Eastern Illinois University The Keep Masters Theses Student Theses & Publications 1-1-1996 Parental Investment in the Red-Eared Slider Turtle, Trachemys scripta Michael D. Marlen Eastern Illinois University This research is a product of the graduate program in Zoology at Eastern Illinois University. Find out more about the program. Recommended Citation Marlen, Michael D., "Parental Investment in the Red-Eared Slider Turtle, Trachemys scripta" (1996). Masters Theses. 1943. http://thekeep.eiu.edu/theses/1943 This Thesis is brought to you for free and open access by the Student Theses & Publications at The Keep. It has been accepted for inclusion in Masters Theses by an authorized administrator of The Keep. For more information, please contact tabruns@eiu.edu.
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Parental Investment in the Red-Eared Slider Turtle, Trachemys scripta (TITLE) BY Michael D. Marlen THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Masters of Biological Sciences IN THE GRADUATE SCHOOL, EASTERN ILLINOIS UNIVERSITY CHARLESTON, ILLINOIS 1996 YEAR I HEREBY RECOMMEND THIS THESIS BE ACCEPTED AS FULFILLING THIS PART OF THE GRADUATE DEGREE CITED ABOVE DATrj' ADVISER DATE DEP AR'fMfNT HEAD
ABSTRACT This study was conducted to determine egg and hatchling components of the red-eared slider (Trachemys scripta). In addition, energy components transferred from egg to hatchling were used to determine the level of parental investment in embryogenesis and hatchling care. Egg8 of the red-eared slider, collected from central lliinois ponds, were obtained by inducing gravid females to lay by an injection of oxytocin. Egg and hatchling lipids were extracted with petroleum ether while egg and hatchling protein content was determined using the micro-kjeldahl procedure. Eggs averaged 70. 7% water by mass, and dry mass of whole eggs and egg yolks averaged 2.4g and 2. lg, respectively. Egg lipids averaged 23.8% of the egg total dry mass and 29.4% of the yolk dry mass, whereas proteins comprised 42.6% of the egg total dry mass and 52.5% of the yolk dry mass. Hatchling somas were comprised of79.6% water and had a mean dry weight of 0.67g. Hatchling yolk sacs were comprised of 50.3% water and had a mean dry weight of0.69g. Hatchling somas and yolk sacs averaged 19.00/o and 37.4% lipids respectively. Hatchling somas contained 58.9% proteins while yolk sacs contained 44.4%. The amount of non-polar lipids in the egg transferred to hatchling red-eared sliders was used as a measure of parental investment in care {PIC). 67% of the original egg lipids remained in the hatchling turtle in the form of PIC, while only 33% were catabolized during embryogenesis. Thus, the large amount of PIC may play a role in enhancing offspring survival. I
ACKNOWLEDGEMENTS First and foremost I would like to thank Dr. Robert Fischer for his patience, assistance, and guidance, as well as for helping me obtain funding for the project. Dr Fischer always came through for me when I needed it, and could not have made it through the hard times without him. I am deeply indebted to him for all that he has done for me, and consider it an honor to have worked with him. To my committee members Dr. Kipp Kruse, Dr. Eric Bollinger, Dr. Tom Nelson, and Dr. Charles Costa, thank you for being there when I had questions or needed insight. All of you made my experience at Eastern a pleasurable one. I could not have asked for a better team to guide me in the right direction and facilitate my education. I would like to give special thanks to Dr. Kipp Kruse for taking me on all those Thursday research trips, I never knew research could be so much fun. Thank you to my fellow graduate students Matt Gilg, and Brett Egger for helping me take care of eggs and hatchlings. I could not have done this project without your help and friendship. I would also like to thank Robyn Escobedo for her help as well as for her patience and understanding. Finally, thanks to my family, Dennis, Nancy, and Lori Marlen for all of your support and encouragement. You are the people who truly made this possible. 2
