- Email: [email protected]
Growth rates of Chinese and American alligators
Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
Growth rates of Chinese and American alligators J.D. Herbert*, T.D. Coulson, R.A. Coulson Louisiana State University Health Sciences Center, Department of Biochemistry and Molecular Biology, 1100 Florida Avenue, Building 140, New Orleans, LA 70119, USA Received 17 August 2001; received in revised form 27 December 2001; accepted 3 January 2002
Abstract Growth rates in two closely related species, Alligator mississippiensis (American alligator) and Alligator sinensis (Chinese alligator), were compared under identical conditions for at least 1 year after hatching. When hatched, Chinese alligators were approximately 2y3 the length and approximately 1y2 the weight of American alligator hatchlings. At the end of 1 year of growth in captivity in heated chambers, the Chinese alligators were approximately 1y2 as long and weighed approximately 1y10 as much as American alligator yearlings. When the animals were maintained at 31 8C, Chinese alligator food consumption and length gain rates dropped to near zero during autumn and winter and body weights decreased slightly, apparently in response to the change in day length. At constant temperature (31 8C), food consumption by American alligators remained high throughout the year. Length gain rates in American alligators decreased slowly as size increased, but were not affected by photoperiod. Daily weight gains in American alligators increased steadily throughout the year. In autumn, provision of artificial light for 18 h a day initially stimulated both length and weight gain in Chinese alligators, but did not affect growth in American alligators. Continuation of the artificial light regimen seemed to cause deleterious effects in the Chinese alligators after several months, however, so that animals exposed to the normal light cycle caught up to and then surpassed the extra-light group in size. Even after removal of the artificial light, it was several months before these extra-light animals reverted to a normal growth pattern. These findings may be of interest to those institutions engaged in captive growth programs intended to provide animals for reintroduction to the wild or to protected habitat. 䊚 2002 Elsevier Science Inc. All rights reserved. Keywords: Alligator mississippiensis; American alligator; Alligator sinensis; Chinese alligator; Crocodilians; Growth; Photoperiod
1. Introduction For many years, we have been raising hatchling American alligators (Alligator mississippiensis) in captivity in heated chambers. One of our purposes was to achieve rapid growth at minimal cost and to establish along the way the nutritional and environmental requirements for such rapid growth. It was our assumption that the procedures we developed would be useful not only for commercial enterprises such as alligator farms, but also *Corresponding author. Tel.: q1-504-942-8343; fax: q1504-942-8274. E-mail address: [email protected] (J.D. Herbert).
for rapid repletion of dwindling stocks of other species of crocodilians that were threatened or endangered. One critically endangered crocodilian species is the Chinese alligator (Alligator sinensis), the only other alligator species in the world and the closest known relative of the American alligator. The Chinese alligator reaches only approximately half the length of an American alligator when full grown, i.e. approximately 2 m for the Chinese male compared to approximately 4 m for the American male. There appear to be only approximately 150 Chinese alligators remaining in the wild in small pockets of threatened habitat close
1095-6433/02/$ - see front matter 䊚 2002 Elsevier Science Inc. All rights reserved. PII: S 1 0 9 5 - 6 4 3 3 Ž 0 2 . 0 0 0 2 7 - 2
910
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
to the Yangzi River in south-eastern Anhui province in China, latitude approximately 318 North (Thorbjarnarson et al., 2000). Although habitat restoration is the major requirement, growth of animals in captivity is needed to supply a healthy population for reintroduction if this becomes feasible. Breeding and growth in captivity is currently occurring at the Anhui Research Center for Chinese Alligator Reproduction (ARCCAR) where several thousand alligators are presently maintained. Breeding and hatching success has been greatly improved in recent years, but rates of growth seem to have been low compared to growth rates in American alligators (Coulson et al., 1973, 1987; Coulson and Coulson, 1986; Coulson et al., 1990, 1996; Coulson and Hernandez, 1964, 1983; Joanen and McNease, 1987; Zhang et al., 1986; Wang and Liang, 1990). Comparison of growth rates using values from the literature is complicated by the fact that we have maintained constant temperature all year long, while the Chinese workers usually drop temperatures during the winter months. For the present report, we maintained individuals of both species under identical conditions for direct comparison of their growth rates. 2. Materials and methods 2.1. Animals Both Chinese alligator and American alligator hatchlings were obtained from Rockefeller Wildlife Refuge in south-west Louisiana (latitude approx. 308 north) within a week or so of hatching. Eggs there were harvested from nests in the marsh and hatched in incubators under controlled temperature and humidity. American alligator eggs were derived from nests of free-roaming alligators in the refuge. Chinese alligator eggs were produced by a breeding pair housed in a fenced enclosure in semi-wild conditions. The Chinese alligator breeding pair was obtained on loan in a cooperative enterprise with John Behler at the Bronx Zoo in New York City. The hatchlings included both sexes. There were six Chinese alligator hatchlings in 1997, received from Dr Ruth Elsey at the Rockefeller Wildlife Refuge, of which four were male and two female. These six animals had been hatched at relatively high temperatures (3 at 33 8C and 3 at 34.5 8C) in an attempt to study the effect on sex determination, and most of the eggs did not hatch successfully at such high temperatures.
