Camp. Eiochem. Physiol. Vol. 93A, No. 2, pp. 473475, 1989 Printed in Great Britain

0300-9629189 $3.00 + 0.00 c 1989 Pergamon Press plc

THE EFFECTS OF TEMPERATURE ON OXYGEN

CONSUMPTION OF EGGS AND HATCHLINGS OF THE

NILE CROCODILE (CROCODYLUS NILOTICUS)

A. hLIE,*t T. I. KANUI and G. M. 0. MALOIY

Department of Veterinary Physiology, College of Agriculture and Veterinary Sciences, University of Nairobi, Nairobi, Kenya; and *Department of Physiology and Nutrition, The Norwegian College of Veterinary Medicine, PO Box 8146, 0033 Oslo 1, Norway

(Received 22 November 1988)

Abstract-l. The oxygen uptake (PO,) of eggs and hatchlings of the Nile crocodile was measured at ambient temperatures (T,) of 25, 32 and 37’C.

2. At a T, of 32”C, VO, was found to be stable during the last month of incubation. It, however, increased with 15% after hatching.

3. Embryos and hatchlings increased their riO,, following exposure to increasing temperatures. At T, between 25 and 32°C Q,, were 1.83 and 2.3 for the eggs and hatchlings, respectively. The Q,, decreased to 1.27 for the eggs but that of hatchlings increased to 2.59 at r, between 32 and 37°C.

4. Exposure of eggs to pure oxygen at r, of 37°C was accompanied by an increase in metabolism (VO,) to a level similar to that of hatchlings kept in air.

5. It is postulated that a combination of high temperature and environmental oxygen may reduce the incubation time of the crocodile eggs.

INTRODUCTION

The incubation time of crocodile eggs decreases with increasing incubation temperature. Eggs incubated at 28, 31 or 34°C hatch after 110, 88 and 84.4 days, respectively (Hutton, 1987). Thus, the temperature effect on the incubation time is larger between 28 and 31”C, than between 31 and 34°C. Since increasing the incubation temperature from 31 to 34°C hardly has any effect on the incubation time, the metabolic rate of the embryo may be close to its maximum at 31°C. In order to test this hypothesis, the oxygen con- sumption was measured at 25,32 and 37°C during the last month of the incubation period. Furthermore, in order to see whether the temperature effect was different before and after hatching, the oxygen uptake of hatchlings was determined at the same temperatures.

Due to limited diffusion across the egg shell, chicken embryos experience mild hypoxia during the last week of the incubation (Hsiby et al., 1983). By

increasing the oxygen content in the incubator, both the oxygen uptake and the growth rate increase (Metcalfe et al., 1981; Hsiby et al., 1983; Stock et al., 1985). In order to see whether increased environ- mental oxygen has an effect on the oxygen uptake of crocodile embryos, the eggs were exposed to pure oxygen at 37°C.

MATERIALS AND METHODS

Nile crocodile eggs (Crocodylus niloficus) were obtained from a local dealer in Mombasa, Kenya. The eggs were from two different nests and arrived 5 and 3 weeks before hatching. The mean weight of the first batch of three

?-Author to whom all correspondence should be addressed.

eggs was 83.7 + 2.4 g, while the remaining batch weighed 89.1 f 8.5 g. The mean weight of the hatchlings was 56.1 f 3.3 g and 67.5 & 5.4 g for the first and the second group, respectively.

Eggs were incubated in a chicken incubator (Glarus 8750). Part of the floor area was covered with a 34 cm thick layer of sand, the rest with water. In order to increase the water vapour pressure of the air, the walls were covered with wet filter paper. The air temperature was kept at 32.0 & 0.5”C and the relative humidity at about 85%. The eggs were placed on the wet sand and allowed to hatch in the incubator. The hatchlings were kept in the incubator for one week, after which they were transferred to an 80 1 aquarium with sand and flat stones in one part and 8cm deep water in the other part. A 250 W infrared lamp, placed 40 cm above the sand, kept the temperature in the water at about 27°C. The hatchlings were fed once a day by hand with pieces of raw fish meat and calf liver.