TABLE OF CONTENTS Title page.... i Abstact............. -.......................... 1 Acknowledgements..... -...... 2 Table of contents....... 3 Introduction.............................................. 4 Methods............................ 7 Results... 10 Discussion..... 12 Literature cited... 17 Tables... 23 Figures.. 25 3
INTRODUCTION Many aspects of an organisms' life history phenotype are determined by how that organism divides resources among the competing compartments of maintenance, growth, and reproduction (Congdon et al., 1982; Nagy, 1982). The energy devoted to reproduction can be divided into two major allocation categories. The first category, reproductive effort, is the proportion of an animals' total resource (Fisher, 1930) or energy budget (Hirshfield and Tinkle, 1975) that is allocated to reproduction. The second category is parental investment or the amount of energy allocated to each offspring (Williams, 1966; Trivers, 1972, 1985; Congdon, 1989, and optimal egg size: Smith and Fretwell, 1974; Brockleman, 1975). Within a single reproductive bout, the result of these two allocation categories may be the major determinant of the number of offspring that will be produced (Brockleman, 1975). Trivers (1972) defined parental investment as "any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring." There are two major categories of parental investment (PI) that are primarily based on the timing of development. They are: (1) pre-ovulatory Pl, investment made before ovulation of an egg; and (2) post-ovulatory Pl, investment made after ovulation of an egg (i.e., care and protection of young; Kaplan, 1980; 4
Congdon, 1989; Congdon and Gibbons, 1985, 1989). Post-ovulatory PI consists of nest defense and feeding of nestlings and fledglings in birds (Breitwisch et al., 1986; Grandel, 1987) and milk transfer from mother to offspring in mammals (van Jaarsveld et al., 1988; Mendl and Paul, 1989), but is rare in oviparous reptiles (Yaron, 1985; Congdon and Gibbons, 1990). The problem with Trivers' (1972, 1985) definition of PI is that it does not distinguish between pre-ovulatory parental investment for embryogenesis and that made for hatchling care. Thus, to better understand pre-ovulatory PL the original definition should be further subdivided into: (1) Parental Investment in Embryogenesis (PIE), or resources allocated to an egg that are used to produce a complete embryo; and (2) Parental Investment in Care (PIC); the energy or material allocated to an egg that is used to fuel the hatchling after it leaves the egg (Congdon, 1989; Fischer et al., 1991). For many species of oviparous reptiles the cost of nest construction probably represents a small portion of the total cost of reproduction (Congdon and Gatten, 1989). Therefore, oviparous reptiles characteristically allocate the majority of their total reproductive investment to eggs (Fischer et al., 1991). In many mammals and birds, investment in embryogenesis and subsequent investments to offspring care after hatching or birth are very distinct in timing and form. However, in most ectothermic vertebrates, investments for embryogenesis and offspring care are made simultaneously prior to ovulation of the egg (Y aron, 1985; Congdon and Gibbons, 1990; Fischer et al., 1991, 1994). Investment in 5
offspring care is usually in the form of lipid reserves that can be found as a yolk plug or fat bodies ofhatchlings (Kraemer and Bennet, 1981; Troyer, 1983; Congdo~ 1989; Fischer et al., 1991, 1994). These reserves play an indirect role in increasing hatchling survivorship by providing energy for hatchlings after leaving the egg (Troyer, 1983; Congdon and Gibbons, 1989; Fischer et al., 1991). Thus, natural selection for a given level of pre-owlatory PIC may be influenced by the feeding abilities of the hatchlings (Fischer et al., 1991). Although the initial proportions of various components in bird eggs have been studied in relation to altricial and precocial development (Nice, 1962; Ricklefs, 1974; Kendeigh et al., 1977; Carey et al., 1980), little is known of the proportions of egg components allocated to PIE and PIC in most reptiles (Ricklefs and Burger, 1977: Congdon and Tinkle, 1982; Congdon et al.,1983a: Congdon and Gibbons, 1985). Thus, to determine energy allocation in reptile eggs and hatchlings I asked_ two questions. (1) What are the components of an owlated redeared slider egg and newly emerged hatchling?; and (2) How is energy allocated to PIE and PIC in a red-eared slider hatchling? 6