Thus it seemed possible that our early growth results might be affected by damage to the animals. The nine American alligator hatchlings in 1997 included five males and four females. Of the 16 1999 Chinese alligator hatchlings (product of the same breeding pair as the 1997 hatchlings and thus full siblings), 10 were male and 6 female. These 16 animals were hatched at what is presumed to be a more favorable temperature (31 8C), i.e. one allowing greater hatching success and viability, and these grew at similar rates to the 1997 hatchlings (see below). We were unable to detect any significant, or even consistent, differences in growth rates between males and females during the first year after hatch for either Chinese alligators or for American alligators, so data for both sexes of the same species were pooled. 2.2. Animal quarters During these experiments the animals were housed in concrete tanks, 75 cm by 135 cm surface area with a water depth of 25 cm. Animals were fed and the tanks cleaned once a day. A 30 cm by 50 cm feedingybasking platform extends just above the water level. The tanks were insulated, kept covered most of the time by insulated lids, and heated (to 31"2 8C) by immersion heaters; they were located outdoors, however, and there was sufficient light ‘leakage’ so that the animals could experience the normal day length cycle throughout the year. Artificial light was provided for 18 h a day in some experiments on both Chinese and American alligators (see below). 2.3. Diet All animals were fed our ‘control’ diet, which is a combination of half a volume of wet meat we.g. fish (mixed marine species), chicken liver, or ground nutria (Myocastor coypus), providing approx. 40% of the total proteinx; one-quarter volume of a dry commercial alligator chow whose protein content is mostly of animal origin wBurris Mills’ 47% protein, providing approx. 60% of total protein (Burris Mill and Feed, Inc., Franklinton, LA 70438)x; and one-quarter volume of water. In the early autumn of 1997, we had some difficulty inducing the Chinese alligator hatchlings to begin eating, even resorting to force-feeding small amounts of meat to each animal. After a week or so of force-feeding, the hatchlings began to eat the
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
911
Fig. 1. Rate of length gain (in cmyday) vs. average length (in cm) of A. mississippiensis. These data are derived from several consecutive years of measurements (ns1225) on alligators fed our control diet during the first year of growth after hatching. Length was measured twice a month or sometimes once a month. The heavy line is a theoretical curve (cmydays0.712ey0.01179(average length in cm)), fitted by inspection, which approximates the average length gain rate at any given length. The light line is also a theoretical curve (cmydays 0.725ey0.01033(average length in cm)) that we have used as a standard for ‘high’ length gain rates.
control diet and to grow for a short while before their food consumption dropped radically as autumn progressed. This was not what we had observed in American alligators at any time; length gain rates gradually decreased as size increased, but there was no discernable seasonal effect on growth rates (or appetite) as long as tank temperatures were maintained at approximately 31 8C at all times (see Fig. 1). 2.4. Measurements Length (in cm), weight (in kg), average length during the growth period, and growth rate (in cmy day and in gyday) of each animal were determined at 15–30-day intervals and group mean and standard deviation determined. 3. Results Fig. 1 summarizes results from several years of experiments and shows length gain rates, in cmy day, of American alligators kept at 31 8C and fed our control diet for 1 year after hatching, plotted against their average lengths for each measurement
period in cm. Newly hatched animals in early September were 25–30 cm in length, weighed approximately 50 g, grew in length at nearly 0.5 cm per day and in weight at approximately 2.5 gy day. At the end of the year animals were approximately 120 cm long, weighed approximately 7.5 kg, were growing in length by approximately 0.15 cm per day and in weight by approximately 40 gy day. We compared growth rates in Chinese and American alligators directly in 1997–1998. Six Chinese alligators and nine American alligators were raised under the same conditions (temperature, diet, even in the same tank for a while) for 1 year. The results are shown in Fig. 2a (length vs. days fed), b (weight vs. days fed), and c (length gain rate in cmyday vs. days). It is apparent that the American alligators grew into much larger animals in the first year. In Fig. 2c, note that American length gain rates started high, then fell steadily over the 1-year period, while the Chinese entered a period of ‘hibernation,’ or at least slow growth, during the winter months, even though temperature was main-
912
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
Fig. 2. (a) Length (in cm) of Chinese (ns6) and American (ns9) alligators (1997 hatchlings) kept at the same temperature and given the same diet during the first year after hatch. Vertical bars indicate standard deviation. Time zero was August 30, 1997, and the final time point was August 25, 1998. (b) Weight (in kg) of the same Chinese (ns6) and American (ns9) alligators during the first year after hatch. (c) Length gain rates (in cmyday) of the same Chinese (ns6) and American (ns9) alligators during the first year after hatch. Time points run from September 4, 1997 to August 16, 1998.
