Emu Emu Society
Journal of BirdLife Australia

Adaptation and function of the bills of Darwin’s finches: divergence by feeding type and sex

Anthony Herrel A B I , Joris Soons C , Peter Aerts B D , Joris Dirckx C , Matthieu Boone E , Patric Jacobs F , Dominique Adriaens G and Jeffrey Podos H
+ Author Affiliations
- Author Affiliations

A Département d’Ecologie et de Gestion de la Biodiversité, Museum National d’Histoire Naturelle, 57 rue Cuvier, Case postale 55, F-75231, Paris Cedex 5, France.

B Department of Biology, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium.

C Department of Biomedical Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.

D Department of Movement and Sports Sciences, Ghent University, Watersportlaan 2, B-9000 Gent, Belgium.

E Department of Subatomic and Radiation Physics, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgium.

F Vakgroep Geologie en Bodemkunde, Ghent University, Krijgslaan 281, S8, B-9000 Gent, Belgium.

G Evolutionary Morphology of Vertebrates, Ghent University – UGent, K.L. Ledeganckstraat 35, B-9000 Gent, Belgium.

H Department of Biology and Graduate Program in Organismic and Evolutionary Biology, University of Massachusetts, Amherst, MA 01003, USA.

I Corresponding author. Email: anthony.herrel@mnhn.fr

Emu 110(1) 39-47 https://doi.org/10.1071/MU09034
Submitted: 4 May 2009  Accepted: 5 September 2009   Published: 18 February 2010


Darwin’s finches are a model system for studying adaptive diversification. However, despite the large body of work devoted to this system, rather little is known about the functional consequences of variation in the size and shape of bills. We test, using two methods, if natural or sexual selection, or both, has resulted in functional divergence in bill and head morphology. Firstly, we compare data on head-shape and bite-forces across nine species of Darwin’s finches. Secondly, we use micro-CT scans and finite-element models to test the prediction that the shape of the bill in representatives of the different feeding types is adaptively related to use of the bill. Sexual dimorphism in head-shape and bite-force was detected, with females having longer bills than males for a given body size. Moreover, our results show strong differences in bill- and head-morphology between feeding types, with base-crushers having higher bite-forces and also relatively high bite-forces at the tip compared to probers and tip-biters. Finally, our finite-element models suggest that the shape of the bill in the tip-biters and base-crushers confers mechanical advantages by minimising stress in tip-loading and base-loading conditions, respectively, thus reducing probabilities of fracture. Our data support the contention that bill-shape is adaptive and evolves under selection for mechanical optimisation through natural selection on feeding mode.

Additional keywords: bird, bite-force, finite-element modeling, sexual dimorphism.


We thank S. Maas and J. Weis (University of Utah) for allowing us to use the FEBio software package. Field work was coordinated through the Charles Darwin Research Station and the Galápagos National Park Service (GNPS). We are particularly grateful for the generosity of the GNPS in granting a salvage permit. The authors thank Luis De Leon, Ana Gabela, Andrew Hendry, Mike Hendry, Eric Hilton, Sarah Huber, Katleen Huyghe, and Bieke Vanhooydonck for their assistance in the field. This project was supported by NSF grant IBN-0347291 to J. Podos; by an interdisciplinary research grant of the special research fund of the University of Antwerp to P. Aerts, J. Dirckx, J. Soons and A. Herrel; and by an Aspirant fellowship of the Research Foundation – Flanders to J. Podos.


Abzhanov, A. , Protas, M. , Grant, B. R. , Grant, P. R. , and Tabin, C. J. (2004). Bmp4 and morphological variation of beaks in Darwin’s finches. Science 305, 1462–1465.
CrossRef | CAS | PubMed |

Abzhanov, A. , Kuo, W. P. , Hartmann, C. , Grant, B. R. , Grant, P. R. , and Tabin, C. J. (2006). The calmodulin pathway and evolution of elongated beak morphology in Darwin’s finches. Nature 442, 563–567.
CrossRef | CAS | PubMed |

Boag, P. T. , and Grant, P. R. (1981). Intense natural selection in a population of Darwin’s finches (Geospizinae) in the Galapagos. Science 214, 82–85.
CrossRef | PubMed |

Bowman, R. I. (1961). Morphological differentiation and adaptation in the Galapagos finches. University of California Publications in Zoology 58, 1–302.

Currey J. D. (2006). ‘Bones: Structure and Mechanics.’ (Princeton University Press: Princeton, NJ.)

Evans F. G. (1973). ‘Mechanical Properties of Bone.’ (Thomas: Springfield, IL.)

