Conductance and Structure of Eggs of Adelie Penguins, Pygoscelis Adeliae, and Its Implications for Incubation ’

The Condor92304-312 0 The CooperOrnithological Society 1990

CONDUCTANCE AND STRUCTURE OF EGGS OF ADELIE PENGUINS, PYGOSCELIS ADELIAE, AND ITS IMPLICATIONS FOR INCUBATION ’

MICHAEL B. THOMPSON~ School of Biological Sciences,Victoria Universityof Wellington,P.O. Box 600, Wellington,New Zealand

KENNETH N. GOLDIE Faculty of Science, Victoria Universityof Wellington, P.O. Box 600, Wellington,New Zealand

Abstract. Structureand conductanceof eggshellsof Adelie Penguins,Pygoscelis adeliae, were studied to addressthe question:How is the eggshelladapted to cope with the extreme aridity of the Antarctic, and in particular, what is the significanceand function of the organic cuticleon the eggshell?Adelie Penguinshell structurehas a basicavian pattern with relatively straight, unbranched pores that are occluded by an organic cuticle on the outside of the shell. Mean shell conductance of 15.4 mg.day-l.Torr- ’ of six intact eggs is significantly lower than one would predict on the basis of fresh egg mass, and conductanceafter cuticle removal (18.6 mg day-l. Torr+) from three eggsis significantlyhigher. Theoretical consid- eration of tensions of 0, and CO, in intact eggsand cuticle-removed eggsindicates that, whereas 0, tensions inside the egg may not be stressfulto the embryo prior to hatching if the cuticle remained intact throughout incubation, CO, tensionsmay be. We concludethat the cuticle reduces loss of water during the early part of incubation, but that its erosion during incubation increasesconductance thereby maintaining CO, tensions close to levels expectedin eggsof this size. Calculationsof functional pore radius from conductancemea- suresfor cuticle-removed eggsare almost identical to real mean pore radius measuredfrom pore casts. Key words: Eggshell; conductance;pore structure:Adelie Penguin; Pygoscelisadeliae; incubation;egg; Antarctic.

INTRODUCTION mour 1987). Conductance is modified by selec- The porosity of avian eggshellis extremely im- tion in such a way that desiccation does not en- portant becausethe avian embryo is isolatedfrom danger the embryo but that water loss during its environment by the shell, and the only ex- incubation is sufficient for proper air space for- changesduring incubation occur by diffusion of mation and the ability to supply sufficient oxygen gasesthrough pores in the shell (Wangensteen et and get rid of excesscarbon dioxide is not com- al. 1970/197 1). Oxygen is taken up and carbon promised. The eggshell must also be strong dioxide and water vapor given off during incu- enoughfor the brooding bird not to break it when bation. Shell conductance is one of the factors incubating, yet not so strong that the chick can- which determines the rate at which gasesmay be not break free (Ar et al. 1979). transmitted acrossthe shell and is the inverse of As gas exchange through the eggshell occurs resistance(Ar et al. 1974). Conductance depends, by diffusion, conductance is inversely propor- in part, on the number and structure of pores tional to the partial pressuregradient of the gas (Rahn and Ar 1974). Pore structure may change acrossthe shell (Paganelli 1980). Selection acts through incubation by embryonic erosion of the to modify conductance of eggshellsof birds at inner surface of the shell (e.g., Booth and Sey- high altitude, where partial pressure of oxygen and water vapor is very different from that at sea level, by acting on functional pore area (Wan- gensteenet al. 1974, Carey et al. 1983). Thus, it I Received 1 May 1989. Final acceptance1 February is clear that conductance of avian eggshells 1990. 2 Present address:Zoology (A08), School of Biolog- matches the physiological requirements of the ical Sciences,University ofsydney, N.S.W. 2006, Aus- egg with its normal incubation environment. tralia. As water moves through the shell as vapor (Ar

13041 ADELIE PENGUIN EGGS 305

TABLE 1. Physicalcharacteristics of six Adelie Penguineggs used in the study.(See text for calculationof mass,volume, and surfacearea.) Thickness measures are meansof the mean of four measuresat eachof 20 or 2 1 equidistantpositions between the blunt and pointedpoles of eachegg.

