Gordon memorial lecture. Physics and physiology of incubation.

This article was downloaded by: [The University Of Melbourne Libraries] On: 15 September 2013, At: 08:01 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

British Poultry Science Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/cbps20

Physics and physiology of incubation A. H. J. Visschedijk

a

a

Rijksuniversiteit te Utrecht, Yalelaan 2, Utrecht, 3584 CM, Netherlands Published online: 08 Nov 2007.

To cite this article: A. H. J. Visschedijk (1991) Physics and physiology of incubation , British Poultry Science, 32:1, 3-20, DOI: 10.1080/00071669108417323 To link to this article: http://dx.doi.org/10.1080/00071669108417323

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/ page/terms-and-conditions

British Poultry Science (1991) 32: 3-20

GORDON MEMORIAL LECTURE PHYSICS AND PHYSIOLOGY OF INCUBATION1

Downloaded by [The University Of Melbourne Libraries] at 08:01 15 September 2013

A. H. J. VISSCHEDIJK Rijksuniversiteit te Utrecht, Yalelaan 2, 3584 CM Utrecht, Netherlands

Abstract 1. The earliest mention of artificial incubation occurs in Aristotle's Historia Animalium written in the 4th century BC. A brief survey of the history of incubation is given from that time to the present. 2. Artificial incubation is also practised by birds belonging to the family of the Megapodes: the Brush Turkey and the Mallee Fowl build a mound and maintain the required temperature of the eggs laid in it. 3. The importance of functional eggshell porosity and incubator ventilation rate for maintaining optimal gas tensions in the embryonic medium (the gas space below the shell) is discussed. 4. Among the early scientific studies (reviewed by Landauer, Lundy, Freeman) particular attention is paid to Barott's (1937) systematic work on temperature, relative humidity and oxygen concentration. 5. The requirements of the embryo with regard to temperature, humidity and gaseous environment are defined. The importance of using gas tensions instead of gas concentrations is once again emphasised. 6. The problems of incubation at high altitude are explained and a successful method for hatching eggs at any terrestrial altitude is described. 7. Although hens can be selected for the functional porosity of their eggs, the procedure does not offer any worthwhile advantages. 8. If functional eggshell porosity and embryonic oxygen uptake are known, then optimal incubator ventilation rate can be predicted when a given optimum gas space oxygen tension is assumed. HISTORICAL SURVEY

The history of incubation has been extensively reviewed by Landauer (1967). So me of the other information in this survey is taken from Romijn (1978) and Kuiper (1959). The earliest reference to artificial incubation occurs in Aristotle's Historia 1

This lecture, the eighth in the series was delivered at the School of Pharmacy, University of London, on 5th April 1990. 3


4

A. H. J. VISSCHEDIJK

Animalium, written in the 4th century BC. Aristotle reported that eggs in Egypt were hatched in the ground by being buried in dung heaps. The Roman Emperor, Hadrian, who visited Egypt around the year 130, wrote to his brother-in-law about this technique as follows: "I wish them no worse than that they should feed on their own chickens, and how foully they hatch them I am ashamed to say". A more hygienic method was practised in China as early as 200 BC. In the lower part of the incubator, called the k'ang, a low fire warmed the upper bowl-shaped part. The bottom of the latter was covered with a layer of broken straw. Over this straw was a 5 to 7 cm deep layer of warmed rice husks. Then followed alternate layers of eggs and husks until the incubator contained 8000 to 10,000 eggs with another layer of husks and straw on top. When after a few days the temperature became too low the eggs were transferred to a second k'ang. This to-and-fro shift went on for about a fortnight, before the eggs were partitioned over both k'angs. At the time of hatching the eggs were spread out in a single layer and covered with husks. Later the Egyptians built huge incubators which were described by Reaumur in 1751. The low two-storeyed buildings had a corridor with incubating ovens on both sides. Glowing bean straw served as a source of heat in the upper compartments, heating the eggs in the lower compartments by radiation from above. After 7 d the amount of heating was reduced, and discontinued after l i d . On the 13th day of incubation half of the eggs were moved to the upper chamber. Temperature and ventilation were regulated by opening and closing the holes to the outside. The temperature was regularly checked by placing eggs close to the eyeball. The eggs were turned three times a day during the first week, and twice daily thereafter. Hatchability seems to have been 80 to 90% of fertile eggs. The production of chicks was a fiscal privilege accorded to a limited number of Egyptians. This was resented by the poor and was abolished by Sultan Mohamed ben-Kelaoum in 1316 AD. The Romans had curious ways of incubating eggs. Pliny, in the first century AD, tells us about a notable drunkard of Syracuse who kept drinking until the eggs he covered with his body were hatched. Pliny concluded that a man or a woman might hatch eggs with only the heat of the body. The Empress, Livia Augustus, being pregnant, was desirous to know whether she would give birth to a boy or a girl. She found out that it would be a boy because a cock chicken hatched from the egg that she incubated for three weeks under her bosom. In 1644 Ferdinand II invited two Egyptians to build incubating ovens in Florence. Unfortunately, hatchabilities were low—less than 50%—, presumably because the Italian climate differed considerably from that of Egypt. However, the Grand Duke was not discouraged. He designed and used a thermometer as well as a hygrometer and thereby improved the results. Reaumur in 1751 returned to the dung method. Barrels containing baskets of eggs were surrounded by dung, the heat from them warming the eggs. He measured the temperature with his newly invented mercury thermometer. However, hatchabilities were only 50% of all eggs set.


