señas Nicaragüense (ISN). They conclude that the results show that zebra finch song is not just a product of environmental influences, but is at least partly genetically encoded. Evidently, it is an extended developmental process, which emerges over multiple generations. In other words, zebra finch songs seem to exhibit what Noam Chomsky referred to as universal grammar:
...it is natural to expect a close relation between innate properties of the mind and features of linguistic structure; for language, after all, has no existence apart from its mental representation. Whatever properties it has must be those that are given to it by the innate mental processes of the organism that has invented it and that invents it anew with each succeeding generation, along with whatever properties are associated with the conditions of its use.
Essentially, Chomsky argues that the brain contains a limited set of rules for structuring language, which are not learnt, but are present at birth. These rules are flexible, but ultimately constrain the diversity of human language. Thus, all of the approximately 6,000 human languages share a basic grammatical structure, which facilitates their acquisition. Applied to the new study, this innate language structure sets limits upon the variations in the pupils' songs, and perhaps drives those variations - towards the wild-type song structure.
The study focused on the acoustic properties of the songs produced by the isolated birds and the multiple generations of birds they tutored. The effects of song variation on mating behaviour were not explored, so it remains to be seen whether females have an increasing preference for the songs of successive generations of pupils over those of their tutors. And, as it is well established that the vocal centres in the song bird brain are regenerated anew every mating season, it would be interesting to investigate whether the progressive changes in the pupils' songs are associated with changes in neurogenesis, cellular organization, or gene expression.