Mass (g) Volume (cc) surfacearea (Crn~) Thickness(mm) x 117.5 108.66 110.97 14.8 0.57 SD 10.9 10.08 6.95 1.7 0.06 Range 102.0-l 30.0 94.37-120.26 101.09-118.97 12.7-16.6 0.47-0.64

et al. 1974) nest humidity is an important pa- ber and maintained in a constant temperature rameter in determining how much water is lost cabinet for 36 hr at 25.5”C. Three copper con- from an egg during incubation. Eggs incubated stantan thermocouples were placed adjacent to in conditions creating unusual vapor pressure the eggsin different parts of the chamber. Eggs gradients, e.g., high altitude (Rahn et al. 1977, were removed from the chamber and weighed to Carey 1980, Carey et al. 1983), buried (Seymour 1 mg on the electronic pan balance after 0.5, 1, and Ackerman 1980) dry (Rahn and Hammel 1.5, 2, 3, 6, 12, 24, 30, and 36 hr. They were 1982), or in wet environments (Ackerman and replaced within 30 set of removal. Temperature Platter-Rieger 1979, Sotherland et al. 1984) have was a constant 25.5”C in the first measurement conductancesdifferent from those of eggsof sim- of conductance and averaged 25.6 f O.l”C (n = ilar size from “normal” environments (i.e., in 30) in the second measurement. Rate of mass open nests,close to sealevel), and in which water loss fell until it was constant after 6 hr, so cal- loss is maintained within acceptable limits (ca. culations were based on the rate of movement 15%, Ar and Rahn 1980). of water vapor acrossthe shell (Ii&,,) over the Arguably, one of the driest environments in final 30 hr of measurement. the world in which birds breed is that of the Three of the eggswere than washed for 90 set Antarctic. The dry conditions result from ex- in Janola (active ingredient 31.5 g.l-I sodium tremely cold temperatures. Adelie Penguins, Py- hypochlorite; Reckitt and Coleman [N.Z.], goscelisadeliae, nest at Cape Bird on Ross Island Avondale, Auckland) at 42°C to remove the cu- adjacent to the Antarctic mainland in early No- ticle (Deeming 1987). Completeness of cuticle vember (Spurr 1975) when conditions are still removal was checked using a binocular dissect- very cold. A previous study showedthat eggshell ing microscope.The eggswere rinsed in tap water of this speciesis adapted to low humidities with at 50°C for several minutes and then in tap water a reduced shell conductance and fewer than pre- at room temperature. All six eggs(three washed dicted pores (Rahn and Hammel 1982). We fur- and three intact) were re-equilibrated to 25.5”C ther studied the significance of the organic cover overnight before being placed back into the bell of the shell (we refer to it as the cuticle), which chamber over fresh silica gel. All eggs were occurs commonly throughout the Sphenisci- weighed at the same 10 time intervals as before. formes (Tyler 1965) and was postulated by Rahn Conductance to water vapor was calculated and Hammel (1982) to be a factor contributing using a modification of Fick’s first law of diffu- to the lower than expected shell conductance of sion: Adelie Penguin eggs. G ~20 = M,&P,,, (1) METHODS (Ar et al. 1974) Six eggsfrom different clutchesof P. adeliae were where G,,, is the conductance of the shell to collected at Cape Bird (70”13’S; 166”28’E) at the vapor (mg.day-1 .Tort-I), MHz0 is rate of move- very beginning of incubation (9-l 1 November ment of vapor acrossthe shell (mg.day-I), and 1987). They were kept cool (but not frozen) until AP,,, is the partial pressure gradient of vapor returned to Wellington in February 1988, when acrossthe shell (Torr). P,,, was assumed to be their length and breadth were measured to 0.1 zero outside the egg(over silica gel) and saturated mm with a dial vernier caliper (Table 1). inside the egg. Thus at 25.5”C P,,, is 24.5 Torr All eggswere equilibrated to 25.5”C overnight and at 25.6”c is 24.6 Torr (Weast and Astle 198 1). before being placed over silica gel in a bell cham- Eggs had lost mass prior to arrival in Wel- 306 MICHAEL B. THOMPSON AND KENNETH N. GOLDIE lington so fresh eggmass (W) was predicted from crographs. Other shell was also boiled in 10% length (L) and