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
913
Fig. 3. Seasonal variation in length gain rates in Chinese alligators (1997 hatchlings, ns6) and in hours of daylight during the first 1100 days after hatch.
tained at 31 8C. By mid-March, length gain rates of Chinese alligators rose again, almost matching length gain rates in American alligators during that period. In the spring, daily weight gains in the Chinese alligators were only approximately 1y10 those in the American alligators since the Chinese alligators were much smaller. When length gain rates of Chinese alligators (1997 hatchlings) in cmyday were plotted over several years, it became apparent that the fall-off in growth was indeed seasonal, and might therefore be related to day length cycles. In Fig. 3, we have plotted the length gain rates vs. time as well as the hours of daylight at the latitude and longitude of New Orleans (308 north, 908 west). The correspondence seems a little crude and somewhat out of phase, but obvious enough to warrant a test of photoperiod effects on Chinese alligator growth. When we obtained 16 new Chinese alligator hatchlings in 1999, we grew these together for a while (August 7 to October 19, 1999), until food consumption and growth rates had dropped to the autumnal low level, then separated the animals into two groups, keeping one exposed to the normal light cycle, but using a timer to provide artificial light for 18 h a day to the other group. Food consumption increased almost immediately
in the group exposed to extra light. Fig. 4a,b shows length and weight in the two groups until February 24, 2000. As time progressed, however, length gain rates began to fall in the extra-light group, but to increase slowly in the normal-light group. By late spring, it had become apparent that the normal-light group was going to catch up with the extra-light group and even surpass it. After the size graphs had crossed over, we removed the light (on June 22, after 320 days of feeding) to see if both groups would then grow at the same rate. They did not, and after approximately 1 month (July 27, 355 days of feeding) we combined the groups in one tank again to ensure that we weren’t seeing some kind of ‘tank’ effect, triggered by subtle differences in temperature or possible toxic effects from unknown sources. It was not until the spring of 2001, however, after more than 150 additional days of exposure to normal light that the two groups of animals seemed to be growing at similar rates (see Fig. 5a–c). 4. Discussion The data shown in Fig. 1 indicate that there is no obvious seasonal effect on growth rates in American alligators during the first year of growth
914
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
Fig. 4. Length (in cm) vs. days fed in two groups of Chinese alligators (1999 hatchlings). The two groups were housed together from August 7 to October 19, 1999. Then one-half (ns8) were separated and exposed to light 18 h a day, while the other group (ns8) were exposed to the normal light cycle. The two curves diverge at October 19. Vertical bars indicate standard deviation. (b) Weight (in kg) vs. days fed in the same two groups of Chinese alligators.
as long as the temperature is maintained at a constant 31 8C. When compared to size at hatch, the yearlings exhibited an approximately four to fivefold increase in length and an approximately 150-fold increase in weight. By contrast, the Chinese alligators, already somewhat smaller than American alligators when hatched, increased in length less than threefold, and in weight less than 30-fold during the first year (Fig. 2).
To ascertain whether our Chinese alligator growth was unusually low, we attempted to compare our data to that obtained by workers at the Anhui Research Center for Chinese Alligator Reproduction. Drawing on data published by Zhang et al. (1986), Table 1 provides lengths and weights at various times after hatching when food was provided every day and temperature was controlled. The data on our 1997 and 1999 hatchl-
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
Fig. 5. Length (in cm) in subsets of the original groups of Chinese alligators (1999 hatchlings), an extra-light group (ns 3) and a normal-light group (ns3). The extra-light group returned to a normal light cycle at 320 days fed and was recombined with the normal-light group after 355 days of feeding. Vertical bars indicate standard deviation. (b) Weight (in kg) in the same two groups of Chinese alligators. (c) Length gain rates (in cmyday) in the same two groups of Chinese alligators.
