Selenium, vitamin E, and trace elements in the plasma of wild and farm-reared alligatorsduring the reproductive cycle

p

1744

VALENTINE LANCE

Department of Medicine, Tulane University School of Medicine, New Orleans, LA, U.S.A. 70112AND

TED JOANEN AND LARRY McNEASE

Louisiana Department of Wildlife and Fisheries, Rockefeller Wildlife Refuge, Grand Chenier, LA, U.S.A. 70643Received December 3, 1982

LANCE, Y., T. JOANEN, and L. McNEASE. 1983. Selenium, vitamin E, and trace elements in the plasma of wild and farm-rearedalligators during the reproductive cycle. Can. J. Zoo!. 61: 1744-1751.

Eggs produced by farm-reared alligators have lower fertility and hatchability than eggs collected from wild alligators andincubated under identical conditions. To assess the effect of diet on trace elements essential for reproduction, plasma samplesfrom farm-reared alligators fed fish (Micropogon undulatus) or nutria (Myocastor coypus) meat, and from wild alligators caughtduring the reproductive cycle (April-July) were assayed for vitamin E, selenium, calcium, magnesium, zinc, copper, iron, totalprotein, cholesterol, and estradiol-17 f). Calcium, magnesium, zinc, iron, total protein, cholesterol, vitamin E, and estradiol-I 7 f)all showed significant increases in females during vitellogenesis (April-May), and all returned to levels not significantly differentfrom males by July. There were no significant differences between the captive alligators fed fish or nutria, or between either groupand wild alligators for any of these plasma constituents. Selenium and copper did not increase during egg production and weresimilar in males and females. There were significant differences due to diet in plasma selenium and vitamin E in female alligators.Fish-fed alligators had significantly higher plasma selenium and significantly lower vitamin E levels than nutria-fed and wildfemales which were not significantly different from one another.

LANCE, Y., T. JOANEN et L. McNEASE. 1983. Selenium, vitamin E, and trace elements in the plasma of wild and farm-rearedalligators during the reproductive cycle. Can. J. Zoo!. 61: 1744-1751.

Les oeufs produits par des alligators d'elevage ont une moins grande fertilite et un tau x declosion moindre queles oeufs d'alligators pond us en nature, merne s'ils sont gardcs dans des conditions d'incubation identiques. Desechantillons de plasma ont ete preleves chez des alligators d'elevage nourris de poisson (Micropogon undulatus) ou deragondin (Myocastor coypus) et chez des alligators sauvages au cours de leur periode de reproduction (avril-juillet) dans Ie butd'etudier les effets du regime alimentaire sur Ies elements presents a l'etat de traces et essentiels a la reproduction: vitamine E,selenium, calcium, magnesium, zinc, cuivre, fer, proteines totales, cholesterol et estradiol-l Zjs. Les concentrations de calcium,de magnesium, de zinc, de fer, de proteines totales, de cholesterol, de vitamine E et d'estradiol-l Zjs subissent une augmentationchez les femelles durant la vitellogenese (avril-mai) ; en juillet, ces concentrations sont significativement sembI abIes a celles desmales. II n'y a pas de differences significatives de ces concentrations entre les alligators d'elevage nourris de poisson et lesalligators nourris de ragondins, ni entre les alligators d'elevage et les alligators sauvages. Le selenium et Ie cuivre n'augmententpas durant la production des oeufs et leurs concentrations demeurent semblables chez Ies males et les femelles. II y a desdifferences significatives reliees ala diete dans les concentrations plasmatiques de selenium et de vitamine E chez les femelles.Les femelles nourries de poisson ont des concentrations de selenium plus elevees et des concentrations de vitamine E plus faiblesque les femelles nourries de ragondin et les femelles sauvages ; il n'y a pas de difference significative entre ces deux derniersgroupes de femelles.

IntroductionCrocodiles have been maintained in zoological col-

lections probably since the time of the ancient Egyp-tians, but it is only within the last 20 years or so thatcrocodiles and alligators have been "farmed" for eco-nomic purposes. Crocodile and alligator farming forskins and meat is now increasing on a world wide basis,with at least 15 established alligator farms in Louisianaand Florida alone. However, many of these farms relyon eggs collected from the nests of wild crocodiles andalligators for their stock. With the increasing pressure onavailable habitat for wild crocodilians and the virtualextinction of a number of species, it is obvious that

[Traduit par Ie journal]

reproduction of captive-raised animals would be bene-ficial from both an economic and conservation point ofview.