GORDON MEMORIAL LECTURE

A. H. J. VISSCHEDIJK—8th Gordon Memorial Lecturer.

Cornelis Drebbel was born in Alkmaar, the Dutch cheese town in 1572. He introduced thermostatic control to a still-air incubator which he built around 1629 for the king of England (Fig. 1). The water mantle between the walls of the oven was heated by an oil-burner (A). On the bottom of the inner side of


A. H. J. V1SSCHEDIJK

FIG. 1.—The incubator of Cornelis Drebbel. After Landauer (1967).

the oven was a glass vessel filled with alcohol. Its shrinking and expansion caused, via B and H, movements of the valve (F) that controlled the opening of the hole (E) through which heated air escaped from the oven. Thermostats based on this principle remained popular into this century, the well-known Hearson still-air incubators of about 1900 are a good example. Finally the Schmidt patent was published. Egg trays were placed in a cylindrical drum, so that thousands of eggs at a time could be turned. A motordriven fan rotated around the drum, causing air movement, temperature equilibration and ventilation. Electrification aided temperature control, turning and ventilation of these force-draught incubators. Our modern walk-in incubators have racks with egg trays which are moved in and out of the incubator. They are connected to an automatic turning device. The latter turns the eggs hourly through an angle of 90°. Temperature and humidity are controlled by suitable sensors. Some recent models offer control of ventilation rate by means of a carbon dioxide sensor and all data are recorded and stored in a computer.

AVIAN ATTEMPTS AT ARTIFICIAL INCUBATION

Not only do human beings practise artificial incubation. Some birds also do. They belong to the family of the Megapodes, the Mound builders or incubator birds. They bury their eggs in mounds or simple excavations in the ground to hatch them. There are 13 species of Indo-Pacific Megapodes, but only two have yielded significant information on incubation biology. Both are



7

native to Australia: one is the Brush Turkey, Alectura lathami, the other the Mallee fowl, Leipoa ocellata (Seymour and Ackerman, 1980). The heat required for embryonic development is generated by the decay of plant matter in the mounds built by the Brush Turkey (Seymour and Rahn, 1978). The Mallee fowl supplements solar radiation with the fermentation of vegetation (Frith, 1962). The male bird builds the mound, which can take almost a year. The result is a low mound, rising about 1 m above the ground and up to 4 m across. The mound consists of leaves and twigs which are soaked with rain and then covered with half a metre of sandy soil. The large mass is important for ensuring relative thermal stability. It has been estimated (Baltin, 1969) that a Brush Turkey mound contains about 3*6 metric tons of leaf litter and soil, a mixture with good insulating properties. Brush Turkeys, relying on organic heat for incubation, must incorporate fresh material in a mound each year. They accomplish this by constructing a new mound each breeding season. However, the Mallee fowl opens previously used mounds and adds fresh litter (Seymour and Ackerman, 1980). The rate of decomposition of vegetation and thus of heat generation is clearly dependent on adequate moisture in the litter. The plant material in the Mallee fowl mound is usually sufficiently moistened by winter and spring rains. The Brush Turkey forms a funnel in the top of its mound prior to rain so that the rain water soaks into it (Fleavy, 1937). When the heat of fermentation reaches the required incubation temperature (34° and 37°C in case of Mallee fowl and Brush Turkey, respectively) the female lays the first egg of a long series. An adult female Mallee fowl weighs about 1-8 kg. It produces about 35 eggs, but requires 5 to 9 d to produce each egg (Frith, 1959; Baltin, 1969; Seymour and Rahn, 1978). If the average laying interval is 7 d the Mallee fowl is engaged in egg laying for about 250 d. Throughout this time the male maintains a mound temperature astonishingly close to the required value of 34°C, even in face of daily and seasonal weather variation. Once or twice a day he opens the mound to measure the temperature in the egg layer of the mound. The temperature sensing organ is not precisely known, but is most likely to be the tongue. The frequent opening and closing of the mound may be as important for ventilation as for temperature control. The Mallee fowl egg hatches in 7 weeks. The hatchlings dig upward through the mound, run off, and forage themselves. They are extremely precocious, fully feathered at hatching and even able to fly within a day or two (Frith, 1962). Measurements have shown that the oxygen consumption of the developing embryos is equivalent to only 3% of the oxygen uptake of the decomposing organic material in the mound. This means that mound metabolism is the major determinant of the gas tensions and temperature in it (Seymour and Ackerman, 1980). The low oxygen and high humidity in a mound suggests that the eggs must be very porous in order to obtain acceptable gas tensions within the egg and to achieve sufficient water loss during incubation. Indeed, Brush Turkey eggs are almost three times more porous than would be predicted on the basis of their mass and incubation time (Seymour and Rahn, 1978).