breadth (B) using the equation: NaOH (by weight) for 10 min to eliminate all organic components (especially shell mem- W = 0.548L.B2 (2) branes),rinsed in distilled water, and dried. Some (Hoyt 1979). of the NaOH treated shell was examined using This value was used to predict shell conductance the SEM, and some was set in resin for prepa- (G,,J using: ration of casts as described by Tompa (1980). Resin blocks were cut to reveal eggshell,which G HZO= 0.384 W0.814 (3) was then etched in 2M HCl, rinsed in distilled (Ar and Rahn 1978). water, dried over silica gel, and mounted for SEM Egg volume (V) was calculated using: analysis as described for the shell. Pore length and diameter at 10 equidistant points along the V = 0.507L.B2 (4) pore were measured from micrographs of pore (Hoyt 1979) casts. and this value was used to calculate surfacearea Mean values are given f one standard devia- (S) of each egg using: tion. Means are compared using Student’s t-test and statistical significanceis assumedifP < 0.05. S = [4.393 + 0.394.L,(B-‘)](V0.667) (5) (Hoyt 1976). RESULTS On completion of conductancemeasures, each The six eggsaveraged 68.6 f 2.7 mm long (range egg had 5-mm intervals marked with graphite =65.5-71.6mm)and55.8 f 1.7mmwide(range pencil around its long axis and was cut along this = 53.3-57.9 mm). These values were used to axis with a dental drill. Contents were discarded, predict initial mass, volume, and surface area inner shell membrane wiped free of fluid, and from equations 2, 4, and 5 (Table 1). the shell allowed to dry over silica gel before being weighed. At each 5-mm interval, shell SHELL STRUCTURE thickness with dried membrane intact was mea- The shell of each egg tended to be thinnest near sured to 0.001 mm with a micrometer with one the equator and thickest about 15-30 mm from curved surface,which was placed against the in- the pointed end (Fig. 1). There was little differ- side of the eggshell.The cut edge of each shell ence in shell thickness between each end of the was then soaked in water, the shell membrane egg and the thickness was remarkably similar peeled away (Tyler 1965) and the thickness of throughout the entire egg.The between-eggvari- the shell alone measured at each 5-mm interval. ation was much greaterthan within-egg variation This gave four independent measures of thick- (Fig. 1). ness(with and without membranes) at each 5-mm Combined inner and outer shell membranes circumference interval of each egg except at the were significantly thicker at the pointed end (72 poles where only two measures were available. f 17 pm) of the egg than the equator (49 + 6 We assumedthickness to be constant around one pm) (paired-sample t = 3.04, df = 5). Combined latitude (Tyler 1965) so the four measureswere membrane thickness could not be measured at averaged. the blunt end of the eggbecause of separation of Specimens for structural analysis were dried inner and outer shell membranes over the air over silica gel, mounted on aluminum stubswith space. silver colloidal glue, gold coated in a Dynovac Shellswere typical of avian eggswith a distinct sputter coater, and examined using a Philips 505 mammary zone and thick palisade layer (Fig. scanning electron microscope (SEM), with ac- 2A). The speciesalso had a surface crystal layer celerating voltages of 20-30 kV. Micrographs as describedby Tyler (1965). However, they had were taken on Ilford FP4 film and prints made a relatively thick outer waxy layer over the entire using standard techniques. surface which also covered the external pore Fragments of shell from the blunt and pointed openings (Fig. 2D). The waxy layer consisted of ends and equator of each eggwere examined us- individual units arising from the calcareousshell ing the SEM. Some intact shell, i.e., with cuticle on stalks and expanding to meet the next adja- (unwashed) and shell membranes, was exam- cent unit (Fig. 2E). They formed a uniform sur- ined. Cuticle thickness was estimated from mi- face over the egg (Figs. 2D, E), which from the ADELIE PENGUIN EGGS 307 surface had the appearance of “dried mud” (Deeming 1987) (Fig. 2C). The whole waxy cuticle was about 48 pm thick. The top half of the units were composed of round globular struc- tures (Fig. 2F) whereasthe bottom (“stalk”) had a pitted but relatively featurelessstructure (Fig. 2G). Pores were hourglass-shaped being slightly wider at the outer surfacethan the inner surface, although there was more variation in the former (Figs. 2B, 3). The narrowest point was between 20 and 40% of the distance from the inside surface. There was no difference in the shape or FIGURE 1. Thicknessof eggshellof six Adelie Pen- guin eggsmeasured at approximately 5-mm intervals dimensions of pores from the blunt or pointed from blunt to pointed pole. As eggswere of different ends or equator of the eggs. sizes, either 20 (eggs l-3) or 21 (eggs4-6) positions were measured.Only the 10 measuresfrom each pole CONDUCTANCE to the equator are presentedin the figure. Hence, one There was no significant change in conductance equatorial measure for eggs4-6 is not representedon the figure,but it does not influence the overall picture. of three eggs that were not treated with hypo- Each point representsthe mean of four measures in chlorite between measurements (t = 0.459, df = different placesat the samelatitude on eachegg, except 4, P > 0.6), but there was a significant increase at the poles, where only two measureswere possible. in conductance of hypochlorite-treated eggs(t = 3.894, df = 4, 0.02 > P > 0.01) (Table 2). Pre- dicted G,,, ‘s for each egg (Table 2) (calculated Our measure of cuticular thickness (48 pm) is using equation 3) were significantly higher than much greater than that found by Tyler (1965). those measured in pretreated eggs(t = 3.493, df Although Tyler found that thicknessof cuticle in = 10, 0.01 > P > O.OOl), but not significantly Adelie Penguin eggswas relatively uniform over different from hypochlorite-treated eggs (t = the whole egg surface compared to other pen- 1.742, df = 4, 0.20 > P > 0.10). guins, he was confused by variation between samples. Perhaps some of his specimenshad al- DISCUSSION ready undergone some cuticular erosion (seebe- low). STRUCTURE The pore system of Adelie Penguins fits into Eggs used in this study (117.5 * 10.9 g) were the arbitrary 3 a i classification of Board et al. slightly smaller than those examined by Rahn (1977) i.e., pores are unbranched and are oc- and Hammel (1982) (125 + 2 g SE) and those cluded by an organic cuticle that coversthe whole reported by Shijnwetter (1960). Adelie Penguins egg. Our pore casts show the same variation as usually lay two eggsin a clutch, the secondbeing found by Tyler (1965); the main variation be- smaller and having a shorter incubation than the tween pores is at either end (Fig. 3). We found first (Taylor 1962). However, eggsin clutches of much more variation, and a widening at the base one eggonly are smaller than the first eggof two- (inside end) of pores resulting from our different egg clutches. We assume that some of our eggs techniques. Included in our pore measurements would have been from single eggclutches because is the opening to the inner shell between the shell they are smaller than the recorded range for first units (Fig. 2B), which is not represented in Ty- eggsof two-egg clutches (Taylor 1962). ler’s casts.We found no evidence of an organic Mean shell thickness of 0.57 mm (Table 1) is plug penetrating the pore as found in Aptenodytes similar to values of 0.61 (Rahn and Hammel patagonica (Tyler 1965). 1982) 0.60 (Schonwetter 1960), and 0.58 (Tyler Rahn and Hammel (1982) had shown that the 1965). We found shell thickness to vary in the number of pores in Adelie Penguin eggsis sig- same way from pole to pole in all eggs(Fig. l), nificantly lower than predicted for eggsof similar unlike Tyler (1965) who found variation in this mass (9,108 compared to 10,700 derived from pattern in Adelie Penguins. This anomaly war- the equation of Tullett and Board [ 19771).Using rants further investigation. the equation of Rahn and Hammel (1982): 308 MICHAEL B. THOMPSON AND KENNETH N. GOLDIE