Foster, D. , Podos, J. , and Hendry, A. P. (2008). A geometric morphometric appraisal of beak shape in Darwin’s finches. Journal of Evolutionary Biology 21, 263–275.
CAS | PubMed |

Grant, P. R. (1981). The feeding of Darwin’s Finches on Tribulus cistoides (L.) seeds. Animal Behaviour 29, 785–793.
CrossRef |

Grant P. R. (1999). ‘The Ecology and Evolution of Darwin’s Finches.’ (Princeton University Press: Princeton, NJ.)

Grant, P. R. , and Grant, B. R. (2002). Unpredictable evolution in a 30-year study of Darwin’s finches. Science 296, 707–711.
CrossRef | CAS | PubMed |

Grant, P. R. , and Grant, B. R. (2006). Evolution of character displacement in Darwin’s finches. Science 313, 224–226.
CrossRef | CAS | PubMed |

Herrel, A. , Podos, J. , Huber, S. K. , and Hendry, A. P. (2005a). Evolution of bite force in Darwin’s finches: a key role for head width. Journal of Evolutionary Biology 18, 669–675.
CrossRef | CAS | PubMed |

Herrel, A. , Podos, J. , Huber, S. K. , and Hendry, A. P. (2005b). Bite performance and morphology in a population of Darwin’s finches: implications for the evolution of beak shape. Functional Ecology 19, 43–48.
CrossRef |

Lack D. (1947). ‘Darwin’s Finches.’ (Cambridge University Press: Cambridge, MA.)

Loeb G. E. , and Gans C. (1986). ‘Electromyography for Experimentalists.’ (University of Chicago Press: Chicago, IL.)

Maas S. , and Weiss J. A. (2008). ‘FEBio: Finite Elements for Biomechanics. User’s Manual, Version 1.0.’ Available at http://mrl.sci.utah.edu/uploads/FEBio_um.pdf [Verified 14 January 2010].

Nigg B. M. , and Herzog W. (1999). ‘Biomechanics of the Musculo-skeletal System.’ (Wiley: New York.)

Nuijens, F. W. , and Zweers, G. A. (1997). Characters discriminating two seed husking mechanisms in finches (Fringillidae:Carduelinae) and estrildids (Passeridae: Estrildinae). Journal of Morphology 232, 1–33.
CrossRef |

Price, T. D. (1984a). The evolution of sexual size dimorphism in a population of Darwin’s finches. American Naturalist 123, 500–518.
CrossRef |

Price, T.D. (1984b). Sexual selection on body size, plumage and territory variables in a population of Darwin’s finches. Evolution 38, 327–341.
CrossRef |

Richmond, B. G. , Wright, B. W. , Grosse, I. , Dechow, P. C. , Ross, C. F. , Spencer, M. A. , and Strait, D. S. (2005). Finite-element analysis in functional morphology. Anatomical Record 283A, 259–274.
CrossRef |

Ross, C. F. (2005). Finite-element modeling in vertebrate biomechanics. Anatomical Record 283A, 253–258.
CrossRef |

Si H. (2008). TetGen: a quality tetrahedral mesh generator and three-dimensional Delaunay triangulator. Available at http://tetgen.berlios.de [Verified 14 January 2010].

Slatkin, M. (1984). Ecological causes of sexual dimorphism. Evolution 38, 622–630.
CrossRef |

van der Meij, M. A. A. , and Bout, R. G. (2004). Scaling of jaw muscle size and maximal bite force in finches. Journal of Experimental Biology 207, 2745–2753.
CrossRef | CAS | PubMed |

van der Meij, M. A. A. , and Bout, R. G. (2008). The relationship between shape of the skull and bite force in finches. Journal of Experimental Biology 211, 1668–1680.
CrossRef | PubMed |

Vanhooydonck, B. , Herrel, A. , Gabela, A. , and Podos, J. (2009). Wing shape variation in the medium ground finch (Geospiza fortis): an ecomorphological approach. Biological Journal of the Linnean Society 98, 129–138.
CrossRef |

Vlassenbroeck, J. , Dierick, M. , Masschaele, B. , Cnudde, V. , Van Hoorebeke, L. , and Jacobs, P. (2007). Software tools for quantification of X-ray microtomography at the UGCT. Nuclear Instruments & Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment 580, 442–445.
CrossRef | CAS |

Vogel S. (2003). ‘Comparative Biomechanics: Life’s Physical World.’ (Princeton University Press: Princeton, NJ.)

Yamada H. (1970). ‘Strength of Biological Materials.’ (Williams and Wilkins: Baltimore.)

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