915
ings were estimated by interpolation, since our measurements were not made at the same times as those made by the Anhui group. For the first 117 days, our temperatures were very similar to those maintained by Zhang et al. (31 8C vs. 30.5 8C); for days 118 through 199, however, we maintained 31 8C while Zhang et al. dropped the temperature to 10 8C before restoring the 30.5 8C temperature thereafter. It is apparent that our growth was somewhat better than that obtained by the Anhui group. For Chinese alligators, the seasonal variation in length gain rates, apparently correlated with day length, suggested an effect of photoperiod (Fig. 3). When artificial light was supplied for 18 h a day in autumn and winter, the initial effects on the Chinese alligators were dramatic (Fig. 4). Food consumption, length and weight gain increased immediately. It seemed obvious that we had solved the low growth ‘problem’ and would be able to provide very large animals in a very short time. The subsequent decline in growth rates in the extra-light group, however, was not immediately remedied by removal of the light or by recombining the two groups in the same tank; it seemed that the ‘extra-light’ animals, after the initial stimulus, had been inhibited in some way. When extra light was provided to American alligators, there was no initial stimulus (above the already high growth rate) and no later inhibition of growth when the artificial light was continued for several months (data not shown). One wonders whether we might be able to ‘tweak’ the lighting cycle (fewer than 18 h of lightyday, fewer days with extra light) to improve growth during ‘hibernation’ without causing damage. The difference in growth rates between Chinese and American alligators and the different responses to photoperiod are two of the more dramatic differences between the two species. It may also be worth sounding a cautionary note for research establishments and for institutions, such as zoos, that house crocodilians for display. Our ‘extralight’ animals showed signs of emaciation for several months before restoration of their normal growth cycles. It is possible that use of 24-h security lights in zoos and animal display houses could have deleterious effects on these and perhaps other crocodilians that may be affected by photoperiod.
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
916
Table 1 Comparison of growth in different groups of Chinese alligators Days fed Group
0
60
117
199
262
(cm)
(kg)
(cm)
(kg)
(cm)
(kg)
(cm)
(kg)
(cm)
(kg)
Mean " Mean "
22.6 1.6 22.1 0.8
0.021 0.002 0.023 0.004
24.8 1.2 24.2 1.2
0.043 0.006 0.043 0.004
25.8 1.2 26.0 1.3
0.052 0.006 0.051 0.005
25.5 1.2 25.9 1.8
0.046 0.005 0.046 0.004
27.4 1.5 28.0 1.9
0.057 0.009 0.059 0.009
Herbert ’1997 ns6 Mean "
23.1 0.9
0.032 0.003
28.7 0.9
0.070 0.007
31.3 0.9
0.095 0.009
37.7 1.5
0.180 0.028
47.3 2.0
0.370 0.060
0.030 0.002
30.0 1.5
0.076 0.009
31.6 1.4
0.093 0.011
36.1 1.7
0.149 0.023
47.3 2.0
0.338 0.064
Zhang ’1986 ns262 ns78
Herbert ’1999 (normal light cycle) ns8 Mean 22.9 " 0.4
Acknowledgments We are grateful to the Louisiana Department of Wildlife and Fisheries and in particular, to Dr Ruth Elsey and other scientists and workers at Rockefeller Wildlife Refuge for provision of the animals and for their generous support over the years. We also thank Mr David Burris of Burris Mill and Feed, Inc., for the gift of various alligator diets for many years. References Coulson, R.A., Coulson, T.D., 1986. Effect of temperature on the rates of digestion, amino acid absorption and assimilation in the alligator. Comp. Biochem. Physiol. 83A, 585–588. Coulson, R.A., Hernandez, T., 1964. Biochemistry of the Alligator, A Study of Metabolism in Slow Motion, Louisiana State University Press, Baton Rouge pp. 11–13. Coulson, R.A., Hernandez, T., 1983. Alligator metabolism: Studies on chemical reactions in vivo. Comp. Biochem. Physiol. 74, i-182. Coulson, T.D., Coulson, R.A., Hernandez, T., 1973. Some observations on the growth of captive alligators. Zoologica 58, 47–52.
Coulson, R.A., Coulson, T.D., Herbert, J.D., Staton, M.A., 1987. Protein nutrition in the alligator. Comp. Biochem. Physiol. 87A, 449–459. Coulson, R.A., Coulson, T.D., Herbert, J.D., 1990. How do digestion and assimilation rates in alligators vary with temperature? Comp. Biochem. Physiol. 96A, 441–449. Coulson, R.A., Coulson, T.D., Herbert, J.D., 1996. Metabolic rate, nutrition, and growth of the alligator. Monograph distributed by Louisiana Dept of Wildlife and Fisheries, 127 pp. Joanen, T., McNease, L., 1987. Alligator Farming Research in Louisiana, USA. Wildlife Management: Crocodiles and Alligators, Surrey Beatty and Sons, Chipping Norton, NSW, Australia pp. 329–340. Thorbjarnarson, J., Wang, X., McMurry, S.T., 2000. Conservation status of wild populations of the Chinese alligator. Results of a survey in southern Anhui Province, July– August, 1999. A Report to the Anhui Province Forestry Department, 87 pp. Wildlife Conservation Society, Bronx, New York. Wang, C., Liang, B., 1990. Research on the Growth Rates of Chinese Alligator Hatchlings in Environmentally Controlled Chambers. From Water onto Land, Chinese Forestry Press House, pp. 257–261. Zhang, Z., Ding, J., Zhao, Y., Pan, H., 1986. The growth rates of young Chinese alligators in captivity. Acta Herpetol. Sin. 5, 217–222.
