Length Of The Incubation Period

Divergence in the length of the incubation period among species occurs late in embryogenesis when different species need different lengths of time to pass through maturational stages. Indeed, only precocial species exhibit an extended plateau in oxygen consumption immediately prior to hatching[10] as well as increased thyroid and adrenal hormones in circulation.1-11'12-1 It has been suggested that both the plateau in oxygen uptake[13] and the thyroid and adrenal hormones[12] are associated with the maturational processes at hatching.

The availability of oxygen may constrain growth during development1-14-1 when gas exchange is limited by the gas conductance1-10-1 and the plateau stage. Ar and Rahn[15] suggested that eggshell conductance, the functional characteristic of an eggshell, might also be the determinant of the length of the incubation period and embryonic growth as well. Three conditions are determined by the eggshell conductance and were proposed to induce hatching. First, the incubating egg should lose approximately 15% of its initial mass as water vapor. Second, the fractional concentration of carbon dioxide in the air space of eggs should have increased from 0.25% to

6%. Third, the relative fractional concentration of oxygen should have declined from 20.9% to 14%. Eggshell conductance establishes these conditions at a precise time in development that initiates tissue maturation and the hatching response, thus determining the length of the developmental period. Therefore, despite differences in egg size and incubation periods that may exist among species, gas tensions determined by the ratio of metabolic rate to diffusive gas conductance across the pores of the eggshell may be the primary determinant of the length of the incubation period. It has been postulated that the gas tensions in the airspace of the egg may constitute an adequate stimulus to cause pipping and terminate the embryonic developmental period.[16] These differences may exist to ensure that tissues mature properly during the plateau stage in oxygen consumption at the termination of embryonic development by ensuring the proper length of the incubation period.

Selection of poultry species for improved reproduction results in a decrease in egg weight and eggshell conductance and a prolonging of the length of the incubation period.[17'18] Conversely, selection for increased adult growth increases egg weight, decreases eggshell conductance, and prolongs the length of the developmental period.[17'19] When growth was measured relative to egg size, selection for improved reproduction increased the weight of hatchlings relative to egg weight, but selection for growth did not. Thus, within a species, plasticity in embryonic growth exists but may also have impact upon the survival rates of embryos in the eggs with varied lengths of incubation periods.

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