Oxygen uptake was measured using constant pressure differential manometric respirometer as described by Dixon (1934). The measuring device consisted of an air-tight respiratory chamber (0.5 1) connected to a similar compen- satory chamber through an M-shaped tube. The tube was partly filled with coloured soap water. During the egg experiments, about 40g of soda lime was placed at the bottom of the respiratory chamber and the egg placed on a screen above the absorber. During experiments on hatch- lings, the absorber was kept in perforated plastic tubes fastened to the lid. When oxygen was consumed and carbon dioxide absorbed, the pressure in the respiratory chamber decreased. By injecting oxygen into the respiratory chamber by means of a 2 ml air-tight precision syringe, the pressure difference between the two chambers was abolished. The oxygen uptake was calculated by taking the exact time for the consumption of 1 ml oxygen. All values were adjusted to 0°C and 760mmHg. The respirometer was placed in a thermoregulated water bath.

The egg or the hatchling was prewarmed to 25, 32 or 37 + 0.2”C before it was placed in the spirometer which was kept at the same temperature. The oxygen uptake was measured until stable values were obtained, thereafter at

473

474 A. AULE et al.

16 -

halchling

12 - i, embryo

8

TEMPERATURE

(“C)

Fig. 1. Oxygen consumption of Nile crocodile eggs and hatchlings at different ambient temperatures. point is based on 4-19 measurements and the vertical bars indicate the SE of the means.

Each

least six measurements were made and the mean value used. At 37”C, the effect of incubation in pure oxygen was tested by flushing the spirometer with oxygen. The measurements were made after 30 min, or once the uptake was stable. The air, egg surface and cloaca1 temperature were measured with a thermocouple connected to digital thermometer (Thermo- electro 2105) with an accuracy of &O.l”C.

Values are expressed as mean + 1 standard deviation. Statistical comparisons were made by means of Student’s r-test, and P values co.05 were accepted as significant differences.

RESULTS

The oxygen uptake of the eggs was stable during the last month of the incubation period. After hatch- ing there was a small, but significant increase in the uptake (Table 1).

Eggs and hatchlings exposed to 25°C decreased their oxygen uptake to 63 and 56% of that at 32°C but there was no appreciable difference between the LO, of eggs and hatchlings at this temperature. Exposure to 37°C increased the oxygen uptake of the hatchlings to 161% of that at 32”C, while the effect on the eggs was minor as the PO2 only increased to 118% (Fig. 1, Table 2).

Eggs kept at 37°C increased their uptake with 29% when they were exposed to oxygen, while such exposure had no effect on the hatchlings (Table 3).

Table I. The oxygen uptake of eggs and hatchlings kept at an ambient temperature of 32.0 k 0.2% N indicates number of obser-

vations

Oxygen uptake

Age (days) ml O,/hr ml O,/g*/hr

Eggs -31 to -1 8.11 + 1.26 0.135~0.022 (N=l9) Hatchlings 0 to I9 9.32 + 0.66 0.158 f 0.025 (N = 8)

P < 0.02 P < 0.05

*Weight of the hatchling at hatching.

DISCUSSION

The oxygen uptake (PO,) of the crocodile embryos kept at 32°C seems to be at its maximum the last l/3 part of the incubation period. This agrees with the findings of Lynn and von Brand (1945) on four species of small turtle eggs kept at 25°C. Ackerman (1981), on the other hand, found that PO, increased until hatching in both green and loggerhead turtle eggs kept at 30°C. Using PO, data from turtle and snake eggs, weighing from 7 to 62 g, he related maximal PO, to egg weight by the equation: PO, = 13.01 f 1.95 W, where PO, is ml O,/day at a temperature of 30°C and W is egg weight in g (Ackerman, 1981). The oxygen uptake of our croco- dile eggs was 194.6 ml O,/day at 32°C. According to the above equation, an egg of 89.1 g (mean weight of six eggs) should have a maxima1 PO2 of 186.8 ml O,/day at 3O”C, which is close to the observed value.

