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Oxygen consumption of brown noddy (Anous stolidus) embryos in a quasiequilibrium state at lowered ambient temperatures
Camp. Eiochem. Physiol. Vol. 93A. No. 4, pp. 707-710, Printed in Great Britain
1989 0
0300-9629/89 $3.00 + 0.00 1989 Maxwell Pergamon Macmillan plc
OXYGEN CONSUMPTION OF BROWN NODDY (ANOUS STOLIDUS) EMBRYOS IN A QUASIEQUILIBRIUM STATE AT LOWERED AMBIENT TEMPERATURES C. MATSUNAGA,*
*Department tDepartment
P. M.
MATHIU,?
G. C. WHITTOW?
and H.
TAZAWA*$
of Electronic Engineering, Muroran Institute of Technology, Muroran 050, Japan and of Physiology, John A. Burns School of Medicine, University of Hawaii, Honolulu, HI 96822, U.S.A.
(Received 3 January 1989) Abstract-l. The oxygen consumption (hiIoz) of the semi-precocial Brown Noddy embryos at different stages of development was measured at 36’C and again after 5-hr exposure to lowered ambient temperatures (30 and 32’C). 2. The MO2 measured in a quasiequilibrium state was equal to the value predicted by a temperature coefficient of 2. 3. In contrast to precocial chickens, the semi-precocial Noddy had no apparent metabolic response to cooling before hatching.
INTRODUCTION
Precocial
chickens
(Callus
domesticus)
exhibit
a
feeble, incipient metabolic response to cooling, indicating endothermic homeothermy before hatching in terms of changes in oxygen consumption and catabolism (Romijn and Lokhorst, 1955; Freeman, 1964, 1967; Tazawa et al., 1988, 1989a, b). This coincides with enhanced activity of the thyroid gland and increasing concentrations of peripheral thyroid hormones during the last stages of incubation in chickens and quails (Thommes and Hylka, 1977; Decuypere et al., 1979; Ockleford et al., 1983; McNabb, 1987). However, the capacity of metabolic compensation for lowered ambient temperatures is not large enough to maintain egg temperature or even to maintain oxygen consumption (MO,) in the face of further decreases in ambient temperature. The detection of homeothermic capacity thus requires a procedure in which a heat loss from the egg does not overwhelm the heat production of the embryo. The “gradual cooling test” fulfilled this requirement and it showed that prenatal chicken embryos near term responded to lowered temperature with a &IO1maintained at the 38°C level until ambient temperature fell below 35°C (Tazawa et al., 1988). This response in late embryos was evidently different from that in young embryos. In another test, “prolonged cooling test” (Tazawa et al., 1989a), when the late chicken embryos were exposed to lowered temperatures for a prolonged period, the ti,, was maintained at a level above that predicted by a temperature coefficient (Q,,) of 2, and this response also differed from that shown by young embryos. This homeothermic response of 0, consumption to lowered temperatures before hatching may be a feature of precocial birds and it has been suggested that it is absent from altricial species (Tazawa et al., $Author
to whom
all correspondence
should
be addressed. 707
1988). It seems worthwhile to examine a semiprecocial species for metabolic responses to lowered temperatures to see if it is intermediate between precocial and altricial species in this respect. Three species of semi-precocial seabirds (Sooty Tern, Sterna fiscatu; Brown Noddy, Anous stolidus; Wedge-tailed Shearwater, Pujinus pucz$cus) breed in large numbers on Manana Island, a small offshore island of Oahu in the main Hawaiian Islands, within easy access of laboratories at the University of Hawaii. The Brown Noddy was selected for study in the present investigation, because it has been designated to be the least semi-precocial of the three species, on the basis of its egg-yolk content and thermogenic capacity (Mathiu, 1988). Thus, the responses of the Brown Noddy embryo would be expected to be the most different from those of the domestic fowl, among the three species breeding on Manana Island.
