Housing Density and Growth in Juvenile Red- Eared Turtles Scott P. McRobert Published online: 04 Jun 2010.

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3 JOURNAL OF APPLIED ANIMAL WELFARE SCIENCE, 2(2), Copyright O Lawrence Erlbaum Associates, Inc. Housing Density and Growth in Juvenile Red-Eared Turtles Scott P. McRobert Department of Biology Saint Joseph's University Growth rates in juvenile turtles relate strongly to their health and welfare, both in the wild and in captivity. Newly hatched turtles in the wild are subject to such high levels of predation that, in some cases, as few as I % survive their first year of life. Juvenile turtles in captivity succumb more quickly to the effects of poor housing conditions and poor diets than do older adults. In both the wild and in captivity, turtles who grow quickly during their first year of life stand a better chance of survival to adulthood. Part of a long-term project to analyze the growth of juvenile turtles under laboratory conditions, this article seeks to develop a protocol for the successful maintenance of turtles in captivity by defining variables that affect health and survival. This article presents the findings of a study to determine whether variations in housing density affect the growth of juvenile red-eared turtles (Trachemys scripta elegans). Many studies have documented growth rates in turtles (Auffenberg & Iverson, 1979; Bury, 1979; Shine & Iverson, 1995). Typically, these studies involved the growth of turtles under natural conditions (Dunham & Gibbons, 1990). Unfortunately, because juvenile specimens are more difficult to mark, release, and recapture, details on the growth of young turtles are not as well documented as are those for adults (Cox, Haxelrig, Turner, Angus, & Marion, 199 1; Janzen, 1993). Juvenile turtles, subject to much higher predation rates than adults (Ernst, Lovich, & Barbour, 1994), may not survive long enough for researchers to gather information on their growth in the wild (Pritchard, 1980). Studying the growth ofjuvenile turtles in captivity resolves this problem (Bobyn & Brooks, 1994; Judd & McQueen, 1980; Witzell, 1980). Such data, although not necessarily representing the growth rates seen in nature, can (a) provide valuable information on juvenile growth rates, Requests for reprints should be sent to Scon P. McRobert, Department of Biology, Saint Joseph's.... -,em

4 134 McROBERT (b) allow comparisons of different species or different populations under controlled conditions, and (c) enable researchers to identify environmental factors that influence growth. Furthermore, studies such as this one may help in developing better methodologies for maintaining turtles in captivity. One factor expected to impact the growth of young turtles, both in captivity and in the wild, is population density. Because turtles can be aggressive competitors, differences in population density may dramatically affect the ability of an individual turtle to acquire critical resources such as food. The effects, however, of population density on growth under natural conditions are difficult to analyze. Other differences, such as food quality, between two populations of different density may compromise a comparison of growth rates between the populations. Accordingly, this study was undertaken to analyze the effects of density on growth in red-eared turtles (Trachemys scripta elegans) under strict laboratory conditions. It attempted to control as many factors as possible-from incubation conditions to the environment in which the juveniles were raised. The turtles were studied for a period of 1 year to determine whether differences in captive population density affected juvenile growth rates. METHOD The turtles in this study were obtained from eggs laid and incubated in my laboratory. In this way I was able to ensure that all turtles entered the study at the same age and, a critical factor, that all eggs were subject to identical incubation environments. Numerous studies have shown that the conditions under which turtle eggs are incubated have significant effects on growth rate (Bobyn & Brooks, 1994; Brooks, Bobyn, Galbraith, Layfield, & Nancekivell, 1991; McKnight & Gutzke, 1993; Roosenburg & Kelley, 1996; Spotila et al., 1994). My breeding colony of adult turtles, obtained from pet shops, reptile shows, and scientific supply companies, provided the turtle eggs. A 15 x 8-ft enclosure in a greenhouse on the roof of the Science Center at St. Joseph's University housed the breeding colony. The enclosure contained two 400-L pools, rock basking platforms, and a land area composed of soil and beach sand. To keep the land area relatively moist, which encourages egg-laying, sprinklers turned on three times each day. For the study, the land area was checked weekly for eggs. When nests were discovered, the eggs were carefully removed and placed in trays of moist vermiculite in 40-L aquaria, each one containing about 12 cm of water, a submersible aquarium heater, and a glass cover. Until hatching, all eggs were maintained at 27 C and between 85% and 90% humidity. A total of 28 juvenile turtles were moved to experimental aquaria within 1 month of hatching. Turtles were randomly assigned to one of three housing situations. In Grouv 1 conditions. a 160-L aauarium (91.4 x 45.7 x 33 cm) housed eieht

