Beak and skull shapes of human commensal and non-commensal house sparrows Passer domesticus
1 Department of Biology, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran
2 Current address: Evolutionary Ecology Associate Research Unit (CSIC), Natural History Museum of Barcelona, Passeig Picasso s/n, Barcelona 08003, Spain
3 Natural History Museum, University of Oslo, Blindern, P. O. Box 1172, Oslo N-0318, Norway
4 Institute of Pharmacy and Molecular Biotechnology, Department of Biology, Im Neuenheimer Feld 364, Heidelberg D-69120, Germany
5 Catalan Institute of Paleontology, Campus de la UAB, Cerdanyola del Vallès 08193, Spain
6 Naturalis Biodiversity Center, Vertebrate Department, Darwinweg 2/PO Box 9517, RA Leiden 2300, Netherlands
7 Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biological Sciences, University of Oslo, Blindern, P. O. Box 1066, Oslo N-0316, Norway
BMC Evolutionary Biology 2013, 13:200 doi:10.1186/1471-2148-13-200Published: 17 September 2013
The granivorous house sparrow Passer domesticus is thought to have developed its commensal relationship with humans with the rise of agriculture in the Middle East some 10,000 years ago, and to have expanded with the spread of agriculture in Eurasia during the last few thousand years. One subspecies, P. d. bactrianus, residing in Central Asia, has apparently maintained the ancestral ecology, however. This subspecies is not associated with human settlements; it is migratory and lives in natural grass- and wetland habitats feeding on wild grass seeds. It is well documented that the agricultural revolution was associated with an increase in grain size and changes in seed structure in cultivated cereals, the preferred food source of commensal house sparrow. Accordingly, we hypothesize that correlated changes may have occurred in beak and skull morphology as adaptive responses to the change in diet. Here, we test this hypothesis by comparing the skull shapes of 101 house sparrows from Iran, belonging to five different subspecies, including the non-commensal P. d. bactrianus, using geometric morphometrics.
The various commensal house sparrow subspecies share subtle but consistent skeletal features that differ significantly from those of the non-commensal P. d. bactrianus. Although there is a marked overall size allometry in the data set, the shape difference between the ecologically differentiated sparrows cannot be explained by differences in size alone. Relative to the size allometry commensal house sparrows exhibit a skull shape consistent with accelerated development (heterochrony), resulting in a more robust facial cranium and a larger, more pointed beak.
The difference in skull shape and robustness of the beak between commensal and non-commensal house sparrows is consistent with adaptations to process the larger and rachis encapsulated seeds of domesticated cereals among human associated populations.