Although growth rates are higher in captive alligatorsthan in wild alligators (Coulson et al. 1973; Chabreckand Joanen 1979; Joanen and McNease 1979), andcaptive animals are usually healthy and free of disease,the eggs produced by captive alligators are generallyinferior to those of wild alligators. Clutch size issmaller, percent fertility is lower, and hatching rates areconsiderably lower than hatching rates of eggs collectedfrom wild alligators and artificially incubated underidentical conditions (Joanen et al. 1983). The low

LANCE ET AL. 1745

hatching rates in eggs from captive alligators could bedue to a number of different factors: (a) acceleratedreproductive maturity (captive raised alligators canreach sexual maturity in 6 years, whereas wild alligatorsdo not reach sexual maturity until at least 10 years ofage) (Joanen and McNease 1975); (b) too high astocking density in the breeding pens leading to stress-induced reproductive failure; (c) nutritional deficien-cies. The first two of these possibilities will take anumber of years of carefully controlled breeding experi-ments to test. The third possibility, nutrition, is an areaeasily amenable to experimental manipulation and onein which there is a wealth of data from other commer-cially bred animals and a small amount of informationon crocodilians. Several instances of mineral andvitamin deficiency states have been reported in crocodi-lians in zoological collections, including instances offatal vitamin E deficiency (Wallach and Hoessle 1968;Frye and Schelling 1973).Joanen and McNease (1979) studied the effect of two

different diets on growth rates in hatchling alligators andshowed that a ground meat diet supplemented with apoultry vitamin mix is superior to a diet of ground fishplus the vitamin mix. Adult alligators fed these samediets show differences in reproductive success. Fish-fedanimals produce fewer eggs than meat-fed animals, andthe eggs they do lay have lower fertility and hatchingrates than those from meat-fed animals. Neither grouphowever, produces eggs with the fertility or hatchabilityof eggs from wild alligators (Joanen et al. 1983).Since a number of trace elements and vitamins have

been shown to be essential for egg production andhatchability in poultry (Savage 1968; Underwood1977), we decided to measure a number of thesesubstances in the blood plasma of wild and captivealligators during the reproductive season: (a) to estab-lish baseline values for these elements in this species,and (b) to see if low egg production and poor egg qualityin captive animals is associated with a deficiency orexcess of any of these plasma constituents.

Materials and methodsAnimalsWild alligators were captured at night at the Rockefeller

Refuge, Grand Chenier ,Louisiana, during the months of April,May, June, and JUly. Reproductive state was determined bylaparotomy, and a 50-mL blood sample was taken by heartpuncture using a heparinized syringe fitted with an 18 guageneedle. The blood was centrifuged and the plasma storedfrozen until assayed.In southern Louisiana the reproductive cycle begins in late

March and early April when the animals move from theirwinter habitat to the deep water canals and lakes. Activecourtship takes place during late April and early May; nestconstruction and oviposition begin 2 to 3 weeks later. DuringApril and May ovarian follicles increase in size from about

5 mm to a preovulatory diameter of between 40 and 45 mm. Asingle clutch of between 30 and 40 eggs is laid during a singlenight. There is no evidence of more than one clutch per year,thus the ovary remains quiescent for the remainder of the year.The male cycle follows that of the female with spermatogen-esis commencing in late March and early April, culiminatingin spermiogenesis in May, with rapid testicular regression inmid-June (Joanen and McNease 1975; Lance 1983; V. Lance,unpublished) .A total of 58 wild, adult females and 25 wild, adult males