5 DENSITY AND GROWTH IN RED-EARED TURTLES 135 turtles. In Group 2 conditions, a 160-L aquarium housed three turtles. In Group 3 conditions, an 80-L aquarium (76.2 x x cm) housed three turtles. Two replicates of each group were observed during this experiment. All experimental aquaria contained brick basking platforms and 20 cm of water. The space available to the turtles (water depth x aquarium width x aquarium length) was 13, cm3 for the 160-L aquaria and 7620 cm3 for the 80-L aquaria. Therefore, in terms of density, Group 1 conditions provided 1,645.9 cm3/turtle, Group 2 conditions provided 4,389.1 cm3/turtle, and Group 3 conditions provided 2,540 cm3lturtle. All experimental aquaria were filtered by both internal and external filters and cleaned weekly. Air and water temperature were maintained between 22 and 26 C. Both incandescent and Vita-Lite fluorescent lights (20-watt; Duro-Lite Lamps, Inc., North Bergen, NJ) on a 12-hr light and 12-hr dark cycle provided lighting. The turtles received pelleted food (Reptile- 10, Wardley Corporation, Secaucus, NJ) 5 days each week, Monday through Friday. At the beginning of the experiment, each aquarium received 0.5 g of foodtturtle. A feeding test was conducted every 2 weeks. During the feeding test, a measured amount of food was added to each tank, and the turtles were observed for 30 min. If uneaten food remained in all aquaria at the end of 30 min, the measured amount of food remained unchanged for another 2 weeks. If the turtles in any aquarium finished all of the food before the 30-min period ended, all tanks received a larger amount of food the next day and were monitored for another 30 min. In this way, all tanks received more food per day than could be eaten in 30 min. As the turtles grew, the amount of food increased (see Table 1). Each aquarium received the same amount of food per turtle each day throughout the experiment. To monitor growth, each turtle was identified by notching the marginal scutes of the carapace (Cagle, 1939). Measurements were taken on the first day of each month for a period of I year. Carapace length was measured as the straight-line distance between the notch in the nuchal scute to the notch in the supracaudal scute. To measure mass, turtles were dried, then placed on a top-loading balance. Percentage growth, Month(n + 1) - Month(n)/Month(n) x 100, was determined on a monthly basis for both carapace length and mass. A one-way analysis of variance, followed by a Tukey honestly significant difference test, determined the signifi- TABLE I Quantities of Food Added to Each Experimental Tank Week Amount of FooaPurtle (g)

