Hermann Rahn

The Avian Egg: Water Vapor Conductance, Shell Thickness, and Functional Pore Area

Gas exchange in the avian embryo has been shown to be dependent on, and limited by, the diffusive properties of gases across the resistance offered by the shell and shell membranes ( Wangensteen and Rahn 1970-71). For simplicity, we shall use the term “shell’ to denote the entire barrier to diffusion between the interior of the egg and the environment. It has also been shown that under normal conditions the diffusion of water vapor across the egg shell approximates the diffusion equations set forth for ideal gases (Paganelli et al. 1971) and that the weight loss in eggs is almost entirely due to diffusion of water through the shell (Romanoff and Romanoff 1949). The diffusive rate of water loss from eggs is:

Relation of Avian Egg Weight to Body Weight

JUST over 50 years ago Heinroth (1922) published the first extensive list of egg weights and adult body weights for 427 species of birds. Since then additional data have appeared and the most extensive list can now be found in Schonwetters (1960-72) monumental description of bird eggs. With these as a major backlog, and data for the Fringillidae (Amadon 1943); the Sphenisciformes, Anseriformes, and Procellariiformes (Lack 1968); and Falconiformes (Mebs 1964), more than 800 egg weightbody weight correlates are now available. Our endeavor has been to describe the relationship between egg weight and body weight in mathematical terms that are amenable to further refinements when additional data became available. We have not been concerned with explaining the relationship as Lack (1968) has done in his elegant analysis, but have rather attempted to find out what common principles might emerge from this particular relationship. Our analytical approach is basically the same as suggested originally by Huxley (1923-24), namely to plot log (egg weight) against log (body weight) and to derive a regression equation that expresses egg weight, W, as a function of body weight, B, raised to a power: W = aBb. The additional data now available allow one to obtain individual regression equations for many orders and families. As Amadon (1943) had anticipated in his review of Huxleys analysis and as Lack (1968) has recently shown for many orders and families, each group of related birds has its characteristic proportionality constant, a. On the other hand, our analysis indicates that the power, b, is most likely the same for all groups, inamely 0.675. In addition, the relation of incubation time to body weight is derived (Rahn and Ar 1974); incubation time is shown to be proportional to body weight raised to the 0.166 power. Thus a 10-fold increase in body weight is in general associated with a 4.73-fold increase in egg weight and a 1.47-fold increase in incubation time.

Aquatic gas exchange: Theory

The gas exchange of water-breathing animals is described in terms of the O2 and CO2 tensions. Equations are developed which describe the gill ventilation requirements in terms of gill O2 and CO2 tensions, exchange ratio, metabolic rate, and inspired gas tensions. When these are plotted on an O2CO2 tension diagram, they allow one to predict the various limitations of gas exchange in water and to compare these directly with the limitations imposed when gas or air is breathed.

Respiratory gas exchange by the avian embryo

Abstract The mechanism of gas exchange by an avian embryo is discussed in terms of the embryos changing metabolic rate, a constant egg shell gas permeability and the changing gas tensions inside the shell. Since gas transport across the shell is by diffusion, equations are developed which predict the gas composition in the air cell of the egg as well as the rate of exchange of O2 CO2 and water vapor. PO2 and PCO2 values were obtained from the air cells of chick embryos of various ages, and when these results are considered together with metabolic rate and measured egg shell permeability data they are found to be consistent with the hypothesis that gas exchange by the avian embryo is limited by diffusion through its porous shell. The implications of this are discussed in terms of water vapor loss from the egg during incubation, development of the embryos blood buffer system and the importance of the permeability of the egg shell to embryo survival.

The avian egg: air-cell gas tension, metabolism and incubation time

Abstract The O 2 and CO 2 tensions in the air cell of incubating eggs just pripr to the pipping stage were analyzed in nine species of birds. These appear to be very similar and average 104 and 37 torr, respectively, for O 2 and CO 2 . The oxygen conductance of the egg shell for each species was calculated from the water vapor conductance previously established, which allows one to calculate the oxygen consumption of the egg as the product of O 2 conductance and ΔP O 2 , across the egg shell. The oxygen uptake of the eggs at this stage of development is proportional to the egg weight raised to the power O.78. A previously derived relationship shows that the incubation time is proportional to the egg weight raised to the power O.22. Combining these two weight functions, one obtains a general relationship between metabolic rate and incubation time; namely in different species of birds that have the same egg weight the natural incubation period is inversely related to the metabolic rate or the egg shell gas conductance.