8



NEEDS/EXPECTATIONS OF ARTIFICIAL INCUBATION

A freshly-laid egg contains all the nutrients, minerals, energy sources and water required for the production of a viable, free-living chick. Actually the egg needs only warming, periodic turning and oxygen to utilise part of the nutrients. In this metabolic process carbon dioxide and water are produced. Thus an egg shell must be porous to allow the inward movement of oxygen and the outward movement of water vapour and carbon dioxide. An egg loses water for 2 reasons. Firstly, it provides an air space of sufficient volume to allow the embryo near term to breathe inside the shell and to pip the latter about one day before hatching. Secondly, an egg must lose sufficient water to ensure that the newly-hatched chick has the same water content as the freshly-laid egg (Ar and Rahn, 1980). For precocious birds, such as the domestic fowl, that value is about 72%. Too great a loss of water would cause dehydration of the embryo; conversely the embryo would drown in the water synthesised during metabolic activity if the water loss from the egg were too small. Normal water loss, occurring by diffusion through the pores of the shell, is 12-6% of the fresh egg mass during the full incubation period (Tullet, 1981; Meir et al., 1984). Thus for the domestic fowl, where incubation takes 21 d, it is 0-6% per day. To accomplish this, a relative humidity within the incubator must be set that takes into account the so-called "functional porosity" of the average egg. We will return to this term later. A typical domestic fowl egg shell contains about 10,000 gas-filled pores. Below the shell we find the outer and inner membranes. An air space is formed between them at the blunt end of the egg as a result of the loss of water from the egg. However, gas is also present between the fibres from which the membranes are made (Rahn et al., 1979). It follows that the embryo with its gas exchanger, the chorioallantois, is entirely surrounded by gas. Thus the embryo develops in a gaseous medium (Visschedijk, 1987). An optimum gas composition of this medium is of primary importance to the successful development of the embryo. A gas such as oxygen encounters two forms of resistance on its way from the atmosphere to the embryo. The first is convedive resistance, offered by the inlet and exhaust valves of the incubator. This resistance limits ventilation or convective gas transport. The second is diffusive resistance, offered by the pores in the egg shell. It limits diffusive gas transport across the pores. Therefore, the oxygen content decreases if there is a flow of this gas from the outside to the embryonic medium. After 17 to 19 d of incubation, when there is a plateau in the otherwise increasing oxygen uptake of the embryo, one could have the situation given in Fig. 2, when the barometric pressure is 760 torr. In order to create optimal gaseous conditions in the embryonic medium at all times one must, according to Fig. 2, take into account: the gaseous composition and barometric pressure of the ambient fresh air, the incubator ventilation rate, the functional porosity of the egg shell and the rate of embryonic oxygen uptake. Any change in any one of these factors requires compensatory changes in the others in order to maintain optimum water loss and gaseous conditions in the embryonic medium.

GORDON MEMORIAL LECTURE Barom. Pr. : 760 torr I

FRESH AIR

O2 FLUX

O 2 Conc. [%]

O2Tension [torr]

21.0

149

19.7

140

|

|

14.0

100

CONVECTIVE RESISTANCE


INCUBATOR GAS DIFFUSIVE RESISTANCE EMBRYONIC MEDIUM

FIG. 2.—Typical gas concentrations and tensions in air, incubator and embryonic medium at metabolic plateau stage.

It is clear that such changes for practical reasons should be calculable. One must be able to express the convective resistance of the incubator, as well as the diffusive resistance of the egg shell, in simple figures. Moreover, one should get used to thinking in terms of partial pressure or gas tension instead of gas concentration. Lundy (1969) deserves great credit for recognising this need and urging its adoption as early as 1969. Thus far I have used the word "resistance". The inverse of resistance is conductance. Resistance is a measure of the difficulty, while conductance is a measure of the ease, with which gases are transported. This transport occurs by convection through the vents of the incubator (convective conductance) and by diffusion through the egg shell pores (diffusive conductance). The function of the pores is the transport of gases, including water vapour. Therefore diffusive conductance can also be called functional porosity. It increases if the pores are more numerous, wider or shorter. However it can be assessed without laborious and destructive measurement of numbers, diameters and lengths, by merely measuring the water loss (that is, weight loss) of an egg in bone-dry pure air at known temperature and barometric pressure (Ar et al., 1974). Humidity must then be expressed in terms of water vapour pressure rather than as a percentage. In the air space the relative humidity is 100%, representing a water vapour pressure of 49 torr at the temperature of the incubator, 37-8°C (Fig. 3). The difference in water vapour pressure between the wet inner and dry outer side of the shell is therefore 49 torr. Because of this driving force an amount of water will diffuse through the pores of the egg to the outer side. Typically this is around 735 mg/d. Thus the conductance of such a shell is 735 mg water/d per 49 torr partial pressure difference, or 15 mg/d per torr. If this egg is incubated in an incubator at 51% humidity (Fig. 4), representing a water vapour pressure of 25 torr, (0-51 X49), then this egg will lose 360 mg water/d,


10


(15 X (49 — 25)). Thus the flux of a gas or vapour is determined by conductance times partial pressure difference. The conductance values for oxygen and carbon dioxide are calculated by using conversion factors (Paganelli et al., 1978): 15-5 ml/d per torr for oxygen, 12 ml/d per torr for carbon dioxide. Similarly, the convective conductance of the incubator is given by its carbon dioxide elimination over the partial pressure difference of this gas between the gas mixtures entering and leaving the incubator. Why should partial pressures instead of gas concentrations be used? Quite simply, life depends on the first, not on the latter. At sea level the effective oxygen tension of air is 149 torr at incubator temperature. At higher altitudes %RH

49 torr

100%

0

735 mg/day

H2O-CONDUCTANCE = ^ = 1 5

FIG. 3.—Relative humidities, water vapour tensions and water loss when an egg is incubated in dry air.