MATERIALS
AND METHODS
The breeding season of the Brown Noddy (Anous stolidus) extends from April to August: the incubation period of the egg is 36 days. The birds lay their eggs directly on the ground; they favor rocky ridges as a nest site. Eggs were collected in May and June, brought to the laboratory and numbered in sequence. They were incubated at 36’C in a forced-draft incubator and turned three times a day, Because the age of embryos was not known when the eggs were collected, the day of the first shell fracture (i.e. initiation of external pipping) was noted for all eggs and the age of prepipping embryos was calculated with reference to the external pipping. Every day after collection, some eggs were candled, embryonic development estimated and the Ao, at 36 ‘C determined. The capacity of the homeothermic response was examined by the prolonged cooling test (Tazawa et al., 1989a); the &lo> of the eggs was first determined at control temperature (36”(Z), and the eggs were exposed to a stepwise lowered ambient temperature for a prolonged period. The tie: was
708
900 800 t 700 _ 600ci 0” 9 500-
I
.
prepipped egg
*
shell - fractured egg
e
internally-pipped
0
pip-holed
egg
egg
5 0” GOO3 I
OL,,““““““,‘.‘,‘. -16
-14
-12
-10
-8
-6
-4
SF
-2
Fig.
I The 0, consumption
(A&) of Brown Noddy embryos incubated artificially the first day of shell fracture. See text for explanation.
measured again after egg temperature had reached a quasiequilibrium state. which was compared with the value corresponding to that for a Q10 of 2 (referred to as “Q10 = 2 value”). The quasiequilibrium state was previously defined as the egg temperature did not change by more than 0.1 C during 1 hr at a lowered ambient temperature (Tazawa and Rahn, 1987). In chicken eggs, it was attained after at least 4 hr of exposure and the ho, also reached a quasiequilibrium value (Tazawa et al.. 1989a). In the present study, it was assumed that the quasiequilibrium state would have been attained after 5 hr of cooling. The metabolic response to prolonged cooling was determined for eggs in the following stages of development: unpipped eggs, shell-fraetured eggs, internally pipped eggs. eggs with a pip hole and ~tchlings (Pettit and Whittow, 1983). In the light of previous experiments on chicken eggs (Tazawa er al.. 1989a) and of preliminary tests with Brown Noddy eggs. the eggs were exposed to a temperature of 30 and 32’C. Because the number of eggs available for the experiment was limited, eggs were subjected to prolonged cooling one to three times. That is, eggs were replaced in an incubator at 36 C after determining the &lo, at a lowered temperature (30 or 32 C). and some of them were cooled again on other days. The cooling procedure for each egg was designed so that the first exposure to cold was randomized among eggs at different stages of development. Measurement of I’&,, was made with a modified volumetric microrespirometer (Scholander. 1942; Ackerman et ni., 1980). which was submerged in a thermos~ticaliy controlled water bath. Procedures for ambient temperature alteration and Ii?,, determination followed them in &he preceding report (Tazawa er ul., 1989a).
Table
I. Oxygen
uptake (Fir,,,).
M,.
No:
Prepip
1st
Is1
336 _+ 34 x
385 * 39
552 i_ 57
.+ SD
PrepiP.
prepipped
ofevent:2nd. hatching.