FIGURE 2. A. Radial view of intact Adelie Penguin eggshellshowing fibrous shell membranes(f) on the inside of the shell, mammary cones (c), thick spongy layer (s), pore (p), and part of the organic cuticle (0) occluding the pore. Bar = 0.1 mm. B. Cast of pore through Adelie Penguin eggshell.Note expansion of pore to inside of shell (bottom) as well as outside (top). Bar = 0.1 mm. C. Outside surface(tangential) view of cuticle over eggshell showing its appearancedescribed as “dried mud” (Deeming 1987). Bar = 1.0 mm. D. Close up of the outer organic cuticle showing its complete cover of the surfaceand occlusionof a pore (p). Bar = 0.1 mm. E. Organic cuticle is composedof individual units which arise on a stalk (b) from the calcareousshell, and then mushroom out to meet the adjacent unit. Bar = 0. I mm. F. Globular structureof the top, expanded portion of the organic cuticle. Bar = 10 pm. G. Relatively smooth stalk of a shell unit. Apart from pitting, no regular structure is visible. Note globular structureof part of top portion of unit (as in Fig. 2E) at top of picture. Bar = 10 pm. ADELIE PENGUIN EGGS 309

TABLE 2. G,, (mg.day-l.Torr+) for six eggs of Adelie Penguins. Initial values were obtained before treatment. Treated values were obtained after eggs l- 3 were scrubbedwith sodium hypochlorite. Eggs4-6 were not treated with hypochlorite to act as controls.

n R SD RCUlge

Pretreatment 6 15.4 1.0 14.2-17.1 Predicted 6 18.6 1.4 16.6-20.2 Treated 18.6 - 17.7-20.0 Controls : 15.7 - 14.9-16.6

Ap (mm2) = 0.447.G,,,.T (6) (Ap is functional pore area, T is shell thickness [mm]), and an estimated pore number of 9,108, our mean calculated functional pore radius of 11.7 (kO.9) pm is very close to the 11.3 pm of Rahn and Hammel (1982) and is only a little INSIDE more than the real mean minimum pore radius FIGURE 3. Stylized pore through shell of Adelie (Fig. 3). On the basis of conductance measures Penguin egg based on measurementsof pore casts at on the three eggsbefore and after treatment with 10 equidistant points from the inside to the outside of the shell. Right: mean values for pores from pointed hypochlorite, we know that functional pore ra- (dashed)and blunt (solid) ends, and equator (dotted). dius increasesabout 12.6%. The functional pore Left: overall mean value with range and standard de- radii estimates in this study and that of Rahn viation at each point; sample sizesare given. Two axes and Hammel (1982) become 12.7 and 13.2 pm, are not to the same scale:x-axis shows pore radius in which are almost identical to the mean minimum pm; y-axis has not been given units, but mean shell thickness(= pore length)is 569 k 60 pm (range= 454- radius of the major portion of an average pore 660 pm). Inset shows real proportions of pore. (Fig. 2B). This supports the finding of Toien et al. (1987) that it is the minimum pore radius that contributes most to the resistance to diffusion bation in nature and this is thought to be a result through eggshells.However, unlike the chickens of increased metabolic heating with embryonic they studied, the intact organic cuticle of Adelie development combined with the erosion of the Penguins contributes significantly to total resis- organic cuticle on the surface of the egg (Rahn tance. and Hammel 1982). Since embryonic mass and Using the equation: metabolism is small during the first half of incubation (C. Vleck et al. 1980) an increase in Ap = 9.2.10-5.~1.23 (7) I’&, causedby higher metabolism will not occur (Ar et al. 1974) until the latter half of incubation. As the circu- to predict functional pore area (Ap) for an eggof latory system develops there will be an increase 117.5 g (Table 1) gives 3.3 mm*, which is lower in heat flow from the brood patch through the than our value of 3.9 mm2 for untreated and 4.6 egg, increasing mean egg temperature, also mm* for treated eggs. (Turner 1987). Field data presentedby Rahn and Hammel (1982) for the mass of eggsfor the first CONDUCTANCE 23 days of an approximately 35day incubation Conductance clearly increases with removal of period actually showed a greater increase in the the cuticle in eggs of Adelie Penguins, thus con- rate of water lossin the first part of the measuring firming that the cuticle is an important means of period than in the last part. As the organic cuticle reducing the rate of water loss, presumably in is a significant contributor to the resistance of response to the arid Antarctic environment (R&m the shell to water loss, it seems likely that con- and Hammel 1982). The rate of water loss of ductance increasesduring the first half of incu- Adelie Penguin eggs increases throughout incubation by erosion of the cuticle, increasing con- 310 MICHAEL B. THOMPSON AND KENNETH N. GOLDIE