was collected. The females ranged in length from 163 to255 ern, and from 15.9 to 46.3 kg in weight. Males rangedfrom 185 to 325 em and 17.5 to 146 kg. The captive alligatorsconsisted of two groups that had been hatched from artificiallyincubated eggs in 1972 and 1973 and maintained in seminat-ural outdoor enclosures from the 4th year after hatching. Pendesign and egg incubation techniques have been described indetail elsewhere (Joanen and McNease 1971, 1975, 1979;Joanen et al. 1983). Both groups had successfully mated andlaid eggs in the year prior to the study. One group (1973 hatch)was fed ground nutria meat (Myocastor coypus) with a poultryvitamin supplement, and the other group (1972 hatch) was feda diet of whole fish (mostly Atlantic croacker, Micropogonundulatus) with the same vitamin supplement. A feeding rateof 7 to 8% body mass per week was established as optimal(Joanen and McNease 1971), though food was presented onlyonce per week. The captive alligators were noosed, restrainedand bled via cardiac puncture when they emerged to feed. Thefish-fed group were more reluctant to feed in the presence ofthe investigators than the nutria-fed group; hence the numberof samples obtained was smaller. After a blood sample wastaken the animal was measured to the nearest ern and its tagnumber recorded. The nutria-fed group consisted of 43females (189-232 cm body length) and 22 males (230-326 ernbody length). Only two males and 30 females (190-243 ernbody length) were sampled from the fish-fed group.

AssaysCalcium, magnesium, zinc, copper, and plasma iron were

assayed on a Perkin-Elmer atomic absorption spectrophoto-meter, model 305B, using methods specified in the handbooksupplied by the manufacturer. Selenium was measured in 1- to2-mL plasma samples using the fiuorometric method of Olson(1969), and vitamin E by the spectrophotometric method ofFabianek et al. (1968). Total plasma protein was measured bythe biuret method and cholesterol by the method of Clark et al.(1968). Plasma estrogens were measured in the plasmasamples from the female alligators by radioimmunoassay aspreviously described (Lance and Callard 1978).

StatisticsOnly about 60% of adult-size, wild female alligators

reproduce in a given year (Joanen and McNease 1975). Toavoid variation not associated with reproduction, the plasmasamples from the nonbreeding females were excluded from thestatistical analyses for the months of April, May, and June.The data were subjected to analysis of variance on a complete-ly randomized design 2 x 2 factorial arrangement of treat-ments (diet and month), followed by Duncan's multiple rangetest using the SAS computer package. Significant levels aregiven at P < 0.05.

1746 CAN. 1. ZOOL. VOL. 61. 1983

TABLE I. Metals and trace elements in female alligator plasma

Calcium, Magnesium, Zinc, Copper, Iron, Selenium,Month and diet mMIL mMIL ug/ml. p.g/ml. ug/rnl, p.g/rnl,

AprilWild 6.62±0.5S 1.69±0.OS 1.42±0.14 0.66±0.OS 2.35±0.42 0.19±0.02

(n = S) (n = S) (n = S) (n = 13) (n = S) (n = 12)Nutria 7.95±0.4S 1.66±0.06 I.SI±O.11 0.66±0.04 2.17±0.IS 0.23±0.0l

(n = 15) (n = 15) (n = 15) (n = 14) (n = 14) (n = 14)Fish 7.52±0.60 1.60±0.07 I.S4±0.IS 0.55±0.04 2.17±0.10 0.24±0.02

(n = II) (n = II) (n = II) (n = 9) (n = S) (n = 7)

MayWild 6.69±0.51 1.35±0.OS 1.41±0.09 0.55±0.04 2.10±0.36 0.20±0.02

(n = II) (n = II) (n = II) (n = IS) (n = 9) (n = 15)Nutria S.51±0.92 1.73±0.13 1.6S±0.22 0.5S±0.04 2.30±0.34 0.16±0.03

(n = 10) (n = 10) (n = 10) (n = 10) (n = 10) (n = 5)Fish 7.20±0.59 1.47±0.13 1.55±0.19 O.64±O.OS 1.99±0.34 0.23±0.02

(n = 9) (Il = 9) (n = 9) (n = 7) (Il = 7) (n = 7)

JuneWild 3.IS±0.15 1.25±0.1O 0.60±0.06 0.61±0.05 0.S9±0.21 0.15±0.01

(n = 9) (n = 9) (n = 9) (n = 14) (n = 15) (n = 14)Nutria 6.5S±0.64 1.51 ±O.OS 1.39±0.17 0.59±0.03 I.S4±0.36 0.19±0.01