Calories, Water, Lipid and Yolk in Avian Eggs

-The contents of fresh eggs of altricial, semi-altricial, semi-precocial, and precocial birds were compared with values for yolk content gathered from the literature. The continuum of developmental maturity at hatching from altricial to precocial eggs is correlated with an increase in yolk, solids, and caloric contents (per gram wet mass) and a decrease in water content. The proportion of lipid in dry matter and caloric content per gram dry mass does not vary significantly among the four developmental groups. The progressively higher caloric content on a wet mass basis with increasing precocity is a result of a larger solid content and lower water content, rather than variation in caloric value of the dry matter itself. Comparison of values within the same developmental group discloses no significant correlation between relative yolk content and egg mass. The total amount of calories in eggs is more importantly determined by egg mass than by yolk content. A freshly laid avian egg contains the necessary nutrients and raw materials that will eventually produce a hatchling. Although it has been recognized since the study of Tarchanoff (1884) that the initial proportions of yolk and albumen differ considerably in the eggs of altricial and precocial birds, the relations between the energetic and chemical contents of eggs and developmental mode are not completely understood (see Ricklefs 1974, Kendeigh et al. 1977, for review). We report here new values for lipid, water, and caloric contents of eggs of precocial, semiprecocial, semi-altricial, and altricial species. These results are combined with previously published caloric and yolk contents to provide an overview of the variation among these values as a function of embryonic maturity at hatching.

Diffusion of gases across the shell of the hen's egg☆

Abstract Chicken egg shells, with the inner shell membrane removed, have been studied to determine their diffusive permeability to oxygen. This was done by measuring the oxygen flux through a section of the shell across which there was initially a difference in oxygen partial pressure but not in total gas pressure. The measured oxygen permeabilities, corrected to 37 °C, averaged 3.2 × 10 −6 cm 3 STP · sec −1 · cm −2 · mm Hg −1 , and did not appear to change with incubation age. From this value the permeabilities for carbon dioxide and water vapor were calculated to be 2.5 × 10 −6 and 4.0 × 10 −6 cm 3 STP · sec −1 · cm −2 · mm Hg −1 , respectively. Also from the oxygen permeability data we were able to estimate a total pore area of 2.3 mm 2 and an average pore diameter of 17 micra. The latter figure agrees well with direct observations. Finally, calculations were made which show that the inner and outer shell membranes, if dry, are a negligible barrier to gas diffusion compared to the shell. This means that the egg shell, the porosity of which is set when the egg is laid, is of prime importance in determining oxygen uptake and carbon dioxide and water vapor loss by the developing embryo.