%RH

PH2O

49%

24 torr 25 H2O-CONDUCTANCE = 15 2

WATER LOSS

torr

360 mg/day

(= §§f \

24/

15x24 = 360 mg/day

FIG. 4.—Relative humidities, water vapour tensions and water loss when an egg is incubated at 51% humidity.


11

the oxygen concentration remains 21%, but because barometric pressure is reduced, the partial pressure of oxygen is also reduced. Thus at an altitude of 5-5 km, where the barometric pressure is reduced from 760 to 380 torr, the effective oxygen tension is only 70 torr. Anyone who has been at high altitude will have noticed the effect of low oxygen tension, through breathlessness and difficulty in doing physical work.


EARLY SCIENTIFIC STUDIES

This lecture does not lend itself to an extensive review of early scientific studies, but fortunately several good reviews have already been published including Barott's "Effect of temperature, humidity, and other factors on hatch of hens' eggs and on energy metabolism of chick embryos" from 1937, Landauer's "The hatchability of chicken eggs as influenced by environment and heredity" published in 1967 and Lundy's "Review of the effects of temperature, humidity, turning and gaseous environment in the incubator on the hatchability of the hen's egg" of 1969. Useful chapters on incubation physiology are to be found in Freeman and Vince's book "Development of the Avian Embryo", published in 1974. In 1980 I reviewed the: "Effects of barometric pressure and abnormal gas mixtures on gas exchange by the avian embryo". Let us consider the systematic studies of Barott (1937) on temperature and relative humidity. The range of temperatures within which domestic fowl eggs can be successfully incubated is small, only 4°C. If 37-9°C is the optimum incubator temperature, then hatchability falls to zero at about 35-6 and 39>7°C. Also, the relationship between humidity and hatchability is parabolic. However, relative humidity must be almost zero or 100% to cause zero hatchability. Barott's Figure suggests that a higher temperature must be matched with a lower humidity to get the best hatches. In other words, at a higher temperature the egg should lose more water. This result is strange if eggs should lose only 12*6% of their original mass during incubation. Eggs surviving the higher temperature have a greater metabolic rate, produce more metabolic water and therefore should lose more water, but hatch less well than at the optimum temperature. The hatchability at 38-9°C was indeed 10% lower than at 37-8°C. Is this the right explanation? I cannot be sure. In any case, hatcheries are more interested in high hatchabilities than in optimum relative humidities at abnormal temperatures. Barott's eggs also seemed to be more sensitive to humidity changes than their modern equivalents. A change of 25% relative humidity reduced hatchability by about 50%, but in my experiments (unpublished, see Tullett, 1981) the reduction was only 3%. Moreover, the variability in conductance within a batch of eggs of the same species, strain and origin is very great. The average water vapour conductance is 15 mg/d per torr, yet the range within a batch is typically 8 to 22-5 mg/d per torr (Visschedijk et al., 1985). In order to obtain the same water loss from these eggs the relative humidities should be 51, 8 and 67%, respectively. All such eggs should in fact hatch


12


without adjusting the humidity. This is remarkable, because high and low conductance eggs show, besides an abnormal water loss, also abnormal air space gas tensions. Barott (1937) also established the relationship between the oxygen concentration at normal atmospheric pressure and hatchability. Eggs are more sensitive to lower than to higher concentrations. The best hatches were obtained at 21% oxygen, the normal concentration in air. As mentioned earlier, we should now use effective oxygen tension, rather than concentration, on the horizontal axis of the graph. It is even more important to remind ourselves that we need a graph giving oxygen tensions within the air space on the horizontal axis because gas tensions in the embryonic medium, rather than those in the incubator, are of primary importance to the gas exchange and development of the embryo.