TABLE 3. Conductancevalues measuredand calculatedin Adelie Penguin eggs.n = 6 pretreatment eggsand n = 3 eggswashed in hypochlorite to remove waxy cuticle. R & H representsdata of Rahn and Hammel (1982).

GHO coo, Parameter mgday-tTorF cm’day-G”P .Tor ’ cm’day-G”e .Ton- ’ cm’day-Gs .Torr-’ cm'day-'.Tor+

Temperature (“C) 25.5* 25.5* 35 35 35 Pretreatment 15.4 19.2 19.5 15.4 12.6 Range 14.2-17.1 17.7-21.3 18.0-21.6 14.2-17.1 11.7-14.0 Posttreatment’ 18.6 23.1 23.5 18.6 15.3 Range 17.7-20.0 21.4-24.9 21.7-25.3 17.1-20.0 14.1-16.4 R&H 13.1 16.3 16.6 13.1 10.7 * Posttreatmenteggs were measuredat 25.6”c.

ductance compatible with the respiratory calculate G,, and GcoZ(Table 3) (Paganelli et al. requirements of the embryo. For an eggin a dry 1978). This calculation assumesthat the cuticle environment like the Antarctic, it is advanta- is dry. By substituting into equation 1 we cal- geous for the rate of water loss to be as low as culate AP,,, to be 54 Torr for pretreated eggsand possible without compromising the exchange of 45 Torr for treated eggswhich gives pre-IP oxy- respiratory gases.One way to maximize this is gen tensions of 104 Torr at sea level if conduc- for conductance to increase to match the respi- tance did not changefrom initial values, and 113 ratory demands as incubation proceeds (Sey- Tot-r if all the cuticle was removed. Predicted mour et al. 1986). It seemsthat Adelie Penguins pre-IP 0, tensions of 109 Torr in eggsthe size achieve this by providing the egg with a waxy of those of Adelie Penguins (D. Vleck et al. 1980) outer cuticle which reduces conductance, but are similar to that of treated (cuticle removed) gradually erodes away. The initial “dried mud” eggs.Even if the 0, tension of 98 Torr calculated appearanceis explained as the junction between from the mean conductance value of Rahn and adjacent cuticular units. “Mushroom-shaped” Hammel (1982) is used, it is unlikely that oxygen cuticular units may trap vapor beneath the cap would be limiting to the embryo. In fact, it is as suggestedby Deeming (1987) and perhaps it close to the value found in a variety of birds is just the capsthat break off at the narrow point (Hoyt and Rahn 1980). on the stalk (Fig. 2G). As the cuticle erodes,con- By assuming an RQ of 0.7 1 (Rahn et al. 1974) ductance increases.Although we were unable to to calculate V,,, from V,, of 24.6 cm3.hr-l, Pco2 obtain shell from incubated Adelie Penguin eggs, inside the pre-IP eggwould be 47 Torr. If all wax those of Ring Penguins, Aptenodytespatagonica,was removed, CO, tension would be 39 Torr, which live in much less severe climates than closeto the predicted value of 34 Torr (D. Vleck Adelie Penguins, lose a mean of 48% of their et al. 1980). However, levels of47 Torr are higher cuticle through erosion during incubation and than any levels recordedfor eggsof this size (Rahn this results in increasing conductance through et al. 1974, D. Vleck et al. 1980). Conductance incubation (Handrich 1989). data from Rahn and Hammel (1982) predict in- By knowing the conductance, incubation tem- ternal pre-IP