Growth rates of Chinese and American alligators J.D. Herbert*, T.D. Coulson, R.A. Coulson Louisiana State University Health Sciences Center, Department of Biochemistry and Molecular Biology, 1100 Florida Avenue, Building 140, New Orleans, LA 70119, USA Received 17 August 2001; received in revised form 27 December 2001; accepted 3 January 2002
Abstract Growth rates in two closely related species, Alligator mississippiensis (American alligator) and Alligator sinensis (Chinese alligator), were compared under identical conditions for at least 1 year after hatching. When hatched, Chinese alligators were approximately 2y3 the length and approximately 1y2 the weight of American alligator hatchlings. At the end of 1 year of growth in captivity in heated chambers, the Chinese alligators were approximately 1y2 as long and weighed approximately 1y10 as much as American alligator yearlings. When the animals were maintained at 31 8C, Chinese alligator food consumption and length gain rates dropped to near zero during autumn and winter and body weights decreased slightly, apparently in response to the change in day length. At constant temperature (31 8C), food consumption by American alligators remained high throughout the year. Length gain rates in American alligators decreased slowly as size increased, but were not affected by photoperiod. Daily weight gains in American alligators increased steadily throughout the year. In autumn, provision of artificial light for 18 h a day initially stimulated both length and weight gain in Chinese alligators, but did not affect growth in American alligators. Continuation of the artificial light regimen seemed to cause deleterious effects in the Chinese alligators after several months, however, so that animals exposed to the normal light cycle caught up to and then surpassed the extra-light group in size. Even after removal of the artificial light, it was several months before these extra-light animals reverted to a normal growth pattern. These findings may be of interest to those institutions engaged in captive growth programs intended to provide animals for reintroduction to the wild or to protected habitat. 䊚 2002 Elsevier Science Inc. All rights reserved. Keywords: Alligator mississippiensis; American alligator; Alligator sinensis; Chinese alligator; Crocodilians; Growth; Photoperiod
1. Introduction For many years, we have been raising hatchling American alligators (Alligator mississippiensis) in captivity in heated chambers. One of our purposes was to achieve rapid growth at minimal cost and to establish along the way the nutritional and environmental requirements for such rapid growth. It was our assumption that the procedures we developed would be useful not only for commercial enterprises such as alligator farms, but also *Corresponding author. Tel.: q1-504-942-8343; fax: q1504-942-8274. E-mail address: [email protected] (J.D. Herbert).
for rapid repletion of dwindling stocks of other species of crocodilians that were threatened or endangered. One critically endangered crocodilian species is the Chinese alligator (Alligator sinensis), the only other alligator species in the world and the closest known relative of the American alligator. The Chinese alligator reaches only approximately half the length of an American alligator when full grown, i.e. approximately 2 m for the Chinese male compared to approximately 4 m for the American male. There appear to be only approximately 150 Chinese alligators remaining in the wild in small pockets of threatened habitat close
1095-6433/02/$ - see front matter 䊚 2002 Elsevier Science Inc. All rights reserved. PII: S 1 0 9 5 - 6 4 3 3 Ž 0 2 . 0 0 0 2 7 - 2
910
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
to the Yangzi River in south-eastern Anhui province in China, latitude approximately 318 North (Thorbjarnarson et al., 2000). Although habitat restoration is the major requirement, growth of animals in captivity is needed to supply a healthy population for reintroduction if this becomes feasible. Breeding and growth in captivity is currently occurring at the Anhui Research Center for Chinese Alligator Reproduction (ARCCAR) where several thousand alligators are presently maintained. Breeding and hatching success has been greatly improved in recent years, but rates of growth seem to have been low compared to growth rates in American alligators (Coulson et al., 1973, 1987; Coulson and Coulson, 1986; Coulson et al., 1990, 1996; Coulson and Hernandez, 1964, 1983; Joanen and McNease, 1987; Zhang et al., 1986; Wang and Liang, 1990). Comparison of growth rates using values from the literature is complicated by the fact that we have maintained constant temperature all year long, while the Chinese workers usually drop temperatures during the winter months. For the present report, we maintained individuals of both species under identical conditions for direct comparison of their growth rates. 2. Materials and methods 2.1. Animals Both Chinese alligator and American alligator hatchlings were obtained from Rockefeller Wildlife Refuge in south-west Louisiana (latitude approx. 308 north) within a week or so of hatching. Eggs there were harvested from nests in the marsh and hatched in incubators under controlled temperature and humidity. American alligator eggs were derived from nests of free-roaming alligators in the refuge. Chinese alligator eggs were produced by a breeding pair housed in a fenced enclosure in semi-wild conditions. The Chinese alligator breeding pair was obtained on loan in a cooperative enterprise with John Behler at the Bronx Zoo in New York City. The hatchlings included both sexes. There were six Chinese alligator hatchlings in 1997, received from Dr Ruth Elsey at the Rockefeller Wildlife Refuge, of which four were male and two female. These six animals had been hatched at relatively high temperatures (3 at 33 8C and 3 at 34.5 8C) in an attempt to study the effect on sex determination, and most of the eggs did not hatch successfully at such high temperatures.