DEFINITION OF THE REQUIREMENTS OF THE EMBRYO

Temperature

Optimum incubation temperature in wild birds may vary from 33 to 39°C. Our domestic species seem to fall within the narrow range of 37 to 38°C. Unfortunately we do not know of any allometric relationship between egg mass and/or incubation time on the one hand, and optimum incubator temperature on the other, as we do for egg shell conductance, water loss and metabolic rate. Optimum temperature can thus be determined only by trial and error. Not only hatchablity, but also time of hatching and its synchronisation are important criteria in determining optimum temperature. Too low and too high temperatures will not only affect hatching time, but will also cause greater variability in it. The most recent systematic study of optimum incubation temperature was done using the Guinea fowl. Ancel (1989), working at the CNRS in Strasbourg, used temperatures of 36, 37, 38, and 39°C and found that 37*2°C was optimum according to the parabolic relationship: Pipped eggs in % of fertiles=69-9-13-8 ft-37-2)2 Thus, if the temperature *=37-2°C the result is 69-9%. A hazardous extrapolation would predict zero hatchabilities at 34*9 and 39>5°C. Humidity

In combined experiments on optimum temperature and humidity Ancel (1989) obtained the best results at 37-2°C and 53% relative humidity. It is interesting to check the latter value on the basis of allometric relationships (Table 1) between water loss (M), water vapour conductance (G), egg mass (W) and incubation time (/) in more than 150 wild bird species (Ar and Rahn, 1978; Hoyt etal., 1979). According to the allometric relationships the required difference in water vapour tension across the egg shell, that is M/G, is 24 torr, whereas according


13

TABLE 1

Comparison between predicted and measured water vapour conductance, water loss and water vapour pressure difference across the shell. Data from Ancel (1989)


Allometric Af = 126xW/I G=5-25XW/I

Ancel (1989) 126x48-9/27=228 Predicted G =9-5 Measured G =10

Units mg/d mg/d/torr mg/d/torr

to Ancel's measurements it would be 22*8 torr. The water vapour tension in the incubator should then be 47-6 —22-8 = 24-8 torr, and the relative humidity 100x24-8/47-6 = 52% at 37-2°C, in excellent agreement with Ancel's findings. Although optimum water vapour tension difference across the eggshell is 24 torr as predicted by the allometric relationship, the mean water vapour conductance of the eggs of a particular species, as well as its optimum incubation temperature should be known to determine the best incubator relative humidity. Gaseous environment

In most early scientific reports barometric pressure was not given when gas concentrations were discussed. The cri de coeur of Lundy (1969) about this omission is highly justified. It is greatly to his credit to state for the first time in the poultry literature that gas percentage concentration is a relative term. I quote: "It does not give the chemically effective concentration; this is its partial pressure. Few workers give partial pressures. This restricts the validity of their results to the local experimental situation". In addition, as we have seen, the oxygen tension in the air space, the embryonic medium, rather than its effective value in the incubator, is of importance to embryonic development. The term effective gas tension was introduced by Wangensteen and Rahn (1970/71). It is calculated on the basis of a volume of gas that is saturated with water vapour at incubator temperature. The saturated water vapour tension at this temperature is 49 torr. Not only the gas tensions in the embryonic medium, but also those in the incubator and in the fresh air should be given as "effective" values if one makes calculations on gas transport. The simple reason for this is that gas tensions may be compared only if they are given in the same dimensions under identical conditions of temperature, humidity and barometric pressure. Thus if the barometric pressure is 760 torr the effective oxygen tensions at incubator temperature are calculated from the oxygen concentrations as follows: In fresh air (21%) (760-49) X 0-21 =149 torr In the incubator (19-7%) (760-49) X 0-197= 140 torr In the embryonic medium (14%) (760-49)X0-14 =100 torr From these figures the oxygen uptake of an egg can be calculated if the convective and/or the diffusive conductance is known. Because the latter may vary between species the optimum convective conductance, a measure of the


14


FIG. 5.—Barometric pressure (PB), effective oxygen tension (PI02) and expected hatchability at various altitudes (km).

TABLE 2

Air space gas tensions at metabolic plateau stage in eggs of some domestic species. From Rahn et al. (1974)

Species Japanese quail Pheasant Domestic Fowl Pekin duck Turkey Embden goose Mean

Air space oxygen tension (torr) 108 100

/HO 1101 98 101 114 104

ventilation rate, cannot be given without knowing the mean diffusive conductance of a random sample of eggs. Data for this variable are, with very few exceptions, not available. Some data are available on air space oxygen tensions in domestic species (Table 2), measured at the metabolic plateau stage (Rahn et al., 1974). However, it has been known for some time that the mean oxygen tension in the embryonic medium is lower than in the air space (Paganelli et al., 1988; Seymour and Visschedijk, 1988). Therefore a value of 100 torr may be


15

assumed to be normal, and possibly represents the optimum gas space oxygen tension at metabolic plateau stage in most birds.


PROBLEMS OF ALTITUDE

It has long time been recognised that the reduction in the barometric pressure at altitude, with the attendant reduction in the oxygen partial pressure, is the basic problem to successful incubation at altitude. Moreover there is an increase in the rate of water loss across the shell. This was first ascribed to the lower absolute humidity of the air at altitude. In Fig. 5 an indication is given of the impairment of the hatchability at increasing altitudes. The attendant barometric pressures and effective oxygen tensions are also given. In addition to reduced hatchability and excessive water loss from the egg at altitude, oxygen consumption and growth are also reduced and hatching time is delayed (Visschedijk, 1980). Reduced barometric pressure not only causes a decrease of the effective oxygen tension but also an increase of the effective shell conductance (Paganelli et al., 1975; Erasmus and Rahn, 1976; Visschedijk et al, 1980). The latter effect is explained as follows (Fig. 6): At sea level the oxygen molecules Sea level, Okm, 1atm. 21%O2

• !