egg
I
9
day before
2nd day of event:
measured
7
shell fracture; F, immrd~ately
different stages of pipping and hatching
Pip hole
____---
II
SF. shell-fractured before emergence
at 36’C. SF indicates
The hiI,, of eggs measured at 36°C at different stages of development, are presented in Fig. 1. Because the first.day of shell fracture (SF) was noted for all eggs, the MO2was plotted in relation lo SF, and the solid lines connect the same eggs. Eggs in which the Mo, was measured only once are not presented in Fig. 1, but they are included in Table 1 which shows the average value (with SD) for individual stages of development from the last day of the prepipping period (referred to as Prepip) to emergence from the egg. There was some variation in the events during pipping: pip hole was not necessarily made on the 3rd or the 4th day after SF and birds did not necessarily emerge from the shell after the 2nd day of pip-hole formation. Some birds hatched 3 days after the first SF. The hatchlings indicated in column F (Table 1) hatched during cooling and thus their hilo, at 36°C had been measured during pip-hole formation. Column Od indicates hatchhngs in which their cooling responses were measured within half a day after hatching, and hatchlings which were kept overnight after hatching are shown in column Id. Figure 2 shows the &lo, at 32 or 30°C measured after 5 hr of exposure, which is plotted as percentage of its own control value at 36°C. The metabolic response to both environment temperatures was determined for unpipped (O), shell-fractured
2nd ~~-I____ 642+61
4
RESULTS
at 36 C. for
IP
SF --
2
Days after SF
Days prior to shell fracture
Hatchling
1st
2nd
F
Od
Id
696 + 64
808 * 33
859 2 99
898 + 98
870 rt I I I
*
egg; IP, internally from
‘4
pipped
the shell; 0 d. within
h
egg: Pip hole, 12 hr after
c
1.
pip hole in egg; 1st. 1st day
hatching;
I d.
within
36 hr after
Metabolic
PrepWed
response
egg
shell -fractured internally-pipped pip-holed
d
egg egg 0 0 :
egg
120P x ‘ii
llO-
p
loo-
5 z
go-
.-”
$-
32%
A
30%
A
I A
8070
*
-!y
60-
-1Q 0
.
50Fig. 2. The O2 consumption (Mo?) in a quasiequilibrium state at lowered ambient temperature, plotted as percentage of control value (36 C). Hatchlings are shown by triangles and circles. The thick solid line indicates the Mo, corresponding to Q,, of 2 for individual temperatures.
(externally pipped) (Jlc), internally pipped (a), pipholed eggs (0) and hatchlings. For hatchlings. closed triangles indicate birds which hatched after their control MO, at 36 C was measured at the pip-hole stage: open triangles represent newly hatched birds in which control Mel was determined after hatching; open circles are l-day-old birds. The large symbols at the individual stages of development indicate the eggs which were subjected to cooling for the first time, and the eggs which were cooled previously are shown by small symbols. There was no marked difference between them. DISCCSSION
Comparison of Mo, during prepipping developmet (Fig. I) with that measured previously for naturally incubated eggs (Pettit and Whittow, 1983) indicates that the present values are slightly higher; e.g. preexternal pipping Mo, for I4 naturally incubated eggs was 283 ml O?/day + 47 (SD), while it was 336ml O,/day + 34 (SD, n = 8) in the present eggs. This may be attributed to the measuring temperatures. The previous measurement was made at an average natural incubation temperature of 35C and the eggs in the present study were incubated artificially at 36-C. In contrast to the domestic fowl, the Brown Noddy pips the shell first (external pipping or SF) and then penetrates the air cell with its beak (internal pipping) (Pettit and Whittow. 1983). While the MO2 at control temperature (36°C) increases with SF and during pip-hole formation, the predominant increase occurs during internal pipping (Fig. I and Table 1). This finding is in accord with unpublished data for the Brown Noddy obtained by Mathiu, Dawson and Whittow using a different technique for measuring MO,. Because the chorioallantoic membrane is still intact even after the shell is fractured, there is no ventilation of the lungs and the O2 supply is
of Noddy
embryos
709