Thus it seemed possible that our early growth results might be affected by damage to the animals. The nine American alligator hatchlings in 1997 included five males and four females. Of the 16 1999 Chinese alligator hatchlings (product of the same breeding pair as the 1997 hatchlings and thus full siblings), 10 were male and 6 female. These 16 animals were hatched at what is presumed to be a more favorable temperature (31 8C), i.e. one allowing greater hatching success and viability, and these grew at similar rates to the 1997 hatchlings (see below). We were unable to detect any significant, or even consistent, differences in growth rates between males and females during the first year after hatch for either Chinese alligators or for American alligators, so data for both sexes of the same species were pooled. 2.2. Animal quarters During these experiments the animals were housed in concrete tanks, 75 cm by 135 cm surface area with a water depth of 25 cm. Animals were fed and the tanks cleaned once a day. A 30 cm by 50 cm feedingybasking platform extends just above the water level. The tanks were insulated, kept covered most of the time by insulated lids, and heated (to 31"2 8C) by immersion heaters; they were located outdoors, however, and there was sufficient light ‘leakage’ so that the animals could experience the normal day length cycle throughout the year. Artificial light was provided for 18 h a day in some experiments on both Chinese and American alligators (see below). 2.3. Diet All animals were fed our ‘control’ diet, which is a combination of half a volume of wet meat we.g. fish (mixed marine species), chicken liver, or ground nutria (Myocastor coypus), providing approx. 40% of the total proteinx; one-quarter volume of a dry commercial alligator chow whose protein content is mostly of animal origin wBurris Mills’ 47% protein, providing approx. 60% of total protein (Burris Mill and Feed, Inc., Franklinton, LA 70438)x; and one-quarter volume of water. In the early autumn of 1997, we had some difficulty inducing the Chinese alligator hatchlings to begin eating, even resorting to force-feeding small amounts of meat to each animal. After a week or so of force-feeding, the hatchlings began to eat the
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
911
Fig. 1. Rate of length gain (in cmyday) vs. average length (in cm) of A. mississippiensis. These data are derived from several consecutive years of measurements (ns1225) on alligators fed our control diet during the first year of growth after hatching. Length was measured twice a month or sometimes once a month. The heavy line is a theoretical curve (cmydays0.712ey0.01179(average length in cm)), fitted by inspection, which approximates the average length gain rate at any given length. The light line is also a theoretical curve (cmydays 0.725ey0.01033(average length in cm)) that we have used as a standard for ‘high’ length gain rates.
control diet and to grow for a short while before their food consumption dropped radically as autumn progressed. This was not what we had observed in American alligators at any time; length gain rates gradually decreased as size increased, but there was no discernable seasonal effect on growth rates (or appetite) as long as tank temperatures were maintained at approximately 31 8C at all times (see Fig. 1). 2.4. Measurements Length (in cm), weight (in kg), average length during the growth period, and growth rate (in cmy day and in gyday) of each animal were determined at 15–30-day intervals and group mean and standard deviation determined. 3. Results Fig. 1 summarizes results from several years of experiments and shows length gain rates, in cmy day, of American alligators kept at 31 8C and fed our control diet for 1 year after hatching, plotted against their average lengths for each measurement
period in cm. Newly hatched animals in early September were 25–30 cm in length, weighed approximately 50 g, grew in length at nearly 0.5 cm per day and in weight at approximately 2.5 gy day. At the end of the year animals were approximately 120 cm long, weighed approximately 7.5 kg, were growing in length by approximately 0.15 cm per day and in weight by approximately 40 gy day. We compared growth rates in Chinese and American alligators directly in 1997–1998. Six Chinese alligators and nine American alligators were raised under the same conditions (temperature, diet, even in the same tank for a while) for 1 year. The results are shown in Fig. 2a (length vs. days fed), b (weight vs. days fed), and c (length gain rate in cmyday vs. days). It is apparent that the American alligators grew into much larger animals in the first year. In Fig. 2c, note that American length gain rates started high, then fell steadily over the 1-year period, while the Chinese entered a period of ‘hibernation,’ or at least slow growth, during the winter months, even though temperature was main-
912
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
Fig. 2. (a) Length (in cm) of Chinese (ns6) and American (ns9) alligators (1997 hatchlings) kept at the same temperature and given the same diet during the first year after hatch. Vertical bars indicate standard deviation. Time zero was August 30, 1997, and the final time point was August 25, 1998. (b) Weight (in kg) of the same Chinese (ns6) and American (ns9) alligators during the first year after hatch. (c) Length gain rates (in cmyday) of the same Chinese (ns6) and American (ns9) alligators during the first year after hatch. Time points run from September 4, 1997 to August 16, 1998.