6 136 McROBERT cance of differences between mean percentage growth for turtles from each density grouping. At the conclusion of the experiment, experimental turtles either were moved to new aquaria in the Biodiversity Laboratory of Saint Joseph's University or were added to the breeding colony. RESULTS In the 4 months of the study, a turtle from one of the Group 1 tanks died of unknown causes. Data for this animal were not used in the study. All other turtles remained healthy for the duration of the study. All turtles used in this project were males, as determined at the conclusion of the study by the presence of long foreclaws and elongation of the preanal portion of the tail (Gibbons & Greene, 1990). Figure 1 shows growth curves for all turtles. No significant differences between turtles from the three density groupings in terms of mean size (carapace length or mass) appeared either at the onset of the study-carapace length, F(2,24) = 0.15,~ > 0.05, mass F(2,24) = 0.29,~ > 0.05-or at the 12-month mark--carapace length, F(2,24) = 1.98, p > 0.05, mass F(2,24) = 2.43, p > Figure 2 shows the percentage change in carapace length per month for turtles from each group. During the 2nd and 12th months, the mean increase in carapace length for turtles from Group 3 was significantly greater than that for turtles from Group 1 and Group 2: Month 2, F(2,24) = 4.22, p < 0.05; Month 12, F(2,24) = 3.44, p < During the other months of the study, there were no significant differences in the percentage change in carapace length between turtles from the three groups, Fs(2,24) = 0.02 to 4.22, p > 0.05, for each comparison. Figure 3 shows the percentage change in mass per month for turtles from each group. During the 1 lth month, the mean increase in mass for turtles from Group 3 was significantly lower than that for turtles from Group 1 and Group 2, F(2,24) = 3.43, p < No significant differences in the percentage change in mass between turtles from the three groups-fs(2, 24) = 0.14 to 3.27, p > 0.05, for each comparison-occurred in the other months of the study. DISCUSSION These findings show that differences in the three housing densities tested had little or no effect on the growth of captive hatchlings. Although turtles from Group 2 had 2.67 times the amount of space per turtle as turtles from Group 1 and 1.73 times the amount of space per turtle as turtles from Group 3, significant differences in growth among turtles from the three groups appeared at only three points during the study. In studies on snapping turtles (Chelydra serpentina), Galbraith, Brooks, and Obbard (1989) found that the growth rate for a natural population of high density was greater than that for lower density populations. As stated earlier, however,

7 DENSITY AND GROWTH IN RED-EARED TURTLES 137, CARAPACE LENGTH I FIGURE 1 Growth rates for turtles from the three housing density groups. Mean carapace length and mass are presented + SEM. other environmental differences between the compared populations complicate studies of the effects of population density on growth in the wild. In the laboratory, McKnight and Gutzke (1993) found that social interactions were important, although population density itself had no effect on the growth rates of juvenile snappers. Turtles housed in isolation grew more rapidly than turtles housed in arouus. In addition. iuvenile sna~~ine turtles develo~e dominance

8 ?a I Group 3 ( Percen Month FIGURE 2 Mean percentage change (+ SEM) in carapace length for turtles from three housing groups. Month FIGURE 3 Mean percentage change (+ SEM) in mass for turtles from the three housing density groups.

9 DENSITY AND GROWTH IN RED-EARED TURTLES 1 39 hierarchies, with larger individuals competing more successfully for food (Froese & Burghardt, 1974). Whether the red-eared turtles used in this study developed dominance hierarchies like those seen in the snapping turtles remains an unknown, although each experimental group included some large and some small individuals. Despite possible social interactions within each group, however, the results presented in this study show that population density itself had no significant effect on growth rate. The densities used in this study appear to have had no detrimental effect on the health of the turtles housed in captive conditions. All turtles grew quickly through their first year of life. Only one death occurred, and no turtle displayed any sign of disease. In the pet trade, overcrowding has often been noted as a source of disease and death among turtles. It is important, therefore, to document housing densities that support vigorous growth rates and good health. It is equally important to note that the maintenance of turtles in captivity is complex and that the housing conditions used in this study promoted good health in a number of ways: 1. The turtles were maintained at a suitable temperature with a consistent light-dark cycle. 2. The water in each tank was well filtered. 3. Tanks contained basking platforms that permitted the turtles to dry their bodies completely. 4. The turtles received adequate amounts of a nutritious food. Deficiencies in any of these parameters, in addition to problems caused by overcrowding, may account for losses of turtles in captive displays, the pet trade, or even in conservation settings where turtles are bred for eventual release to the wild. Future studies will manipulate the described, basic protocol for maintaining turtles to study other parameters that might affect the health, welfare, and juvenile growth rate of turtles in captivity. ACKNOWLEDGMENTS This work was supported by the Wardley Corporation (Secaucus, NJ) and by Howard Hughes Medical Institute Grant I thank Kelly Lyons, Robert Korbeck, and Elizabeth Dinan for their help in maintaining and observing the turtles.

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