restricted. This restriction is mitigated by pipping the chorioallantoic membrane and breathing air. Subsequently, embryos spend several days in the egg until they escape from the shell. The MO, does not significantly increase upon emergence from the shell (Table I, P > 0.1, Student’s unpaired t-test). The low ambient temperatures to which the Brown Noddy embryo was exposed were determined after taking account of the results obtained for chicken eggs (Tazawa et al., 1989a). In chicken eggs, exposure to an ambient temperature lowered by 10°C from the control (38°C) reduced the MO, to a value equivalent to that predicted by a Qlo = 2, independently of developmental stage, including external pipping. However, the externally pipping embryos responded to an ambient temperature lowered by 8°C (30’C) with a MO, higher than the value predicted from Q,, = 2. When the ambient temperature was lowered by 6C (32°C) the late prepipping chicken embryos (18 days old) increased their MO2 above the predicted value of Q10 = 2. Preliminary experiments made for Brown Noddies with pip holes in their eggs indicated that they showed no homeothermic metabolic response when the ambient temperature was lowered by more than 8’C (28’C). The metabolic response was therefore examined at 30°C (lowered by 6-C) and 32 C (by 4C) for all stages of pipping and hatching (Fig. 2). In contrast to precocial chickens, the semi-precocial Brown Noddy before hatching decreased the MO, to the value predicted from a Q,, = 2 in a quasiequilibrium state at 30°C. Newly hatched birds alone responded to this temperature with increasing MO,. When ambient temperature was decreased by 4C to 32’C, the MO, at the quasiequilibrium state was slightly high, above the value predicted from Q,, = 2. This interpretation is not a simple one, however, because a natural increase in Mo, during 5-6 hr of exposure may not be neglected at this temperature. Nevertheless, corroboration of this result comes from Mathiu, Dawson and Whittow (unpublished data). Repeated exposures of the egg to a lowered temperature might affect their metabolic response, but such effects were not evident in the present investigation. The MO, at 36°C and metabolic response of eggs which were exposed repeatedly on different days to a temperature lowered by 4 or 6-C, were not different from those of eggs subjected for the first time to a low temperature exposure (Fig. 2). In conclusion, the semi-precocial Brown Noddy fails to respond metabolically to lowered temperature before hatching, but it does after hatching, while precocial chickens exhibit incipient homeothermic responses during the late pre-pipping stage (Tazawa et al., 1989a). Of the three nominally semi-precocial species of seabirds breeding on Manana Island, the Brown Noddy has been designated the least semiprecocial of the three species (Mathiu, 1988). The metabolic response of the other two species may be stronger than that of the Brown Noddy. Consequently, it would be worth examining the metabolic responses in the other two species. Acknowledgements-This
study was supported in part by the Grant-in-Aid for Research by the Ministry of Education, Science and Culture, Japan and by a grant from the
710
C. MATSUNAGA
Hawaii Heart Association. We are grateful to the State of Hawaii Division of Forestrv and Wildlife for oermission to work on Manana Island and to the U.S. Fish’and Wildlife Service for permits to study migratory species.
REFERENCES Ackerman R. A., Whittow G. C., Paganelli C. V. and Pettit T. N. (1980) Oxygen consumption, gas exchange, and growth of embryonic wedge-tailed shearwaters (P@cus pacificus chlororhynchus). Physiol. Zool. 53, 2 I O-22 I. Decuypere E., Nouwen E. J., Kuhn E. R., Geers R. and Michels H. (1979) Differences in serum iodohormone concentration between chick embryos with and without the bill in the air chamber at different incubation temperatures. Gen. camp. Endocr. 37, 264-267. Freeman B. M. (1964) The emergence of the homeothermicmetabolic response in the fowl (Gal/us domesticus). Comp. Biochem. Physiol. 13, 413-422. Freeman B. M. (1967) Some effects of cold on the metabolism of the fowl during the perinatal period. Camp. Biochem. Physiol. 20, 179-193. Mathiu P. M. (1988) Development of thermoregulation in three species of tropical seabirds. Ph.D. dissertation, University of Hawaii. McNabb F. M. A. (1987) Comparative thyroid development in precocial Japanese quail and altricial ring doves. J. exp. Zool. Suppl. I, 281-290.
t?t al.