The Avian Egg: Mass and Strength

In a series of recent works, attention has been paid to the functional properties of the avian eggshell: water vapor and respiratory gas conductances, water loss, metabolic rate and incubation time-all these major physiological characteristics of eggs may be closely and intimately related to egg mass, which, in turn, is allometrically related to eggshell structural properties such as thickness, porosity, mass, density and surface area (Wangensteen 1972, Ar et al. 1974, Rahn and Ar 1974, Paganelli et al. 1974, Rahn et al. 1974, Ar and Rahn 1978). These structural and functional relations of bird eggs reveal some variables of importance to the physiology of the embryo, including the gradient in water vapor pressure between egg and nest, the fractional water loss constant, the constancy of gas composition in the air cell, and total oxygen consumption per gram egg during incubation. The ability to hatch successfully is the outcome of a delicate equilibrium among several factors, some of which are inherited in the structure and function of the egg itself, while others are either imposed on the egg by the environment or controlled by the incubating parents. The eggshell provides the egg with an external “skeletal” support that utilizes the dome principle to obtain strength with economy in building material and without need for internal supporting posts. It must satisfy conflicting demands: On the one hand, it must be strong enough to support the incubating bird’ s mass plus the egg’ s own mass and to protect and prevent it from being crushed during incubation. On the other hand, it must not be too strong for the hatchling to break its way out, a problem that may become crucial in bigger eggs where shell thickness increases and the specific metabolic rate of the embryo decreases (Paganelli et al. 1974, Rahn et al. 1974). The ratio of total shell pore area to shell thickness is largely evolved to meet the forthcoming metabolic demands of the growing embryo, which in turn, are a function of mass (Ar et al. 1974). Adding to this the belief that any saving in building material should benefit the laying bird, we hypothesize that eggshell strength should be related to egg mass. Eggshells have been subjected to numerous strength tests in the past. They have been crushed, cracked, pierced, snapped, compressed, bent and deformed in various ways. Force has been applied inwards and outwards, on whole eggs and on pieces of shells. Various methods and instrumentations have been used (Brooks 1960, Tyler and Geake 1963, 1964, Tyler and Coundon 1965, Tyler and Thomas 1966, Carter 1971, Scott et al. 1971). However, most of these studies were designed to establish practical “quality” criteria as they are understood by the poultry industry (Petersen 1965). As a result, most of the research has been concentrated on domestic hen (Gallus domesticus) eggs and little has been published on other species (Romanoff and Romanoff 1949, Brooks 1960, Tyler 1969a, Radcliffe 1970, Peakall et al. 1973). Strength has been correlated with factors such as calcium diet, diet in general, insecticides, shell microstructure, specific gravity, incubation period and shape index (e.g., Sluka et al. 1967, Wells 1967a, b, Vanderstoep and Richards 1969, Connor and Arnold 1972, King and Robinson 1972, Cooke 1973, Carter 1976). However, Tyler (196913) clearly demonstrated that the main factor affecting strength in hen eggs is shell thickness, where strength is a function of shell thickness squared. It is our purpose here to describe how egg strength scales with mass. We do not try to explain the relationship, but rather attempt to define the common principles that emerge from this relationship.

Air breathing of the garfish (Lepisosteus osseus)

Abstract The North American ganoid fish, Lepisosteus osseus, is a facultative air breather, progressing from complete dependence on the gills at low water temperature to utilization of the lung at warm temperatures for extraction of about 70–80% of its total oxygen requirements. CO2 elimination through the lungs is 0% at low temperatures, up to 8% at 20–25 °C. Total lung volume averages 8–10% of the body weight and expired tidal volume is about 40% of the total lung volume. Expiration is accomplished by hydrostatic pressure while inspiration involves a buccal pump. The estimated pulmonary surface area of 800–1000 cm2 · ml O2−1 · min−1 is low when compared to other vertebrates and suggests that the lung is an incompletely developed respiratory organ. Blood pH and PCO2 values indicate that the animal relies strictly on ventilation of the gills for O2 and CO2 exchange at low temperatures. As the temperature increases, the lung becomes the predominant organ for oxygen extraction, the blood PCO2-increases, plasma bicarbonate does not rise and the plasma pH falls.

Respiratory gas exchange of high altitude adapted chick embryos

Abstract Gas exchange by embryos from chickens acclimatized to an altitude of 3800 m (p B = 480 torr) was studied in order to ascertain the nature of the altitude adaptation in this species. The P O 2 and P CO 2 differences across the egg shell were measured and found to be less than the values previously reported for sea-level eggs by about a factor of two. Further measurements of embryonic oxygen consumption ( O 2 ) and shell conductivity to oxygen (G O 2 ) indicated that, compared to eggs at sea level, O 2 was reduced by a factor of 0.58 while G O 2 was increased only by a factor of 1.07 in the high-altitude eggs. These independent measurements predict the ΔP O 2 across the egg shell of the high-altitude eggs to be only 0.54 times that of sea-level eggs; the directly measured factor was 0.53. The authors conclude that at high altitude a major adaptation of the chick embryo is a reduced metabolism which decreases the ΔP O 2 across the egg shell since its gas conductivity remains essentially unchanged.