METHODS Red-Eared Sliders (Trachemys scripta) were captured on nesting grounds or by hoop traps baited with sardines from seven small central Illinois lakes and ponds between the months of May- July 1995. Upon capture, turtles were sexed, plastron and carapace lengths measured to 0.1 cm, and weighed to the nearest 0.1 g. In addition, females were palpated to determine presence or absence of eggs. Gravid females were induced to lay eggs by an injection of oxytocin (1.5 mukg body mass; Ewert and Legler, 1978) into the peritoneal cavity. Each clutch of eggs was separated and the individual eggs assigned a number and mark unique to their clutch to avoid mixing among clutches. Eggs were measured (length and width) to 0.1 mm and weighed to 0.001 g. Eggs within a clutch were then randomly divided among two groups: Group 1 eggs were immediately frozen for later analysis of egg energy components. Group 2 eggs were partially buried in a vermiculite substrate and incubated at 30 C until hatching. The vermiculite substrate was kept continually moist, since water stress has been shown to reduce hatchling size (Packard et al., 1981; Janzen et al., 1990), by first weighing the vermiculite and then adding water until a I. lg: 1.0g (- 150 kpa) ratio of water to vermiculite was reached. The eggs were then placed in the mixture and a weight taken. Water was then added as needed to maintain the weight of the container and eggs and to keep the 7
vermiculite substrate hydrated at the proper ratio. Incubating eggs were monitored daily and at emergence hatchlings were removed, washed, plastron length and carapace length measured to 0.1 mm, and wet mass determined to 0.0 lg. Hatchlings were then frozen for later analysis of energy components and to determine the levels of energy allocated to PIE and PIC. Turtle eggs were separated into shell and yolk by slicing around the egg circumference with a scalpel. Yolk sacs were separated from the frozen hatchling soma by making an incision from the median line between the 6th and 7th plastral scutes to the entrance of the yolk sac. Egg yolks (yolk + albumin) and the separated hatchlings (yolk sac and soma) were dried in an oven at 55 C for a minimum of 72 h or until they reached a constant mass. Yolks were ground with a glass rod and hatchling soma components were macerated with scissors. Nonpolar lipids from egg yolks and hatchlings (yolk sac and soma) were extracted in petroleum ether using a soxhlet apparatus for a minimum of 5 h. The amount of non-polar lipids (NPL) was then determined by subtracting mass of the sample after extraction from the sample mass before extraction. The proportion ofnpl in the egg that remained in the fully developed hatchling was used as a measure of pre-ovulatory parental investment in the form of care. Total nitrogen of the lean dry samples was determined for egg and hatchling components using a micro Kjeldahl procedure. Total protein of each sample was estimated by multiplying the total nitrogen content of the sample by 6.25 (Card and Nesheim, 1966). 8
Eggshells, egg yolk, and hatchling components were ashed at 550 C in a mufile furnace for 24h to determine the inorganic and remaining organic content of the samples. All weight values are presented as means plus or minus one standard error. All percent values are presented as just mean. 9