Ockleford E. M., Davison T. F. and Vince M. A. (1983) Changes in plasma iodohormone concentrations during the day before hatching in Gallus domesticus. Comp. Biochem. Physiol. 75A, 139-140. Pettit T. N. and Whittow G. C. (1983) Embryonic respiration and growth in two species of noddy terns. Physiol. Zool. 56, 455-464. Romijn C. and Lokhorst W. (1955) Chemical heat regulation in the chick embryo. Poult. Sci. 34, 649-654. Scholander P. F. (1942) Volumetric microrespirometers. Rev. Sci. Inst. 13, 32-33. Tazawa H. and Rahn H. (1987) Temperature and metabolism of chick embryos and hatchlings after prolonged cooling. J. e.yp. Zool. Suppl. 1, 105~109. Tazawa H., Wakayama H., Turner J. S. and Paganelli C. V. (1988) Metabolic compensation for gradual cooling in developing chick embryos. Comp. Biochem. Physiol. 89A, 125-129. Tazawa H., Okuda A., Nakazawa S. and Whittow G. C. (1989a) Metabolic responses of chicken embryos to graded, prolonged alterations in ambient temperature. Comp. Biochem. Physiol. 92A, 613-617. Tazawa H.. Whittow G. C., Turner J. S. and Paganelli C. V. (1989b) Metabolic responses to gradual cooling in chicken eggs treated with thiourea and oxygen. Comp. Biochem. Physiol. 92A, 6 19-622. Thommes R. C. and Hylka V. W. (1977) Plasma iodothyronines in the embryonic and immediate post-hatch chick. Gen. camp. Endocr. 32, 417-422.
1989 0
0300-9629/89 $3.00 + 0.00 1989 Maxwell Pergamon Macmillan plc
OXYGEN CONSUMPTION OF BROWN NODDY (ANOUS STOLIDUS) EMBRYOS IN A QUASIEQUILIBRIUM STATE AT LOWERED AMBIENT TEMPERATURES C. MATSUNAGA,*
*Department tDepartment
P. M.
MATHIU,?
G. C. WHITTOW?
and H.
TAZAWA*$
of Electronic Engineering, Muroran Institute of Technology, Muroran 050, Japan and of Physiology, John A. Burns School of Medicine, University of Hawaii, Honolulu, HI 96822, U.S.A.
(Received 3 January 1989) Abstract-l. The oxygen consumption (hiIoz) of the semi-precocial Brown Noddy embryos at different stages of development was measured at 36’C and again after 5-hr exposure to lowered ambient temperatures (30 and 32’C). 2. The MO2 measured in a quasiequilibrium state was equal to the value predicted by a temperature coefficient of 2. 3. In contrast to precocial chickens, the semi-precocial Noddy had no apparent metabolic response to cooling before hatching.
INTRODUCTION
Precocial
chickens
(Callus
domesticus)
exhibit
a
feeble, incipient metabolic response to cooling, indicating endothermic homeothermy before hatching in terms of changes in oxygen consumption and catabolism (Romijn and Lokhorst, 1955; Freeman, 1964, 1967; Tazawa et al., 1988, 1989a, b). This coincides with enhanced activity of the thyroid gland and increasing concentrations of peripheral thyroid hormones during the last stages of incubation in chickens and quails (Thommes and Hylka, 1977; Decuypere et al., 1979; Ockleford et al., 1983; McNabb, 1987). However, the capacity of metabolic compensation for lowered ambient temperatures is not large enough to maintain egg temperature or even to maintain oxygen consumption (MO,) in the face of further decreases in ambient temperature. The detection of homeothermic capacity thus requires a procedure in which a heat loss from the egg does not overwhelm the heat production of the embryo. The “gradual cooling test” fulfilled this requirement and it showed that prenatal chicken embryos near term responded to lowered temperature with a &IO1maintained at the 38°C level until ambient temperature fell below 35°C (Tazawa et al., 1988). This response in late embryos was evidently different from that in young embryos. In another test, “prolonged cooling test” (Tazawa et al., 1989a), when the late chicken embryos were exposed to lowered temperatures for a prolonged period, the ti,, was maintained at a level above that predicted by a temperature coefficient (Q,,) of 2, and this response also differed from that shown by young embryos. This homeothermic response of 0, consumption to lowered temperatures before hatching may be a feature of precocial birds and it has been suggested that it is absent from altricial species (Tazawa et al., $Author
to whom