PQ 2 =149

Altitude, 5.5km, 0.5 atm. 21%O 2

C02| FIG. 6.—Explanation of the increase of the eggshell conductance at altitude (reduced atmospheric pessure).

diffusing from the ambient air into the egg frequently collide with the many bigger nitrogen molecules present in the pores of the shell. At altitude there are fewer oxygen and nitrogen molecules because of the lower barometric pressure. Hence there are fewer collisions. The resistance offered by the nitrogen molecules to the oxygen molecules is less: in other words the effective oxygen conductance is greater. Similarly, the effective conductances of carbon dioxide and water vapour are increased. This explains the excessive loss of water and carbon dioxide at altitude. Thus the combined effects of altitude


16


consist of a lack of oxygen (hypoxia), carbon dioxide (hypocapnia) and water (dehydration). Human beings at high altitude are faced with similar problems. What can be done about it? It is generally assumed that hatchability at altitude can be normalised if the embryonic gas exchange is restored to its sea level value. To normalise oxygen consumption it is necessary to bring the oxygen tension in the embryonic medium back to its sea level value throughout development. For this purpose the effective oxygen tension of the gas mixture supplied to the incubator must be restored to 149 torr (Fig. 7) by increasing the oxygen concentration, for instance to 45% at an altitude of 5-5 km, where the barometric pressure is 380 torr (Visschedijk and Rahn, 1981). Barometric pressure, torr

760

2

380

0 2 conductance, ml(STPD)/day.torr

16

>2

32

Diffusive O2 tension difference, torr

40

2

20

O2 uptake, ml(STPD)/day

6 4 0 - 6 4 0

Sea level, 760 ton-

5.5 km. 380 torr

Standard O2 cond.

Standard 0 2 condjt760/380

4S.0XO 2 IN

\—— Diffusive —« I A P O = 4 0 torr

21XO

Diffusive

119.7X0 OUT

[36.2%O2 'OUT

Convective — A P 0 = 29 torr

FIG. 7.—Required oxygen concentrations and tensions at metabolic plateau stage. Left: at sea level (760 torr). Right: at an altitude of 5-5 km (380 torr).

The oxygen tension in the embryonic medium decreases during incubation from 149 to about 100 torr. The same values must be maintained at any altitude. At the same time the increased shell conductance must be compensated for by a proportional decrease of the oxygen tension difference across the shell. At a barometric pressure of 380 torr the effective oxygen conductance is doubled, thus requiring the oxygen tension difference across the shell to be halved to obtain the same oxygen uptake of 640 ml/d (16 X 40 = 32 X 20). It follows that the incubator oxygen tension at altitude must be lower than at sea level; in other words: the required incubator ventilation rate must be reduced (Visschedijk, 1985), from 47-4 1/d per egg at sea level to only 6*3 1/d per egg at an altitude of 5-5 km.

17

GORDON MEMORIAL LECTURE Barometric pressure, torr CO2 conductance, ml (STPD)/day, torr Diffusive CO2 tension difference, torr CO2 elimination, ml (STPD)/day

760 :2 12 x 2 37% :2 448 =

5.5 km, 380 torr

Sea level, 760 torr Standard CO2cond.


|^

380 24 18% 448

Standard CO2cond. x 760/380

Diffusive 1 4PCO 2 =37Vbtorr '0 95%

0% CO2 IN

I

Diffusive 1 4PCO 2 =18%torr

CO2

7.1% CO2 I OUT

[OUT

I f I I 6.7-I

23.5 -J

I

AP CO2 = 6.7 torr

Convective AP CO2 = 23.5 torr

FIG. 8.—Required carbon dioxide concentrations and tensions at metabolic plateau stage. Left: at sea level (760 torr). Right: at an altitude of 5-5 km (380 torr).

Knowing the ventilation rate and the normalised carbon dioxide output we can calculate the carbon dioxide tensions in the incubator and the gas space (Fig. 8). The altitude value of the latter is slightly lower than at sea level, but experiments have shown that this does not affect gas exchange and hatchability (Girard and Visschedijk, 1987). Similar reasoning applies to water vapour (Table 3). In order to maintain the same water loss as at sea level the water vapour tension, and hence relative humidity, has to be increased in the incubator. In the example it must be 75-5% at an altitude of 5*5 km. Experiments at simulated altitudes of 5-5 and 2-9 km showed that the prescribed measures resulted in almost normal gas exchange, growth and hatchability (Girard and Visschedijk, 1987). The reduced ventilation rate implies considerable savings in the supply of oxygen and electricity, makes the TABLE 3

Required incubator water vapour tensions and relative humidities at sea level (760 torr) and at an altitude of 5-5 km (380 torr)

Barometric pressure (torr) H2O conductance, mg/day (torr) Diffusive H2O tension difference (torr) Water loss from egg (mg/d) Incubator H2O tension (torr) (=49-diffusive H2O tension difference) Incubator relative humidity (%) (=Incubator H2O tension X 100/49)

Sea level

5-5 km

Difference

760 15 24 360

380 30 12

0-5 X2 0-5 =

25

37

51

75-5

360

18


supply of additional carbon dioxide redundant and facilitates the required increase in humidity.