RESULTS Clutch size averaged 9.4 eggs with a range of 3 to 17 eggs, however these values may be inaccurate since full clutches are not always obtained when ovulation is induced by oxytocin injections (Ewert and Legler, 1978). Total wet mass and dry mass of individual eggs averaged 9.53 + 0.36g and 2.44 + 0.lOg, respectively (Table 1). T. scripta eggs averaged 34.4 ± 0.58mm in length and 21.8 ± 0.37mm in width (Table 1). Newly emerged hatchlings had an average carapace length of25.4mm and an average plastron length of23.0mm. Eggs averaged 70.7% water (Table 1) by total mass and shells averaged 15.2% of the total dry mass of the egg with 45.7% of that represented by inorganic material. Egg lipids averaged 23.8% of the total dry mass and 29.4% of the yolk dry mass (0. 72 ± 0.03g; Table 1, Fig. 1), while proteins comprised 42.6% of the egg total dry mass and 52.5% (1.28 ± 0.05g) of the yolk dry mass (Fig. 1). Total hatchling (hatchling and yolk sac) wet mass was 72% of the total egg wet mass, and total hatchling dry mass was 49.6% of the total egg dry mass and 61% of the total yolk dry mass. On average, individual hatchling somas were comprised of79.6% water (Table 2). Hatchling bodies were found to have a mean dry weight of 0.67 ± 0.06g and a mean wet weight of 4.42 ± 0.32g (Table 2). Hatchling somas averaged 19.0% lipids (0.13 ±_0.0lg; Table 2) and 58.9 /o (0.39 ± 0.04g) proteins (Fig. 2). IO
Yolk sacs on the other hand averaged S0.3% water, with mean wet and dry weights of I.SS ± 0.13g and 0.69 ± O.OSg, respectively (Table 2). Yolk sacs averaged 37.4% (0.26 ± 0.02g) lipids and 44.4% (0.30 ± 0.03g) proteins (Fig. 3). Thus, the percent lipids ofhatchling bodies (19.00/o) was much less than the proportion of lipids in the yolk sac (37.4%; Table 2). On average, 22% of the lipids present in the eggs remained in the hatchling body and 46% remained in the yolk sac. Thus, 61 /o of the original egg lipids remained in hatchlings, and 33% of the egg lipids were catabolized during development of the embryo. Total hatchling bodies (soma+ yolk sac) averaged 33% lipids by dry mass. 11
DISCUSSION Egg Characteristics The percent water of T. scripta eggs (70.70%) was nearly identical to values reported for other turtle species with parchment-shelled eggs ( 70.4%; T. scripta = 72.2%; Congdon and Gibbons, 1985), and to values found for alligators (70%; Congdon and Gibbons, 1989; 67%; Fischer et al., 1991), however, this proportion of water is appreciably higher than values reported for snakes (54%; Clark and Siskin, 1956; 41.3%; Stewart and Castillo, 1984; 63.2%; Fischer et al. 1994). The proportion of the egg total chy mass represented by dried eggshell (15.2%) was somewhat lower than values found for turtle species with parchmentshelled eggs (190/o) and 3% lower than values found for T. scripta eggs from South Carolina (18.6%; Congdon and Gibbons, 1985). The percent dried eggshell of T. scripta eggs was considerably lower than values reported for alligators (35%; Congdon and Gibbons, 1989; 32%; Fischer et al., 1994) which have eggshell proportions similar to turtles with rigid shelled eggs ( 41 %; Congdon and Gibbons, 1985). The proportion oflipids in whole T. scripta eggs (23.8%) was similar to values found for other turtles with parchment shelled eggs (24%; Congdon and Gibbons, 1985) and with the values Congdon and Gibbons (1985) found for T. scripta eggs (24.8%). These lipid proportions also coincide with values reported 12
for alligators (26%; Congdon and Gibbons, 1989) and cottonmouths (23.6%; Fischer et al., 1994) but are substantially lower than values reported for diamond back water snakes (32.7%; Stewart and Castillo, 1984) and bull snakes (32.4%; Gutzke and Packard, 1987). The proportion of lipids in the yolk oft. scripta (29.4%), although consistent with the findings of Congdon and Gibbons (1985) are considerably lower than values reported for the proportion of lipids in the yolk of alligators (40%; Congdon and Gibbons, 1989). Whole egg dry mass was comprised of 42.6% proteins which is appreciably lower than values reported by Wilhoft (54.9%; 1986) for the common snapping turtle. The large amount of proteins found in common snapping turtle eggs may be due to the fact that during snapping turtle development a greater amount of proteins are required for the building of hatchling muscle mass, or they may be utilizing proteins instead of lipids as an energy source to fuel embryogenesis. Hatchling characteristics Wet mass of T. scripta hatchlings as a proportion of total egg wet mass (69.8%) was intermediate between values reported for other turtles (78%; Congdon et al., 1983a,b), and alligators (56%; Congdon and Gibbons, 1989). Total dry mass of T. scripta hatchlings as a proportion of total egg dry mass (55. 7%) and yolk dry mass (67.9%) was also similar to those values reported for the chicken turtle (58% and 72o/o, respectively; Congdon et al., 1983a), alligators 13
(50% and 77% respectively; Congdon and Gibbons, 1989; 79%; Fischer et al., 1991), and snakes (77%; Stewart and Castillo,1984; 75%; Stewart et al., 1990; 71%; Fischer et al., 1994). The total dry mass of T. scripta hatchlings was 33% lipids, whereas that for chicken turtle hatchlings was 27% (Congdon et al., 1983a). The lipid proportions of total dry mass reported for alligators (Congdon and Gibbons, 1989) and cottonmouths (Fischer et al., 1994) were 38% and 19%, respectively. The increase in the percent hatchling lipids in the red-eared slider compared to the chicken turtle may be explained by the possibility that the increased lipids in the form of PIC supply the northern red-eared slider turtles with the energy needed to overwinter and ultimately reach a positive energy flow. The proportion of proteins in the hatchling soma and yolk sac were 58.9% and 44.4o/o, respectively. These values are contradictory to the findings ofwilhoft (1986) who reported protein values of 32.2% for hatchling somas and 58.2% for hatchling yolk sacs. Although these numbers are quite different. our values seem more accurate since protein content would be expected to be more abundant in the soma, since it is used for the building of muscle structure, and not usually used as an energy source in reptiles. Parental Investment Approximately 67% of the original non-polar lipids in the egg were transferred to the hatchling in the fonn of care (PIC), with only 33% catabolized 14
during embryogenesis (PIE). These findings are slightly higher than those of Congdon et al. (1983b) who reported PIC values of 50-600/o for aquatic turtles. However, these values are similar to PIC proportions transferred to hatchling snakes (61%; Stewart and Castillo, 1984;) and alligators (74%; Fischer et al.,1991). The high PIC value obtained for the red-eared slider compared to other turtles may be due to the fact that T. scripta overwinter in the nest and require additional lipids to survive the prolonged nesting period. In addition, in the spring when food availability may be limited the additional lipids in the form of PIC may provide the hatchling with energy needed to survive until food availability increases and a positive energy flow can be obtained. In the field, hatchling turtles must somehow fulfill their metabolic requirements until they are able to feed on. their own. During times of negative energy flow, the hatchling turtle must utilize non-polar lipids in the form of PIC for maintenance. The increased level of PIC in T. Scripta would provide a hatchling with enough stored lipids to support a standard metabolic rate at 28 C (0.09 cm 3 0 2 /g/h; R Fischer, unpubl. data) for approximately 58 days. The large amount of non-polar lipids transferred from egg to hatchling (PIC) in T. Scrpta has apparently been made to fuel the prolonged period when hatchlings have a negative energy balance (e.g. overwintering, dispersal from nest to water). Thus, increased hatchling lipid reserves in the form of PIC may have been selected for in northern turtle species to counteract the possible detrimental effects of a stressful environment. 15