all correspondence
should
be addressed. 707
1988). It seems worthwhile to examine a semiprecocial species for metabolic responses to lowered temperatures to see if it is intermediate between precocial and altricial species in this respect. Three species of semi-precocial seabirds (Sooty Tern, Sterna fiscatu; Brown Noddy, Anous stolidus; Wedge-tailed Shearwater, Pujinus pucz$cus) breed in large numbers on Manana Island, a small offshore island of Oahu in the main Hawaiian Islands, within easy access of laboratories at the University of Hawaii. The Brown Noddy was selected for study in the present investigation, because it has been designated to be the least semi-precocial of the three species, on the basis of its egg-yolk content and thermogenic capacity (Mathiu, 1988). Thus, the responses of the Brown Noddy embryo would be expected to be the most different from those of the domestic fowl, among the three species breeding on Manana Island.
MATERIALS
AND METHODS
The breeding season of the Brown Noddy (Anous stolidus) extends from April to August: the incubation period of the egg is 36 days. The birds lay their eggs directly on the ground; they favor rocky ridges as a nest site. Eggs were collected in May and June, brought to the laboratory and numbered in sequence. They were incubated at 36’C in a forced-draft incubator and turned three times a day, Because the age of embryos was not known when the eggs were collected, the day of the first shell fracture (i.e. initiation of external pipping) was noted for all eggs and the age of prepipping embryos was calculated with reference to the external pipping. Every day after collection, some eggs were candled, embryonic development estimated and the Ao, at 36 ‘C determined. The capacity of the homeothermic response was examined by the prolonged cooling test (Tazawa et al., 1989a); the &lo> of the eggs was first determined at control temperature (36”(Z), and the eggs were exposed to a stepwise lowered ambient temperature for a prolonged period. The tie: was
708
900 800 t 700 _ 600ci 0” 9 500-
I
.
prepipped egg
*
shell - fractured egg
e
internally-pipped
0
pip-holed
egg
egg
5 0” GOO3 I
OL,,““““““,‘.‘,‘. -16
-14
-12
-10
-8
-6
-4
SF
-2
Fig.
I The 0, consumption
(A&) of Brown Noddy embryos incubated artificially the first day of shell fracture. See text for explanation.
measured again after egg temperature had reached a quasiequilibrium state. which was compared with the value corresponding to that for a Q10 of 2 (referred to as “Q10 = 2 value”). The quasiequilibrium state was previously defined as the egg temperature did not change by more than 0.1 C during 1 hr at a lowered ambient temperature (Tazawa and Rahn, 1987). In chicken eggs, it was attained after at least 4 hr of exposure and the ho, also reached a quasiequilibrium value (Tazawa et al.. 1989a). In the present study, it was assumed that the quasiequilibrium state would have been attained after 5 hr of cooling. The metabolic response to prolonged cooling was determined for eggs in the following stages of development: unpipped eggs, shell-fraetured eggs, internally pipped eggs. eggs with a pip hole and ~tchlings (Pettit and Whittow, 1983). In the light of previous experiments on chicken eggs (Tazawa er al.. 1989a) and of preliminary tests with Brown Noddy eggs. the eggs were exposed to a temperature of 30 and 32’C. Because the number of eggs available for the experiment was limited, eggs were subjected to prolonged cooling one to three times. That is, eggs were replaced in an incubator at 36 C after determining the &lo, at a lowered temperature (30 or 32 C). and some of them were cooled again on other days. The cooling procedure for each egg was designed so that the first exposure to cold was randomized among eggs at different stages of development. Measurement of I’&,, was made with a modified volumetric microrespirometer (Scholander. 1942; Ackerman et ni., 1980). which was submerged in a thermos~ticaliy controlled water bath. Procedures for ambient temperature alteration and Ii?,, determination followed them in &he preceding report (Tazawa er ul., 1989a).
