Modification of growth and development of muscles of poultry

Growth curves of selected and unselected lines of broilers and Japanese quail show that chicks respond to selection for 8-week or 4-week body mass, respectively, by increasing the exponential growth rate during the first 2 weeks after hatching. Comparative studies indicate that growth rate varies among species of birds according to adult body mass, developmental maturity of the chick at hatching, and the postnatal growth increments of the skeletal muscles of the legs. Differentiation of tissues leading to mature function apparently precludes embryonic function and decreases proliferation and growth. Precocial species, such as the domestic fowl, grow only one-quarter as fast, on average, as altricial species of the same size, whose chicks are less mature at hatching and depend more on parental care for food, warmth, and protection. Among precocial species, those whose legs have the smallest postnatal growth increments grow most rapidly overall. The domestic fowl is among the slowest growing of precocial species. Although diet quality may limit growth rate, this should not be a factor in selection programs, because high quality diets can be provided. Rate of food assimilation apparently also is limiting, but it can be increased experimentally by force feeding and presumably is sufficiently selectable so as not to limit breeding programs designed to increase growth rate. The response of broilers and quail to selection of body mass apparently affects the rate of proliferation of skeletal muscles during the early posthatching period. Muscle quality is not affected. There is inconclusive evidence of a shift in muscle cell nuclei from differentiated to proliferative pools in selected lines. Furthermore, selected broilers apparently are less capable than unselected chicks of generating body heat, which requires functional skeletal muscle during the first week posthatching. Growth performance of broilers might be increased beyond present levels by selecting directly for reduced or delayed maturity (e.g., thermogenesis, flight) of chicks during the first 2 weeks and by selecting smaller legs in adult birds. Any attempt to formulate a selection program on these ideas would require additional basic research and might be thwarted by economic considerations or by offsetting selection caused by phenotypic responses that strain certain functional relationships in the growing chick.

Rates of oxygen consumption in four species of seabird at Palmer Station, Antarctic peninsula

1. We determined standard metabolic rates (SMR) of wild-caught adults of the Adelie penguin, southern giant fulmar, blue-eyed shag, and South Polar skua at Palmer Station, Antarctica, during January and February 1981. Oxygen consumption was measured volumetrically in a closed system at temperatures between 2 and 12 degrees C. 2. Mean SMR varied between 0.82 l O2/kg per hr for male fulmars and 1.30 l O2/kg per hr for unsexed adult skuas. Values were 174-198% of those predicted by the Lasiewski-Dawson equation for nonpasserines. 3. The SMR of the Adelie penguin was considerably higher than that reported in other studies and higher than most values for other species of penguins. 4. Our measurements of oxygen consumption agree with some estimates of metabolism based upon loss of mass by fasting birds during incubation.

Development of homeothermy in the diving petrels Pelecanoides urinatrix exsul and P. georgicus, and the Antarctic prion Pachyptila desolata

Body temperatures of South Georgia diving petrel (P. georgicus) chicks increased from about 37.5 degrees C at hatching to between 38.5 and 39 degrees C within two weeks. Temperatures of common diving petrel P. u. exsul chicks averaged 38.8 degrees C after two weeks of age. Burrow temperatures varied between 5 and 10 degrees C. Measurements of oxygen consumption and body temperature indicated that chicks achieve effective endothermy at 5 degrees C after 9 days in P. u. exsul, 5-6 days in P. georgicus, and 0 days in the Antarctic prion (Pachyptila desolata). The maximum mass-specific, cold-induced oxygen consumption of small chicks that we could measure with our apparatus (ca. 5-6 cc O2/g per hr) was achieved at 5-6 days in P. u. exsul, 3 days in P. georgicus, and 0 days in P. desolata. Mass-specific thermal conductance decreased with age and body size in all 3 species, but was highest in P. u. exsul and lowest in P. desolata. Conductance was similar at the age of effective endothermy in all 3 species (ca. 3 J/g per hr per degrees C). The period required for the development of endothermy is related to age-specific changes in both conductance and capacity for heat production and it closely parallels the length of the brooding period. It is suggested that the length of the period of thermal dependence of the chick is related to the distance between feeding areas and the nesting site.

Clutch Size in Birds: Outcome of Opposing Predator and Prey Adaptations

A model is proposed to explain clutch size in birds as the outcome of the interaction between predatory adaptations of birds to increase their feeding efficiency and adaptations of their food resources to avoid predation. Variations in clutch size are consistent with the model. A modification that incorporates the seasonality of food resources is also discussed.