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
913
Fig. 3. Seasonal variation in length gain rates in Chinese alligators (1997 hatchlings, ns6) and in hours of daylight during the first 1100 days after hatch.
tained at 31 8C. By mid-March, length gain rates of Chinese alligators rose again, almost matching length gain rates in American alligators during that period. In the spring, daily weight gains in the Chinese alligators were only approximately 1y10 those in the American alligators since the Chinese alligators were much smaller. When length gain rates of Chinese alligators (1997 hatchlings) in cmyday were plotted over several years, it became apparent that the fall-off in growth was indeed seasonal, and might therefore be related to day length cycles. In Fig. 3, we have plotted the length gain rates vs. time as well as the hours of daylight at the latitude and longitude of New Orleans (308 north, 908 west). The correspondence seems a little crude and somewhat out of phase, but obvious enough to warrant a test of photoperiod effects on Chinese alligator growth. When we obtained 16 new Chinese alligator hatchlings in 1999, we grew these together for a while (August 7 to October 19, 1999), until food consumption and growth rates had dropped to the autumnal low level, then separated the animals into two groups, keeping one exposed to the normal light cycle, but using a timer to provide artificial light for 18 h a day to the other group. Food consumption increased almost immediately
in the group exposed to extra light. Fig. 4a,b shows length and weight in the two groups until February 24, 2000. As time progressed, however, length gain rates began to fall in the extra-light group, but to increase slowly in the normal-light group. By late spring, it had become apparent that the normal-light group was going to catch up with the extra-light group and even surpass it. After the size graphs had crossed over, we removed the light (on June 22, after 320 days of feeding) to see if both groups would then grow at the same rate. They did not, and after approximately 1 month (July 27, 355 days of feeding) we combined the groups in one tank again to ensure that we weren’t seeing some kind of ‘tank’ effect, triggered by subtle differences in temperature or possible toxic effects from unknown sources. It was not until the spring of 2001, however, after more than 150 additional days of exposure to normal light that the two groups of animals seemed to be growing at similar rates (see Fig. 5a–c). 4. Discussion The data shown in Fig. 1 indicate that there is no obvious seasonal effect on growth rates in American alligators during the first year of growth
914
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
Fig. 4. Length (in cm) vs. days fed in two groups of Chinese alligators (1999 hatchlings). The two groups were housed together from August 7 to October 19, 1999. Then one-half (ns8) were separated and exposed to light 18 h a day, while the other group (ns8) were exposed to the normal light cycle. The two curves diverge at October 19. Vertical bars indicate standard deviation. (b) Weight (in kg) vs. days fed in the same two groups of Chinese alligators.
as long as the temperature is maintained at a constant 31 8C. When compared to size at hatch, the yearlings exhibited an approximately four to fivefold increase in length and an approximately 150-fold increase in weight. By contrast, the Chinese alligators, already somewhat smaller than American alligators when hatched, increased in length less than threefold, and in weight less than 30-fold during the first year (Fig. 2).
To ascertain whether our Chinese alligator growth was unusually low, we attempted to compare our data to that obtained by workers at the Anhui Research Center for Chinese Alligator Reproduction. Drawing on data published by Zhang et al. (1986), Table 1 provides lengths and weights at various times after hatching when food was provided every day and temperature was controlled. The data on our 1997 and 1999 hatchl-
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
Fig. 5. Length (in cm) in subsets of the original groups of Chinese alligators (1999 hatchlings), an extra-light group (ns 3) and a normal-light group (ns3). The extra-light group returned to a normal light cycle at 320 days fed and was recombined with the normal-light group after 355 days of feeding. Vertical bars indicate standard deviation. (b) Weight (in kg) in the same two groups of Chinese alligators. (c) Length gain rates (in cmyday) in the same two groups of Chinese alligators.