Table
I. Oxygen
uptake (Fir,,,).
M,.
No:
Prepip
1st
Is1
336 _+ 34 x
385 * 39
552 i_ 57
.+ SD
PrepiP.
prepipped
ofevent:2nd. hatching.
egg
I
9
day before
2nd day of event:
measured
7
shell fracture; F, immrd~ately
different stages of pipping and hatching
Pip hole
____---
II
SF. shell-fractured before emergence
at 36’C. SF indicates
The hiI,, of eggs measured at 36°C at different stages of development, are presented in Fig. 1. Because the first.day of shell fracture (SF) was noted for all eggs, the MO2was plotted in relation lo SF, and the solid lines connect the same eggs. Eggs in which the Mo, was measured only once are not presented in Fig. 1, but they are included in Table 1 which shows the average value (with SD) for individual stages of development from the last day of the prepipping period (referred to as Prepip) to emergence from the egg. There was some variation in the events during pipping: pip hole was not necessarily made on the 3rd or the 4th day after SF and birds did not necessarily emerge from the shell after the 2nd day of pip-hole formation. Some birds hatched 3 days after the first SF. The hatchlings indicated in column F (Table 1) hatched during cooling and thus their hilo, at 36°C had been measured during pip-hole formation. Column Od indicates hatchhngs in which their cooling responses were measured within half a day after hatching, and hatchlings which were kept overnight after hatching are shown in column Id. Figure 2 shows the &lo, at 32 or 30°C measured after 5 hr of exposure, which is plotted as percentage of its own control value at 36°C. The metabolic response to both environment temperatures was determined for unpipped (O), shell-fractured
2nd ~~-I____ 642+61
4
RESULTS
at 36 C. for
IP
SF --
2
Days after SF
Days prior to shell fracture
Hatchling
1st
2nd
F
Od
Id
696 + 64
808 * 33
859 2 99
898 + 98
870 rt I I I
*
egg; IP, internally from
‘4
pipped
the shell; 0 d. within
h
egg: Pip hole, 12 hr after
c
1.
pip hole in egg; 1st. 1st day
hatching;
I d.
within
36 hr after
Metabolic
PrepWed
response
egg
shell -fractured internally-pipped pip-holed
d
egg egg 0 0 :
egg
120P x ‘ii
llO-
p
loo-
5 z
go-
.-”
$-
32%
A
30%
A
I A
8070
*
-!y
60-
-1Q 0
.
50Fig. 2. The O2 consumption (Mo?) in a quasiequilibrium state at lowered ambient temperature, plotted as percentage of control value (36 C). Hatchlings are shown by triangles and circles. The thick solid line indicates the Mo, corresponding to Q,, of 2 for individual temperatures.