It has been suggested that increased hatchling size (i.e. bigger is better) may play a key role in: 1) reducing hatchling susceptibility to predation (Miller et al., 1987), and 2) increasing foraging efficiency (Froese and Burghardt, 1974) and thus increasing hatchling survival rates (Janzen, 1993). However, this study along with those of Congdon et al (1983a) and Congdon and Gibbons {1985) provide strong evidence that residual yolk reserves may be more important than hatchling size in determining hatchling survival rates. If increased hatchling size is the essential component for determining hatchling survival, stored energy in lipid reserves (PIC) should be allocated to produce a larger hatchling, but this is not the case. The tradeoff made between energy allocated to embiyogenesis and hatchling care is biased towards investment in care. The amount of lipids transferred from egg to neonate red-eared sliders is far from trivial and, as in most reptiles, is greater than 50% of the original egg lipids. Thus, both hatchling size and levels of lipid reserves should be considered as components of neonate quality (Fischer et al., 1994). The increased neonate yolk reserves should enhance offspring survival over a broad range of environmental conditions {Troyer, 1983; Schultz, 1991; Fischer et al. 1991, 1994). Given the importance of the role that post hatching yolk reserves could play in the survival of hatchling reptiles, selection should operate such that an optimal proportion of non-polar lipids is allocated to eggs and hatchlings to ultimately maximize the survival (and fitness) of those hatchlings. 16
LITERATURE CITED Breitwisch, R, P. G. Merritt, and G. H. Whitesides. 1986. Parental investment by the northern mockingbird: male and female roles in feeding nestlings. Auk 103: 152-159. Brockleman, W. Y. 1975. Competition, the fitness of offspring, and optimal clutch size. Am. Nat. 109:677-699. Carey, C., H. Rahn, and P. Parisi. 1980. Calories, water, lipid, and yolk in avian eggs. Condor 82:335-343. Card, L. E., and M. C. Nesheim. 1966. Poultry production. Lea and Febiger, Philadelphia, Pennsylvania. 400 pp. Clark, H., and B. F. Siskin. 1956. Nitrogenous excretion by embryos of the viviparous snake Thamnophis s. sirtalis. J. Exp. Biol. 33:384-393. Congdon, J. D. 1989. Proximate and evolutionary constraints on energy relations of reptiles. Physiol. Zool. 62:356-373. Congdon, J. D., and J. W. Gibbons. 1985. Egg components and reproductive characteristics of turtles; relationship to body size. Herpetologica 45:194-205. Congdon, J. D., and J. W. Gibbons. 1989. Posthatching yolk reserve in hatchling American alligator. Herpetologica 41: 194-205. 17
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Table 1. Components of 33 recently owlated eggsfrom twelve individual redeared sliders from Illinois. Mean Minimum Maximum SE Eggs Width(mm) 21.8 18.8 25.3 0.38 Length(mm) 34.4 29.2 41.7 0.59 Wet wt (g) 9.53 5.52 13.31 0.37 %water 70.7 62.7 78.2 0.01 Dry wt (g) 2.44 1.24 2.91 0.10 Lipids (g) 0.6 0.3 0.9 0.29 23
Table 2. Components of 15 hatchlings from twelve clutches of red-eared sliders from Illinois. Mean Minimum Maximum SE Hatchlings Soma Wet wt (g) 4.42 2.92 7.53 0.32 % water 79.6 71.2 87.9 0.50 Dry wt (g) 0.67 0.39 1.20 0.06 Lipids (g) 0.14 0.08 0.23 0.01 Lipids% 19.0 18.7 23.2 0.37 Yolk Sac Wet wt (g) 1.55 0.74 2.41 0.13 % water 53.7 38.7 75.1 2.34 Dry wt (g) 0.69 0.37 1.05 0.05 Lipids (g) 0.27 0.13 0.43 0.02 Lipids% 37.4 35.2 43.2 0.56 24
Fig 1. Proportion(%) of components of eggs of the red-eared slider turtle.. 25
LIPIDS (29.4o/o) ORGANICS (11.4%) EGG YOLK COMPONENTS N ' PROTEIN (52.5 /o) INORGANICS (9.6%)
Fig 2. Proportion(%) ofhatchling soma components of the red-eared slider turtle. 27
LIPIDS (19.0 /o} ORGANICS (10.7%) INORGANICS (11.4o/o) HATCHLING COMPONENTS (SOMA) N 00 PROTEIN (58.9 /o)
Fig 3. Proportion(%) ofhatcwing yolk sac components of the red-eared slider turtle. 29
LIPIDS (37.4%) ORGANICS (11.2%) HATCHLING COMPONENTS {YOLK SAC)!,H 0 PROTEIN (44.4o/o) INORGANICS (7.0 /o)