CO, tension of 55 Torr, which is perature, and pre-IP V,,, we can calculate the higher than the mean value for birds (Hoyt and partial pressureof oxygen inside the eggprior to Rahn 1980) and higher than recorded in any but hatching. Pre-IP V,, is 34.6 cm3.hr-l in Adelie the smallest avian eggs, and may therefore be Penguins (Bucher et al. 1986). This is remarkably stressfulto the embryos. close to the value of 32.6 cm’. hr’ predicted us- Based on egg mass alone, and on the ratio of ing the relationship between V,, and fresh egg egg mass to incubation time, the predicted con- mass and incubation time determined by Hoyt ductance of eggsof Adelie Penguins is 18 rng. and Rahn (1980). Adjusting the G,,, measures day-‘.Torrl (Rahn and Hammel 1982) which in Adelie Penguinsfrom mg.day-‘.Torr-I to cm3. is very close to the value we obtained for eggsin day-‘.Torr-’ (coefficient = 1.244, Paganelli et al. which the cuticle was removed (Table 2). Con- 1978) and adjusting to the incubation tempera- sequently, we conclude that the cuticle is an ad- ture of 35°C (Rahn and Hammel 1982) we can aptation that reducesthe rate of water loss in the ADELIE PENGUIN EGGS 311 early stagesof incubation, but that its erosion LITERATURE CITED during incubation facilitates proper respiratory ACKERMAN,R. A., ANDM. PLATTER-REIGER. 1979. function of the egglate in incubation. We do not Water lossby pied-billedgrebe (Podilymbus pod- know how much of the cuticle erodes during in- imps) eggs.Am. Zool. 19:921. cubation or whether it has any other function. AR, A., ANDH. RAHN. 1978. Interdependenceof gas About 50% of the cuticle on eggsof King Pen- conductance,incubation length, and weight of the avian egg, p. 227-237. In J. Piiper [ed.], Respi- guins is eroded during incubation (Handrich ratory function in birds, adult and embryonic. 1989). Springer-Verlag, Berlin. Clearly cuticular erosion is not the only way Aa, A., ANDH. RAHN. 1980. Water in the avian egg: of achieving an increase in conductance as in- overall budget of incubation. Am. Zool. 20:373- 384. cubation proceeds.In the buried eggsof mega- AR, A., C. V. PAGANELLI,R. B. REEVES,D. G. GREENE, podes,a similar result is obtained by having fun- AND H. RAHN. 1974. The avian egg:water vapor nel-shapedpores with extremely narrow openings conductance,shell thickness,and functional pore to the inside of the shell. As the shell is etched area. Condor 76: 153-l 58. AR, A., H. RAHN, AND C. V. PAGANELLI.1979. The by the embryo during incubation, the narrowest avian egg:mass and strength.Condor 8 1:33l-337. part of the pore is obliterated, removing the re- BOARD,R. G., S. G. TULLETT,AND H. R. PERROTT. gion of higher diffusion resistanceas well as re- 1977. An arbitrary classificationof the pore sys- ducing pore length (Booth and Seymour 1987). tems in avian eggshells.J. Zool. (Lond.) 182:25l- Adelie Penguins do not have pores of this shape. 265. BOOTH,D. T., AND R. S. SEYMOUR. 