The ri0, after hatching was only 15% higher than before hatching. Turtle hatchling, on the other hand,

Table 2. Thermal sensitivities (Q,,) of resting oxygen consumption of embryos and hatchlings within tem-

perature intervals between 25 and 37°C

Temperature Q,, interval

(“C) Embryo Hatchling

25-32 1.83 2.30 32-37 1.27 2.59

Table 3. The oxygen uptake of eggs and hatchlings kept ar 37.0 f 0.2”C in air and in oxygen

Oxygen uptake

(ml OJhr)

In air In oxygen

Eggs 9.61 It 2.33 (A’ = 8) 12.36 k 1.42 (N = 5) P < 0.05

Hatchlings 15.01 f 4.28 (N = 6) 13.93 + 3.60 (N = 4) NS P <0.02 NS

Metabolism of Nile crocodile eggs 475

showed no increase in PO, after hatching (Lynn and von Brand, 1945; Prange and Ackerman, 1974). In precocial avian hatchlings PO, increases consid- erably after hatching. The specific riOZ of chickens increased from I .24 ml O,/g/hr on the hatching day to 3.25 ml OJg/hr 4 days later, or 2.6 times (Bjtinnes et al.. 1987). The crocodile hatchling had a VOZ of 0.25ml OJg/hr at 37”C, which is about I/S of the chicken. When the chicken reaches its maximal rate on day four, its resting metabolic rate is 13 times the crocodile rate. Else and Hufbert (1981) found that the resting metabolic rate of a mouse was eight times that of a lizard of similar size and a body temperature of 37-c.

Embryonic reptiles behave like ectothermic organ- isms in their response to temperature since metabo- lism increases with temperature and they hatch earlier in a warm than in a cold environment (Packard ef al., 1977). Both crocodile embryos and hatchlings increased their EiOZ when the temperature was increased from 25 to 32°C. The Q,,, was 1.83 for the embryos and 2.3 for the hatchlings, When the temperature was increased further, both embryos and hatchlings increased their uptake, but the temperature effect was much larger for the hatchlings (Q10 = 2.59) than for the embryos (Q,* = 1.27) between 32 and 37°C. Eggs incubated in oxygen at 37°C were, however, able to increase their VO, to the same level as the hatchings. If PO, in air at 32°C is compared with that at 37°C in pure oxygen the Q,, is 2.3. This means that the embryo is governed by the van’t Hoff effect within this temperature range, pro- vided that it is supplied with sufficient oxygen. Whether the embryo turns to anaerobic metabolism during air incubation at 37°C is not known, but since both young alligators and crocodiles (Crocodylus porosus) have large capacities for anaerobic glycolysis (Coulson and Hernandez. 1983; Wright, 1986), one would expect the crocodile embryos to have some ability to turn to anaerobic metabolism when the oxygen supply is limited.

The rigid-shell eggs of American alligator have 4.2 times higher water vapour conductance than avian eggs of similar size (Packard er al., 1979). In com- parison, the conductance of two unfertilized croco- dile eggs (unpublished data) was about 60% of that predicted for an avian egg of similar size using the allometric equation of Ar et af. (1974). The water vapour conductance is proportional to the oxygen conductance across the shell (Paganelli et al., 1978). Thus, a low oxygen conductance across the shell may explain why the embryo in air was unable to increase its aerobic metabolism sufficient to cope with the increased total metabolism at 37°C. It is possible, therefore, that the small temperature effect noted during incubation at temperatures above 31°C (Hut- ton. 1987). could probably be att~butable to some degree of hypoxia. A combination of high ambient temperature and oxygen, could, therefore, be used in commercial farms to reduce the incubation time of crocodile eggs as well as to determine the preferred sex of the crocodile population.

The relationship between oxygen consumption and weight at different temperatures in C. porosus, weigh- ing from 49 to 4078 g, has been described by Wright ( 1986). According to his equations, a 61.8 g crocodile

(mean weight of our hatchhngs) should have a PO, of 5.05 ml O,/hr at 25°C and 7.68 ml O,/hr at 33”C, which gives a Q,, = I .69. Our value was 5.20 ml OJhr at 25°C and 9.32ml O,/hr at 32”C, which gives a Q,, = 2.3. This indicates that the temperature has a larger effect on the metabolic rate of C. niloticus than of C porosus. We would like to suggest that further studies aimed at examining in detail the metabolism, water vapour conductance, shell thickness and func- tional pore area of the Nile crocodile eggs are both necessary and rewarding.

~e~~o~~e~ge~~~~-This work was supported by generous grants from NORAD KEN 046 Project and Leverhulme Trust London. A. Aulie would like to thank the University of Nairobi and NORAD for their hospitality during his stay in Kenya.

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