(Il = 13) (n = 13) (n = 13) (n = II) (n = 12) (n = S)Fish 5.73±0.07 1.41±0.09 1.06±0.21 0.70±0.07 1.37±0.45 0.27±0.03

(n = 5) (n = 5) (n = 5) (n = 5) (n = 5) (n = 6)

JulyWild 2.74±0.07 1.12±0.07 0.49±0.03 0.5S±0.07 0.43±0.09 0.16±0.02

(n = 6) (n = 6) (n = 6) (n = 6) (n = 5) (n = 7)Nutria 3.39±0.19 1.06±0.OS 0.46±0.03 o 52±0.OS 0.5S±0.03 0.16±0.02

(n = 3) (n = 3) (n = 3) (n = 3) (n = 3) (n = 2)Fish 3.41±0.39 1.11 ±0.52 0.6S±0.12 0.70±0.1O 0.32±0.09 0.27±0.03

(n = 4) (Il = 4) (n = 4) (n = 4) (n = 4) (n = 4)

ResultsPlasma calcium, magnesium, iron, and zinc all

showed significant increases in all three groups ofreproductively active females during the folliculargrowth phase (April-May), and declined to levels notsignificantly different from males or nonbreedingfemales by July. Monthly means of plasma values forthese metals for each group are given in Table 1. Sincethere were no significant differences between the captivefemales fed fish or nutria, or between either group andwild females for these plasma constituents, the datafrom all females was pooled and the monthly variationwas plotted as shown in Fig. 1. Plasma copper in femalealligators was not significantly different from that ofmale alligators and did not show any significant changewith season. There were no significant differences inplasma copper among the three groups that could beattributed to diet (Table 1, Fig. 1).

Plasma selenium concentrations in female alligatorsranged from 0.04 to 0.51 p.g/rnl.. There was nodetectable seasonal variation within each group, but thefish-fed animals had significantly higher concentrations

than either the nutria-fed or the wild alligators (Table 1,Fig. 2).

The monthly variations in plasma protein, choles-terol, vitamin E and total estrogens (estradiol) values forthe three groups of female alligators are presented inTable 2. There were no significant differences amongthe three groups for protein, cholesterol, or estradiol,but there were significant seasonal changes associatedwith the reproductive cycle. All four of these constitu-ents were higher during the follicular growth period thanduring the postreproductive period. Pooled monthlymeans, ignoring diet, are shown in Fig. 3.

Plasma vitamin E levels in female alligators showedconsiderable individual variation, with values rangingfrom 1.0 to 48.4 ug/ml.. However, despite this variabil-ity there were significant differences due to season (Fig.3), and significant differences due to diet (Fig. 2).Fish-fed alligators had significantly lower plasma vita-min E concentrations than either the wild or thenutria-fed group.

The percent fertility and percent hatching of the eggslaid by the nutria-fed and fish-fed alligators as compared

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1747

Vitamin E,f-Lg/mL

8 22.3±2.515 18.4±2.811 12.2±2.9

II 14.3±2.29 9.5±4.38 6.2± 1.1

9 5.7±2.413 7.8±2.65 7.6±3.0

5 8.0±3.73 2.8±0.24 3.2±0.5

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FIG. 1. Mean monthly values for metals in the plasma offemale alligators during the reproductive cycle. Bars represent means± SEM of the pooled values from all three groups. Letters at the base of the figure represent the months April to July. Malevalues, pooled means of all samples, are presented for comparison.

TABLE 2. Protein, cholesterol, vitamin E, and estradiol in female alligator plasma

Protein,g/100 mL n

Cholesteroln mg/lOO mL n

Estradiol,n pg/mL

AprilWildNutriaFish

MayWildNutriaFish

JuneWildNutriaFish

JulyWildNutriaFish

7.4±0.79.1±0.8

1O.2± 1.5

7.5±0.57.9±1.27.8±0.9

6.3±0.86.0±0.25.8±0.4

6.6±0.85.7±0.26.2±0.2

8158

101 ±23100±992±17

13 565± 194 814 444±52 159 498±54 10

1188

90±1567±2259±14

17 348±69 119 599±84 108 630± 144 9

15 96±40 I15 114±37 55 48±17 5

6 46±17 63 42±3 34 60±26 4

14125

69±477±12

101±21

534

77±2167±1775±16

1748 CAN. 1. ZOOL VOL 61, 1983

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13.5

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WILD NUTRIA FISH

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FIG. 2. Plasma vitamin E and selenium in female alligators.Bars represent means ± SEM arranged by diet. Values inparentheses are the number of samples in each group.