1987. Effect of The predicted shell thickness for Adelie Pen- eggshellthinning on water vapor conductanceof guins (from equation 7 of Ar et al. 1974) is 0.45 Mallee Fowl eggs.Condor 89:453-459. mm, 2 1% thinner than the actual mean thickness BUCHER,T. L., G. A. BARTHOLOMEW,W. Z. TRIVEL- (Table l), and is reflectedin the shell being heavi- PIECE,AND N. J. VOLKMAN. 1986. Metabolism, er than predicted (Rahn and Hammel 1982). The growth, and activity in Adelie and Emperor penguin embryos. Auk 103:485493. effect of a thicker than predicted shell on con- CAREY,C. 1980. Adaptation of the avian egg to al- ductanceis offsetto some extent by a greaterthan titude. Am. Zool. 20:449459. predicted functional pore area. Perhapsa weaker CAREY,C., S. D. GARBER,E. L. THOMPSON,AND F. C. eggshell, or one with more extremely funnel- JAMES. 1983. Avian reproduction over an alti- shaped pores, would be more likely to break in tudinal gradient. II. Physical characteristicsand water loss of eggs.Physiol. Zool. 56:340-352. a penguin nest, especially when compared to the DEEMING,D. C. 1987. Effect of cuticle removal on protection offered by the mound nest of a mega- the water vapour conductanceof eggshells of sev- pode. Also, with such a thick shell, it is unlikely eral speciesof domestic birds. Br. Poult. Sci. 28: that embryonic dissolution of the shell would be 23 l-237. HANDRICH,Y. 1989. Incubation water loss in Ring sufficientto significantly decreasepore length and penguin egg. I. Change in egg and brood pouch influence pore shapeas in the thinner megapode parameters.Physiol. Zool. 62:96-l 18. eggshells.Consequently, a waxy cuticle may be HOYT,D. F. 1976. The effect of shapeon the surface- better able to provide a different solution to the volume relationships of birds’ eggs. Condor 78: requirement for an increasing conductance 343-349. through incubation in penguins than a different HOYT, D. F. 1979. Practical methods of estimating volume and fresh weight of bird eggs.Auk 96:73- pore shapewould, especially if it has some other 77. function, also. HOYT, D. F., AND H. RAHN. 1980. Respiration of avian embryos-a comparative analysis. Resp. ACKNOWLEDGMENTS Physiol. 39:255-264. This studywas supportedby a New ZealandUniver- PAGANELLI,C. V. 1980. The physicsof gas exchange sitiesGrant PostdoctoralFellowship to M.B.T. and by acrossthe avian eggshell.Am. Zool. 20:329-338. the Departmentof Zoology, Victoria University of PAGANELLI,C. V., R. A. ACKERMAN,AND H. RAHN. Wellington.Approval for eggcollection was given by 1978. The avian egg: in vivo conductancesto RossDependency Research Committee, Antarctic Di- oxygen, carbon dioxide, and water vapor in late vision, New ZealandDepartment of Scientificand In- development, p. 212-218. In J. Piiper [ed.], Re- dustrial Research(DSIR), and eggswere kindly col- spiratory function in birds, adult and embryonic. lectedand returnedto Wellington-byJohn Co&rem, Springer-Verlag,Berlin. EcoloavDivision. DSIR. Final manuscrintorenaration RAHN, H., AND A. AR. 1974. The avian egg: incu- was supportedby the Departmentof &ology, Uni- bation time and water loss. Condor 76: 147-152. versityof Florida. RAHN, H., C. V. PAGANELLI,AND A. 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