Amos Ar

The Avian Egg: Incubation Time and Water Loss

The large difference in incubation time among bird eggs, ranging from a minimum of 11 days to nearly 90, has aroused mans interest since antiquity. In her critical review of the history of our knowledge of incubation periods, Margaret M. Nice (1954) wrote, The people who have been concerned with incubation periods fall into three groups: the guessers, the copyists, the investigators. Guessers came first and these have been busily quoted since Aristotle for more than 20 centuries. It was not until Evans (1891) and Heinroth (1922) made their own observations that reliable data began to accumulate. In spite of the inaccuracies since Aristotles time, and the many exceptions which are now well recognized, there is an obvious general correlation between egg weight and incubation period. These have been presented by Heinroth (1922), Needham (1931), and Worth (1940) in graphic form and were reinvestigated in this presentation on the basis of newer information in the literature. It is of interest to note in retrospect that all these correlations are essentially similar, that the standard error of estimates is large, and that there are many exceptions. This merely illustrates that the many factors which determine the incubation period are not understood. For these reasons we analyzed other correlates of egg size such as the gas conductance of the egg shell and particularly the water loss properties of eggs, problems which Heinroth had already mentioned some 50 years ago in his classical treatise on incubation time. On the basis of water vapor conductivity measurements of the egg shell previously presented (Ar et al. 1974), the daily weight losses of eggs during natural incubation reported by Drent (1970), and the reported incubation periods, one is now able to derive new relationships which apply to eggs in general. These indicate that incubation time for a given egg weight is inversely porportional to the water vapor conductance of the egg shell. Furthermore, during natural incubation all eggs, regardless of size, lose approximately 18% of their initial weight and the mean water vapor pressure difference between the egg and the microclimate of the nest is 35 torr.

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.

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.

Analysis of chorioallantoic gas exchange in the chick embryo

To analyze the gas exchange mechanisms in the chorioallantois, PO2 and PCO2 were measured in air cell gas, in the allantoic artery and in the allantoic vein in chicken embryos on the 16th day of incubation. In addition, the O2 dissociation curve of blood, and O2 uptake and CO2 output of the embryo were determined. From O2 measurements performed in hypoxia (FIO2=0.14), normoxia and hyperoxia (FIO2=0.67), it was concluded that there was a sizable functional arterio-venous shunt amounting to 10-15% of the total chorioallantoic blood flow and that the diffusing capacity of the air cell-blood barrier for O2 was about 7 microliter . min-1. Torr-1. The CO2 measurements are in agreement with the model. In hypoxia, the air cell-blood transfer of O2 was markedly diffusion limited. The diffusion limitation effect was slight in normoxia, and not detectable in hyperoxia. At all oxygenation levels the effect of the shunt on blood arterialization was marked, particularly so in hyperoxia where the air cell-arterialized blood PO2 difference averaged 180 Torr.

Repetitive and simultaneous sampling from the air cell and blood vessels in the chick embryo

A catheterization procedure is described for repeated and simultaneous removal of gas samples from the air cell and of blood from the allantoic vein and artery of the developing chick embryo. The partial pressures of O2 and CO2 determined using this method do not differ from those obtained by conventional methods.

The independent effects of atmospheric pressure and oxygen partial pressure on gas exchange of the chicken embryo

CO2 production and air cell PCO2 were continuously measured during late development in the chicken egg while acutely exposed from one to three hours to various O2 concentrations ranging from 11 to 39%. A small but significant increase in metabolism, ca. 8%, was found when O2 concentration was above normal values, while a reduction to 70% was observed when O2 concentrations were below normal, and fell to 50% when maintained for three hours. These values were also compared with metabolic rates reported by Lokhorst and Romijn (1965, 1967)) who incubated eggs continuously at reduced O2 concentrations as well as under reduced barometric pressure, and showed that at the same ambient PO2 the metabolism was significantly higher in the eggs at reduced barometric pressure. We attribute this difference to the increased diffusion coefficient of O2 which is inversely related to the barometric pressure. It illustrates that the ambient partial pressure of O2 and ambient atmospheric pressure exert an independent effect upon gas exchange of the avian embryo.

Heart rate in developing ostrich embryos

1. A non-invasive condenser microphone was used to detect cardiogenic, acoustic pressure changes (acoustocardiogram, ACG) over the eggshell in order to determine embryonic heart rate (HR) of ostriches in a commercial hatchery. 2. HR measured for 36 eggs at 36.3C was maintained at about 185 bpm during the middle stage of development (days 19 to 23) and then decreased with embryonic development. 3. The daily changes in HR were not related to egg mass, but HR of high water vapour conductance (GspH2O) eggs was found to decrease less during the last stages of incubation relative to low and medium GspH2O groups. 4. The averaged HR at 80% of incubation period was close to the value predicted from the allometric equation determined previously for embryos of domesticated precocial birds.

Gas tension profile of the lung of the viper, vipera xanthina palestinae

Oxygen and carbon dioxide concentrations along the lung of five awake, resting Palestine vipers were continuously measured by mass spectrometry. Ventilatory volumes, body wall movements and heart rate were also measured. In the anterior part of the faveolar (respiratory) lung, oxygen and carbon dioxide concentrations returned to within 1% of inspired composition with each inspiration. Between breaths, changes of 0.5-2% in gas concentrations were seen in the faveolar region but practically no changes occurred in the caudal, non-respiratory lung (air sac) where mean oxygen and carbon dioxide concentrations of 16.4% and 2.5% respectively were recorded. The respiratory exchange ratio dropped from near 1.5 in the anterior faveolar region to zero in the transition zone to the air sac. Instantaneous R values declined with breath-holding time in each location along the length of the lung. Gas exchange appears greatest in the posterior faveolar region near the heart and there is evidence of cardiogenic gas mixing in this region. The posterior air sac may either passively respond to air movements in the anterior lung or it may participate in ventilation. During periods of extended breath-holding (10-15 min) pronounced body wall movements were seen but there was no air flow from the mouth and gas exchange continued in the lung with rapidly decreasing R.