(externally pipped) (Jlc), internally pipped (a), pipholed eggs (0) and hatchlings. For hatchlings. closed triangles indicate birds which hatched after their control MO, at 36 C was measured at the pip-hole stage: open triangles represent newly hatched birds in which control Mel was determined after hatching; open circles are l-day-old birds. The large symbols at the individual stages of development indicate the eggs which were subjected to cooling for the first time, and the eggs which were cooled previously are shown by small symbols. There was no marked difference between them. DISCCSSION
Comparison of Mo, during prepipping developmet (Fig. I) with that measured previously for naturally incubated eggs (Pettit and Whittow, 1983) indicates that the present values are slightly higher; e.g. preexternal pipping Mo, for I4 naturally incubated eggs was 283 ml O?/day + 47 (SD), while it was 336ml O,/day + 34 (SD, n = 8) in the present eggs. This may be attributed to the measuring temperatures. The previous measurement was made at an average natural incubation temperature of 35C and the eggs in the present study were incubated artificially at 36-C. In contrast to the domestic fowl, the Brown Noddy pips the shell first (external pipping or SF) and then penetrates the air cell with its beak (internal pipping) (Pettit and Whittow. 1983). While the MO2 at control temperature (36°C) increases with SF and during pip-hole formation, the predominant increase occurs during internal pipping (Fig. I and Table 1). This finding is in accord with unpublished data for the Brown Noddy obtained by Mathiu, Dawson and Whittow using a different technique for measuring MO,. Because the chorioallantoic membrane is still intact even after the shell is fractured, there is no ventilation of the lungs and the O2 supply is
of Noddy
embryos
709
restricted. This restriction is mitigated by pipping the chorioallantoic membrane and breathing air. Subsequently, embryos spend several days in the egg until they escape from the shell. The MO, does not significantly increase upon emergence from the shell (Table I, P > 0.1, Student’s unpaired t-test). The low ambient temperatures to which the Brown Noddy embryo was exposed were determined after taking account of the results obtained for chicken eggs (Tazawa et al., 1989a). In chicken eggs, exposure to an ambient temperature lowered by 10°C from the control (38°C) reduced the MO, to a value equivalent to that predicted by a Qlo = 2, independently of developmental stage, including external pipping. However, the externally pipping embryos responded to an ambient temperature lowered by 8°C (30’C) with a MO, higher than the value predicted from Q,, = 2. When the ambient temperature was lowered by 6C (32°C) the late prepipping chicken embryos (18 days old) increased their MO2 above the predicted value of Q10 = 2. Preliminary experiments made for Brown Noddies with pip holes in their eggs indicated that they showed no homeothermic metabolic response when the ambient temperature was lowered by more than 8’C (28’C). The metabolic response was therefore examined at 30°C (lowered by 6-C) and 32 C (by 4C) for all stages of pipping and hatching (Fig. 2). In contrast to precocial chickens, the semi-precocial Brown Noddy before hatching decreased the MO, to the value predicted from a Q,, = 2 in a quasiequilibrium state at 30°C. Newly hatched birds alone responded to this temperature with increasing MO,. When ambient temperature was decreased by 4C to 32’C, the MO, at the quasiequilibrium state was slightly high, above the value predicted from Q,, = 2. This interpretation is not a simple one, however, because a natural increase in Mo, during 5-6 hr of exposure may not be neglected at this temperature. Nevertheless, corroboration of this result comes from Mathiu, Dawson and Whittow (unpublished data). Repeated exposures of the egg to a lowered temperature might affect their metabolic response, but such effects were not evident in the present investigation. The MO, at 36°C and metabolic response of eggs which were exposed repeatedly on different days to a temperature lowered by 4 or 6-C, were not different from those of eggs subjected for the first time to a low temperature exposure (Fig. 2). In conclusion, the semi-precocial Brown Noddy fails to respond metabolically to lowered temperature before hatching, but it does after hatching, while precocial chickens exhibit incipient homeothermic responses during the late pre-pipping stage (Tazawa et al., 1989a). Of the three nominally semi-precocial species of seabirds breeding on Manana Island, the Brown Noddy has been designated the least semiprecocial of the three species (Mathiu, 1988). The metabolic response of the other two species may be stronger than that of the Brown Noddy. Consequently, it would be worth examining the metabolic responses in the other two species. Acknowledgements-This
study was supported in part by the Grant-in-Aid for Research by the Ministry of Education, Science and Culture, Japan and by a grant from the
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Hawaii Heart Association. We are grateful to the State of Hawaii Division of Forestrv and Wildlife for oermission to work on Manana Island and to the U.S. Fish’and Wildlife Service for permits to study migratory species.
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