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A Tale of Two Finches. Graham Ritchie. White-backed Munia ( Lonchura striata ). Bengalese Finch ( Lonchura striata var. domestica ). Dramatis Personae. Munia Inhabits south-east Asia Quieter, simple (mainly linear) finite state song which is largely innate. Bengalese Finch

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A Tale of Two Finches

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A tale of two finches l.jpg

A Tale of Two Finches

Graham Ritchie


Dramatis personae l.jpg

White-backed Munia

(Lonchura striata)

Bengalese Finch

(Lonchura striata var. domestica)

Dramatis Personae


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Munia

Inhabits south-east Asia

Quieter, simple (mainly linear) finite state song which is largely innate

Bengalese Finch

Bred in captivity in Japan for ~300 years for plumage

Louder, more complex (but still finite state), song which is largely learned

The Data


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Isolation / Fostering Experiments


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Female Choice

  • Female preference for song was measured by the level of hormones & the number of nesting strings collected

  • Female bengalese finches preferred (artificially standardised) complex syntax over simple syntax

  • Female munias also preferred the complex syntax (with munia phonology) to a male munia song


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Neural Mechanism

  • Okanoya found that a bilateral lesion of the NIf region in 2 bengalese finches with complex song resulted in both singing a munia-like song

  • The same procedure carried out on a bengalese (and a zebra finch) with simple song had no effect

He therefore concludes that that the NIf is responsible for

The more complex phrase to phrase transitions


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Okanoya’s Hypothesis

  • Argues that the louder complex song requires greater cognitive load / increased testosterone / larger brain etc.

  • As song complexity & volume has no (apparent) survival value & may increase predation, this will be selected against in the wild

  • However, females prefer the more complex song, so it seems that sexual selection is the driving force for song complexity


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Okanoya’s Hypothesis

  • Song complexity can therefore be seen as an honest signal for fitness – a fitter male can afford a more complex song

  • When domestication reduces the natural selection pressure, (indirect) sexual selection comes to the fore and the song becomes more complex

  • Argues that this demonstrates how syntax could evolve without survival significance


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Agrees that the song changed as selection pressure eased off, but argues that song complexity evolved without overt selection on the trait

Instead of sexual selection driving the increase in complexity, reduced selection pressure allows random drift to affect e.g. innate note transition probabilities

Other factors (e.g. early auditory experience, mnemonic biases) then come into play in affecting song structure

Deacon’s Hypothesis


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Deacon’s Hypothesis

  • Essentially argues that domestication reduces selection pressure, which, via drift, leads to ‘dedifferentiation’ of the genetic, neural & physiological factors affecting song structure

  • As innate biases degrade, previously minor influences have more effect and different neural systems (including association & motor learning regions) begin to be used to contribute to song

This (perhaps along with ‘self-domestication’) might explain

how we could move from an innate communication system

to something like language


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Proposed Model

  • Evolving population of probabilistic FSMs (or possibly SRNs, or n-grams)

  • Some form of learning which interacts with genetically encoded biases

    • possibly simply adjusting probabilities to match songs heard (this could fit neatly with Deacon’s explanation)


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Proposed Model

  • Costs imposed on FSM complexity

  • Try to reproduce the fostering / isolation results

  • Artificially control natural & sexual selection pressure & mutation rate to test the plausibility of Okanoya’s and Deacon’s (and other) hypotheses

  • Further ideas welcome…


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Why Birdsong?

  • The comparative method is an important technique in studying language evolution

  • Much research has been carried out with primates, but their communication systems seem to have little in common with human language

  • Although birds are more distant relatives, birdsong displays some interesting parallels with language

  • Darwin: “The sounds uttered by birds offer in several respects the nearest analogy to language”


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Parallels with Language & Language Learning

  • Complex syntactic vocalisations

  • Experience interacts with innate biases for learning

  • Birdsong ‘dialects’ exist

  • Some birds undergo a critical period

  • Learning depends on hearing themselves & others

  • Infant birds ‘babble’ in a similar way to human babies

  • Specialised neural mechanisms for song

BUT birdsong is not like language in the

sense of conveying complex meaning


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Final Remarks

  • Birdsong shares important and interesting features with language, especially the ways in which it is learned

  • A better understanding of how birdsong evolves may shed light on language evolution, particularly the emergence of syntax

  • A computer model will allow us to test hypotheses about birdsong evolution & the interaction of learning and evolution under various conditions much more systematically & easily than with real birds


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