915
ings were estimated by interpolation, since our measurements were not made at the same times as those made by the Anhui group. For the first 117 days, our temperatures were very similar to those maintained by Zhang et al. (31 8C vs. 30.5 8C); for days 118 through 199, however, we maintained 31 8C while Zhang et al. dropped the temperature to 10 8C before restoring the 30.5 8C temperature thereafter. It is apparent that our growth was somewhat better than that obtained by the Anhui group. For Chinese alligators, the seasonal variation in length gain rates, apparently correlated with day length, suggested an effect of photoperiod (Fig. 3). When artificial light was supplied for 18 h a day in autumn and winter, the initial effects on the Chinese alligators were dramatic (Fig. 4). Food consumption, length and weight gain increased immediately. It seemed obvious that we had solved the low growth ‘problem’ and would be able to provide very large animals in a very short time. The subsequent decline in growth rates in the extra-light group, however, was not immediately remedied by removal of the light or by recombining the two groups in the same tank; it seemed that the ‘extra-light’ animals, after the initial stimulus, had been inhibited in some way. When extra light was provided to American alligators, there was no initial stimulus (above the already high growth rate) and no later inhibition of growth when the artificial light was continued for several months (data not shown). One wonders whether we might be able to ‘tweak’ the lighting cycle (fewer than 18 h of lightyday, fewer days with extra light) to improve growth during ‘hibernation’ without causing damage. The difference in growth rates between Chinese and American alligators and the different responses to photoperiod are two of the more dramatic differences between the two species. It may also be worth sounding a cautionary note for research establishments and for institutions, such as zoos, that house crocodilians for display. Our ‘extralight’ animals showed signs of emaciation for several months before restoration of their normal growth cycles. It is possible that use of 24-h security lights in zoos and animal display houses could have deleterious effects on these and perhaps other crocodilians that may be affected by photoperiod.
J.D. Herbert et al. / Comparative Biochemistry and Physiology Part A 131 (2002) 909–916
916
Table 1 Comparison of growth in different groups of Chinese alligators Days fed Group
0
60
117
199
262
(cm)
(kg)
(cm)
(kg)
(cm)
(kg)
(cm)
(kg)
(cm)
(kg)
Mean " Mean "
22.6 1.6 22.1 0.8
0.021 0.002 0.023 0.004
24.8 1.2 24.2 1.2
0.043 0.006 0.043 0.004
25.8 1.2 26.0 1.3
0.052 0.006 0.051 0.005
25.5 1.2 25.9 1.8
0.046 0.005 0.046 0.004
27.4 1.5 28.0 1.9
0.057 0.009 0.059 0.009
Herbert ’1997 ns6 Mean "
23.1 0.9
0.032 0.003
28.7 0.9
0.070 0.007
31.3 0.9
0.095 0.009
37.7 1.5
0.180 0.028
47.3 2.0
0.370 0.060
0.030 0.002
30.0 1.5
0.076 0.009
31.6 1.4
0.093 0.011
36.1 1.7
0.149 0.023
47.3 2.0
0.338 0.064
Zhang ’1986 ns262 ns78
Herbert ’1999 (normal light cycle) ns8 Mean 22.9 " 0.4
Acknowledgments We are grateful to the Louisiana Department of Wildlife and Fisheries and in particular, to Dr Ruth Elsey and other scientists and workers at Rockefeller Wildlife Refuge for provision of the animals and for their generous support over the years. We also thank Mr David Burris of Burris Mill and Feed, Inc., for the gift of various alligator diets for many years. References Coulson, R.A., Coulson, T.D., 1986. Effect of temperature on the rates of digestion, amino acid absorption and assimilation in the alligator. Comp. Biochem. Physiol. 83A, 585–588. Coulson, R.A., Hernandez, T., 1964. Biochemistry of the Alligator, A Study of Metabolism in Slow Motion, Louisiana State University Press, Baton Rouge pp. 11–13. Coulson, R.A., Hernandez, T., 1983. Alligator metabolism: Studies on chemical reactions in vivo. Comp. Biochem. Physiol. 74, i-182. Coulson, T.D., Coulson, R.A., Hernandez, T., 1973. Some observations on the growth of captive alligators. Zoologica 58, 47–52.
Coulson, R.A., Coulson, T.D., Herbert, J.D., Staton, M.A., 1987. Protein nutrition in the alligator. Comp. Biochem. Physiol. 87A, 449–459. Coulson, R.A., Coulson, T.D., Herbert, J.D., 1990. How do digestion and assimilation rates in alligators vary with temperature? Comp. Biochem. Physiol. 96A, 441–449. Coulson, R.A., Coulson, T.D., Herbert, J.D., 1996. Metabolic rate, nutrition, and growth of the alligator. Monograph distributed by Louisiana Dept of Wildlife and Fisheries, 127 pp. Joanen, T., McNease, L., 1987. Alligator Farming Research in Louisiana, USA. Wildlife Management: Crocodiles and Alligators, Surrey Beatty and Sons, Chipping Norton, NSW, Australia pp. 329–340. Thorbjarnarson, J., Wang, X., McMurry, S.T., 2000. Conservation status of wild populations of the Chinese alligator. Results of a survey in southern Anhui Province, July– August, 1999. A Report to the Anhui Province Forestry Department, 87 pp. Wildlife Conservation Society, Bronx, New York. Wang, C., Liang, B., 1990. Research on the Growth Rates of Chinese Alligator Hatchlings in Environmentally Controlled Chambers. From Water onto Land, Chinese Forestry Press House, pp. 257–261. Zhang, Z., Ding, J., Zhao, Y., Pan, H., 1986. The growth rates of young Chinese alligators in captivity. Acta Herpetol. Sin. 5, 217–222.