Comparison of the eggshell mammillary microstructure of Galliformes and Anseriformes

In free-living species, the comparison of eggshell microstructure focus mainly on species representing extremely different reproductive strategies or with phylogenetically divergent taxa. The purpose of this study was to compare the microstructure of the mammillary layer in two bird orders (Galliformes and Anseriformes) representing phylogenetically close precocials, and to relate eggshell microstructure to female body weight and egg characteristics. The results showed significant differences between the studied orders. The mammillary layer in Galliformes had more knobs, and consequently, the percent coverage with them was higher than in Anseriformes. The size of individual knobs did not differ significantly between the orders. The obtained results are consistent with the reports that the ossification at the time of hatching is more advanced in Galliformes, so representatives of this order must resorb more calcium from the eggshell. Egg parameters and female body weight were correlated significantly negatively only with the number of knobs per mm² for Anseriformes and, to a lesser extent, for Galliformes, but not with the coverage per cent of knobs or the size of individual knobs. Only for the three largest species, which lay proportionally larger eggs than the other species studied, the number of knobs per mm² was significantly lower, but no difference was found in the per cent coverage of knobs. Our results showed that order was a stronger predictor of microstructure than species. Different ossification patterns of the embryonic skeletal system described in these two orders could be the explanation for the observed differences in eggshell microstructure; however, more research is needed.