with eggs collected from the nests of wild alligators areshown in Fig. 4.In the plasma samples from male alligators there were

no significant seasonal changes detected for any of thesubstances assayed in either the wild or captive animals.The pooled data for the two groups, ignoring season, arepresented in Table 3. There were no significant differ-ences in calcium, zinc, iron, protein, cholesterol, orvitamin E between the wild and the nutria-fed males.Plasma magnesium and copper were slightly, but

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FIG. 3. Mean monthly values for vitamin E, cholesterol, protein, and estradiol-l Zp. Symbols as in Fig. 1.

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significantly, higher in the wild than in the captivemales. Plasma selenium was marginally higher in thecaptive males fed nutria meat than in the wild group, butthe difference was not significant.

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DiscussionThe results of this study show that although eggs from

farm-reared alligators are considerably poorer in qualitythan eggs from wild alligators, this difference is notreflected in any detectable differences in the concentra-tion of metals in the blood plasma. Plasma levels ofcalcium, magnesium, zinc, copper, and iron were notsignificantly different in wild and farm-reared femalealligators during the breeding period. Furthermore,differences in diet in captive alligators, while havingsignificant effects on egg production and egg quality,did not appear to have any effect on the plasmaconcentrations of these substances. It is unlikely there-fore, that poor reproductive success in farm-rearedalligators is due to a deficiency or excess of these metals.Although wild females had significantly lower cal-

cium, zinc and iron than the two groups of captivealligators in the month of June (Table 1), the differenceswere not directly attributable to diet. Most of the wildalligators captured in June had already oviposited andhence, had lower plasma vitellogenin levels. This effectwas compounded by the fact that the captive alligatorsovulated somewhat later than the wild alligators. Recent

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LANCE ET AL

similar to those reported in birds, both in the concentra-tion found during the nonbreeding period and in the

n magnitude of the increase during egg production (Har--------------------- land et al. 1974). Plasma zinc in snakes on the otherCalcium, mMIL 3.12±0.OS. 24 2.S7±0.09 20 hand, is many orders of magnitude higher than inMagnesium, mMIL 1.32±0.OS" 24 1.06±0.OS 23 alligators and does not show any increase duringZinc, u.g/rnl, 0.44±0.02 24 0.42±0.02 22 . 11 . (V bli h dCopper, p.g/ml, 0.76±0.03* 24 0.60±0.OS 23 vite ogenesis . Lance, unpu IS ~ ). P~asma copperIron, ug/rnl, 0.S3±0.07 24 0.S4±0.04 IS level~ -:ere reporte~ to be slightly higher lll.egg-laYlllgSelenium, ug/ml, 0.17±0.01 29 0.20±0.01 22 than III Immature chickens, though the magnitude of theProtein, g/lOO mL S.64±0.19 24 S.S3±0.22 22 increase was slight (Savage 1968). There were noCholesterol, mg/lOOmL SS.I±4.2 24 SO.2±4.3 22 detectable changes in plasma copper levels either byVitamin E, ug/rnl, S.36±0.S9 24 S.2S±0.44 22 season or diet in the female alligators sampled in this

study. The concentration of copper in alligator plasma'Significantly different from captive group (P < 0.05). (0.6 ILg/mL) is about twice that found in birds, but in the

same range, or slightly lower than serum or whole bloodvalues reported in a snake (Deforge et al. 1971), a lizard(Beck 1956), and three species of turtle (Musquera et al.1978). In the male alligators sampled however, plasmacopper was slightly but significantly higher in the wildthan in the captive group. This small difference may bedue to the fact that a large percentage of the diet ofalligators consists of crabs and crayfish (Chabreck 1971;McNease and Joanen 1977) which contain high levels ofcopper in the blood pigment hemocyanin. This differ-ence, however, was not evident between wild andcaptive females.It is well established that estradiol secreted by the