ROLE OF GENETIC SELECTION


We have found it is possible to select hens on the basis of egg conductance (unpublished results). However, eggs with very low and very high shell conductance occur so rarely (Visschedijk et al., 1985) that selection is impracticable.

THE FUTURE

HAVE ALL PROBLEMS BEEN SOLVED?

Theoretically the answer is a qualified "Yes". The relationships between all respiratory factors and how to compensate for abnormal conductances and altitude effects are known. To summarise: Effective diffusive+convective resistance =(149—optimum gas space oxygen tension)/oxygen uptake. This holds at any altitude (Visschedijk, 1987). Whether the optimum gas space oxygen tension is 100 torr at the metabolic plateau stage, or slightly lower or higher, is being investigated. We should also find out if this value is valid for various species and breeds. If it is true one may conclude that the required ventilation rate, and thereby the optimal incubator gas composition, differs greatly between broiler and layer breeder eggs. Measurements of conductance and gas exchange have shown that layer eggs may need only 30% of the ventilation of broiler eggs. Experimental data are given in Table 4.

TABLE 4

Egg mass, oxygen uptake and oxygen eggshell conductance, measured at metabolic plateau stage in broiler and layer eggs, and calculated optimum incubator ventilation rate for identical gas space oxygen tensions

Egg mass O2 uptake/egg O2 uptake/g egg O2 conductance O2 conductance/g egg Gas space PO2 Ventilati

°nrate C(>2 output

Broilers 64-8 714 11-0 15-6 0-241 100

Layers 62-1 680 11-0 16-9 0-272 100

297

108

Units g ml(STPD)/d ml(STPD)/g/d ml(STPD)/d/torr ml(STPD)/g/d/torr torr

CONCLUSION

Artificial incubation has come a long way in two and a half millenia. Whilst we have succeeded in turning it into a science, I believe the achievements of the first incubationists were impressive and should ensure that we remain humble in such matters.


19

REFERENCES ANCEL, A. (1989) L'Oeuf de Pintade (Numida meleagris L.) et son incubation. Thesis, University of Strasbourg.


AR, A., PAGANELLI, C.V., REEVES, R.B., GREEN, D.G. & RAHN, H. (1974) The avian egg: water

vapor conductance, shell thickness, and functional pore area. Condor, 76: 153-158. AR, A. & RAHN, H. (1978) Interdependence of gas conductance, incubation length, and weight of the avian egg, in: PIIPER, J. (Ed.) Respiratory Function in Birds, Adult and Embryonic, pp. 227-236 (Berlin, Springer). AR, A. & RAHN, H. (1980) Water in the avian egg: overall budget of incubation. American Zoologist, 20: 373-384. BALTIN, S. (1969) Zur Biologie und Ethologie des Talegellahuhns (Alectura lathami Gray) unter besonderer Berücksichtigung des Verhaltens während der Brutperiode. Zeitschrift für Tierpsychologie, 26: 524-572. BAROTT, H.G. (1937) Effect of temperature, humidity and other factors on hatch of hen's eggs and on energy metabolism of chick embryos. Technical Bulletin United States Department of Agriculture, 553: 1-45. ERASMUS, B.D. & RAHN, H. (1976) Effects of ambient pressures, He and SF 6 on O 2 and CO 2 transport in the avian egg. Respiratory Physiology, 27: 53-64. FLEAVY, D.H. (1937) Nesting habits of the brush turkey. Emu, 36: 153-163. FREEMAN, B.M. & VINCE, M.A. (1974) Development of the Avian Embryo (London, Chapman and Hall). FRITH, H.J. (1959) Breeding of the Mallee Fowl, Leipoa ocellata Gould (Megapodiidae). C.S.I.R.O. Wildlife Research, 4: 31-60. FRITH, H.J. (1962) The Mallee-Fowl: The Bird that Builds an Incubator (Sydney, Angus and Robertson). GIRARD, H. & VISSCHEDIJK, A . H J . (1987) Altitude hypocapnia at 2,800 m does not affect development of the chicken embryo. Journal of Experimental Zoology, Supplement 1: 365-370. HOYT, D.F., BOARD, R.G., RAHN, H. & PAGANELLI, C.V. (1979) The eggs of the Anatidae:

conductance, pore structure and metabolism. Physiological Zoology, 52: 438-450. KUIPER, J.W. (1959) Over het kunstmatig broeden van kuikens. Overdruk T.N.O Nieuws, 46: 1-5. LANDAUER, W. (1967) The hatchability of chicken eggs as influenced by environment and heredity. Storrs Agricultural Experiment Station, Monograph 1 (revised). LUNDY, H. (1969) A review of the effects of temperature, humidity, turning and gaseous environment in the incubator on the hatchability of the hen's egg, in: CARTER, T.C. & FREEMAN, B.M. (Eds) The Fertility and Hatchability of the Hen's Egg, pp. 143-176 (Edinburgh, Oliver and Boyd). MEIR, M., NIR, A. & AR, A. (1984) Increasing hatchability of turkey eggs by matching incubator humidity to shell conductance of individual eggs. Poultry Science, 63: 1489-1496. PANAGELLI, C.V., ACKERMAN, R.A. & RAHN, H. (1978) The avian egg: In vivo conductances to

