Timing of ossification in duck, quail, and zebra finch: intraspecific variation, heterochronies, and life history evolution.

Skeletogenic heterochronies have gained much attention in comparative developmental biology. The temporal appearance of mineralized individual bones in a species - the species ossification sequence - is an excellent marker in this kind of study. Several publications describe interspecific variation, but only very few detail intraspecific variation. In this study, we describe and analyze the temporal order of ossification of skeletal elements in the zebra finch, Taeniopygia guttata, the Japanese quail, Coturnix coturnix japonica, and the White Pekin duck, a domestic race of the mallard Anas platyrhynchos, and explore patterns of intraspecific variation in these events. The overall sequences were found to be conserved. In the duck, variability is present in the relative timing of ossification in the occipital, the basisphenoid and the otic regions of the skull and the phalanges in the postcranium. This variation appears generally in close temporal proximity. Comparison with previously published data shows differences in ossification sequence in the skull, the feet, and the pelvis in the duck, and especially the pelvis in the quail. This clearly documents variability among different breeds.