Clas Lilja

A comparative study of growth, skeletal development and eggshell composition in some species of birds

Some studies of birds suggest that the development of the skeleton may invoke a constraint on the rate of postnatal growth. Other studies have shown that the eggshell is the major source of calcium for skeletal development of the embryo. To test whether avian growth rate is indeed associated with different patterns of skeletal development, we compared the degree of skeletal ossification of the long bones of the wing and the leg of one slowly growing precocial species (quail Coturnix japonica ) with that of two rapidly growing altricial species (starling Sturnus vulgaris and fieldfare Turdus pilaris ). The degree of skeletal ossification of the long bones of the wings and legs of lines of quails that had undergone long-term selection for high- and low-growth rate, respectively, also was compared with a non-selected control line. Next, the fine structure of the inner eggshell surface (mammillary layer) of both pre- and post-incubated eggs, i.e. before and after embryonic development/calcium removal was compared. The data show that the skeleton of the more rapidly growing species and lines was less ossified than that of the more slowly growing ones. This difference appeared to be associated with different rates of calcium removal from the eggshell. Removal was more extensive in eggs of quail than in eggs of starling and fieldfare, i.e. more extensive in shells with a high number of mammillary tips per unit of surface area than in shells with a lower number. It is therefore concluded that growth rate is of fundamental importance for the pattern of skeletal development. Moreover, the mammillary density varies between different bird species, it is suggested, in order to support the different rates of calcium removal by developing embryos.

Eggshell structure, mode of development and growth rate in birds.

The shell of an egg contributes to successful embryogenesis in many ways, such as through protection, respiration and water exchange. The shell is also the major source of calcium for the development of high-calcium consuming organs, e.g. the skeleton, muscles and brain. Some studies show, moreover, that growth rate may play a fundamental role in the pattern of skeletal development in birds: the faster the growth the less ossified the skeleton is at hatching. We predicted, therefore, that slow (precocial) and fast (altricial) growing bird species should lay eggs encased in shells with different structures adapted to support different rates of calcium removal by developing embryos. We tested this prediction by comparing the fine structure of the inner eggshell surface (mammillary layer) from 36 bird species belonging to 18 orders ranging from Struthioniformes to Passeriformes. Using scanning electron microscopy, we compared the mammillary layer of both non-incubated eggs and eggs at the time of hatching, i.e., before and after embryonic development and the accompanying calcium removal. The results were consistent with the prediction, i.e., the number of mammillary tips per unit of surface area was associated with mode of development and growth rate. The number was higher, and calcium removal was also more extensive, in shells from precocial bird species than in shells from altricial bird species.

A comparative study of embryonic development of Japanese quailselected for different patterns of postnatal growth

Patterns of early embryonic development have traditionally been viewed as invariant within vertebrate taxa. It has been argued that the specific differences which are found arise during the later stages of development. These differences may be a result of allometry, heterochrony or changes in relative growth rates. To test whether early embryonic development is indeed invariant, or whether selection of adult characteristics can alter embryonic growth, we compared embryonic development in birds selected for different patterns of postnatal growth. Using quail lines selected for high and low body mass, we compared somite formation, and muscle and feather development. We obtained data that showed changes in the rate of myotome formation in the brachial somites which contribute to muscle formation in the limbs and thorax. We think these observations are connected with intraspecific changes in adult morphology, ie., breast muscle size. Our findings suggest that selection for late ontogenetic/adult stages affects early embryonic development.