Reduction of pore area of the avian eggshell as an adaptation to altitude

Abstract

Standard measurements of water vapor conductance in units of mg of H20 * day-l * torr-1 (SI equivalent is mg of H20* day-l * pascal-') of fresh eggs of the red-winged blackbird (Agelaius phoeniceus) and the native chicken of India (Gallus gallus) collected at altitude are significantly less than those of eggs of the same species collected near sea level. This decrease is caused by a reduction of the total effective pore area of the eggshell at altitude. It appears to be proportional to the reduction in barometric pressure and the simultaneous increase in the diffusion coefficient of water vapor. Thus, reduction in pore area offsets increased diffusivity at altitude, and water vapor loss through the eggshell at any altitude remains the same as at sea level. The data suggest a structural adaptation of the shell to altered diffusivity of gases at altitude in order to prevent excessive water loss of eggs during natural incubation.During normal incubation bird eggs continuously lose water. The amount lost is proportional to egg weight and inversely proportional to incubation period (1-3) and appears to be regulated in such a fashion that the typical egg loses about 15% of its initial weight prior to pipping. The volume of water lost is replaced by an equivalent gas volume, the air cell, which provides gas for initial inflation of the lungs and the rebreathing maneuvers prior to pipping of the eggshell (4). The water vapor transport across the eggshell takes place by molecular diffusion (5), and shell permeability is determined by the pore geometry (area and length), which is fixed when an eggshell is formed. Thus, a sea level egg transferred to altitude, under otherwise similar nest conditions, should lose water more rapidly in direct proportion to the increase of the diffusion coefficient of water vapor. This was demonstrated by Paganelli et al. (6) by placing eggs in a desiccator and exposing them to various simulated altitudes.Reduction of pore area of the shell would prevent excessive dehydration at altitude. Such a reduction was observed by Wangensteen et al. (7), who showed that the water vapor conductance of chicken eggs acclimated for many generations at an altitude of 3800 m was reduced compared with their sea level controls. More recently Packard et al. (8) of freshly collected eggs at altitude and sea level and measurements of shell thickness we have calculated the total effective pore area of the eggshell. Our results indicate that the shell pore area is reduced in approximate proportion to the reduction in barometric pressure and in proportion to the concomitant increase of the diffusion coefficient of water vapor.In such a case the water vapor flux at altitude (for a given partial pressure difference across the eggshell) remains the same as at sea level and, hence, excessive dehydration due to the increased diffusivity of water vapor at altitude is prevented. METHODS Fresh eggs of the red-winged blackbird (Agelaius phoeniceus)were collected at an altitude of 2400 m in the Rocky Mountains near ...

Gas Exchange of Avian Eggs with Special Reference to Turkey Eggs

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Evolution of eggshell structure during rapid range expansion in a passerine bird

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