oxygen, carbon dioxide, and water vapor in late development, in: PIIPER, J. (Ed.) Respiratory Function in Birds, Adult and Embryonic, pp. 212-218 (Berlin, Springer). PAGANELLI, C.V., AR, A., RAHN, H. & WANGENSTEEN, O.D. (1975) Diffusion in the gas phase: the

effects of ambient pressure and gas composition. Respiration Physiology, 25: 247-258. PAGANELLI, C.V., SOTHERLAND, P.R., OLSZOWKA, A.J & RAHN, H. (1988) Regional differences in

diffusive conductance/perfusion ratio in the shell of the hen's egg. Respiration Physiology, 71: 45-56. RAHN, H., AR, A. & PAGANELLI, C.V. (1979) How bird eggs breathe. Scientific American, 240: 46-55. RAHN, H., PAGANELLI, C.V. & AR, A. (1974) The avian egg: Air-cell gas tension, metabolism and incubation time. Respiration Physiology, 22: 297-309. ROMIJN, C. (1978) Kunstmatig broeden en europese vorstenhuizen. Tijdschrift voor Diergeneeskunde, 103: 629-640. SEYMOUR, R.S. & ACKERMAN, R.A. (1980) Adaptations to underground nesting in birds and reptiles. American Zoologist, 20: 437-447.

20


R.S. & RAHN, H. (1978) Gas conductance in the eggshell of the mound-building Brush Turkey. in: PIIPER, J. (Ed.) Respiratory Function in Birds, Adult and Embryonic, pp. 243-246 (Berlin, Springer). SEYMOUR, R.S. & VISSCHEDIJK, A.H.J. (1988) Effects of variation in total and regional shell conductance on air cell gas tensions and regional gas exchange in chicken eggs. Journal of Comparative Physiology B, 158: 229-236. TULLETT, S.G. (1981) Theoretical and practical aspects of eggshell porosity. Turkeys, 29: 24-28. VISSCHEDIJK, A.H.J. (1980) Effects of barometric pressure and abnormal gas mixtures on gas exchange by the avian embryo. American Zoologist, 20: 469-476. VISSCHEDIJK, A.HJ. (1985) Gas exchange and hatchability of chicken eggs at simulated high altitude. Journal of Applied Physiology, 58: 416-418. VISSCHEDIJK, A.HJ. (1987) Air space: the embryonic medium. Journal of Experimental Zoology, Supplement 1: 193-201. VISSCHEDIJK, A.H.J., AR, A., RAHN, H. & PIIPER, J. (1980) The independent effects of atmospheric pressure and oxygen partial pressure on gas exchange of the chicken embryo. Respiration Physiology, 39: 33-44. VISSCHEDIJK, A.H.J. & RAHN, H. (1981) Incubation of chicken eggs at altitude: theoretical consideration of optimal gas composition. British Poultry Science, 22: 451-460. VISSCHEDIJK, A.H.J., TAZAWA, H. & PIIPER, J. (1985) Variability of shell conductance and gas exchange of chicken eggs. Respiration Physiology, 59: 339-345. WANGENSTEEN, O.D. & RAHN, H. (1970/71) Respiratory gas exchange by the avian embryo.


SEYMOUR,

Respiration Physiology, 11: 31—45.

Gordon Richards Memorial Lecture.

The Gordon Wilson Memorial Lecture.

THE GORDON WILSON MEMORIAL LECTURE.

THE GORDON WILSON MEMORIAL LECTURE.

Gordon memorial lecture. Vaccines and vaccination--past, present and future.

Gordon Bell memorial lecture. Management of soft tissue sarcoma.

Gordon Memorial Lecture. Chicken neoplasia--a model for cancer research.

Memorial. Edgar Stillwell Gordon.

The Gordon-Taylor memorial lecture: metatropoi aurai--the winds of change.

Masahiro Iio Memorial Lecture.

Gordon-Taylor Lecture, 1976. Sir Gordon Gordon-Taylor, surgeon-anatomist and humanist.

The Gordon Wilson Lecture. The new genetics.

The Gordon Wilson Lecture. Congenital adrenal hyperplasia.

The Gillies Memorial Lecture 1975.

Hayes Martin Memorial Lecture. Non desperandum.

Second annual Ludwig von Bertalanffy Memorial Lecture.

The Gordon Wilson Lecture. Plasma cholesterol: atherogenesis and mortality.

First Tigani El Mahi Memorial Lecture.

The Cairns memorial lecture: the Cairns tradition.

Phillip Gett memorial lecture awareness in anaesthesia.

Cricothyroidotomy and ventilation: physics and physiology.

THE GORDON WILSON LECTURE EVOLUTION OF CLINICAL CANCER GENETICS.

The Gordon Wilson Lecture. The ABC's of viral hepatitis.

Karnofsky Memorial Lecture. The immunotherapy and gene therapy of cancer.