developing ovarian follicle initiates vitellogenin synthe-sis in the liver of egg-laying vertebrates, and that thevitellogenin is secreted into the blood and transported tothe ovary where it is taken up by the developing follicle.In birds, reptiles, amphibians, and fishes this estrogen-

data suggests that delayed ovulation in the captive induced protein is present in very high concentration inalligators is probably related to stocking density the plasma during ovarian follicular development. Larg~(T. Joanen and L. McNease, unpublished). i~s in plasma calcium, phosphorus, total protein,The range of values for calcium, magnesium, and iron and total lIpids are charaCteristic of this vitelloenic

in the nonbreeding female and male alligators sampled I2hase. Similar changes in plasma constituents -catI bein this study are in good agreement with previously elicited in immature females or in males by injection ofpublished data for this species (Dessauer et al. 1962; estrogenic hormones (Urist and Schjeide 1961; Ho et al.Dessauer 1974). The dramatic increases in plasma 1982). The alligator is thus similar to all other egg-calcium, magnesium, and iron seen in the plasma of J laYing ve~ebrates in that v~nesis is associatedfemales during the follicular growth phase (April-May) with, increases in plasma calcium~have not previously been shown in crocodilian reptiles, c.!!ole~rol, and that these-'--mereases appeas-to bethough large increases in plasma calcium have been estrogen inducible. Not only are the changes in plasmareported for snakes and lizards during vitellogenesis constituents during the reproductive cycle correlan(Dessauer 1974), and are a well-known index of egg with increases in plasma estradiol (Table 2, Fig. 3), butproduction in the domestic fowl (Urist and Schjeide similar changes can be demonstrated after estrogen1961). injections in immature alligators (Van Brunt and Men-The concentration of iron in the blood plasma of zies 1971; V. Lance, unpublished). The estrogen-

female alligators and the relative increase observed inducible protein vitellogenin is known to bind calciumduring vitellogenesis are remarkably similar to that seen in all species studied. The fact that plasma magnesium,in egg-laying birds (Planas 1970). There are no iron, and zinc are also increased during vitellogenesis inpublished data on plasma iron and reproduction for other birds and alligators suggests that this protein may alsoreptilian species. bind these cations.Plasma zinc levels in the>female alligator are also A large percentage of the vitellogenin molecule is

TABLE 3. Plasma constituents of male alligators

100

Wild Captiven

75

FISH

o WI La

~F.':1I::::J

50

NUTRIA

% HATCH % FERTILITY

FIG. 4. Percent hatching and percent fertility of eggscollected from wild and captive alligators fed fish or nutria.

1749

1750 CAN. J. ZOOL. VOL. 61, 1983

composed of lipid (Dessauer and Fox 1959), hence theincrease in plasma cholesterol seen during the folliculargrowth phase in the alligator. Similar increases inplasma cholesterol have been recorded in the laying hen(Mukherjee et al. 1969). The dramatic differencebetween breeding and nonbreeding female alligatorswith respect to vitamin E levels, however, does notappear to have been reported in other egg-layingspecies. Whether this vitellogenin-associated increasein vitamin E is due to binding of the tocopherol to thevitellogenin molecule, or simply due to the fact thategg-producing alligators have more lipid in the plasmaand hence more tocopherol remains to be determined.By whatever method the vitamin E is transported, it isobvious that large amounts get into the developingfollicle, since plasma of unfed hatchling alligators has avitamin E concentration in excess of 70 u.g/rnl, (V.Lance, unpublished).Although the fish-fed female alligators have sig-

nificantly lower plasma vitamin E levels than both thenutria-fed and wild female alligators, and produced eggsthat were clearly of lower fertility and hatchability thaneggs from the other two groups, the cause of reproduc-tive failure in captive alligators is not entirely due to avitamin E deficiency. Nutria-fed female alligators hadplasma vitamin E levels that were not significantlydifferent from wild female alligators, but produced eggsthat were of significantly poorer quality than those ofwild alligators (Fig. 4). Recent data suggest thatstocking density (overcrowding) is also a major factor inreproductive failure in captive alligators (T. loanen andL. McNease, unpublished). Nonetheless, the data doshow that, as in other animals fed diets of marine fish,reproductive failure almost invariably occurs (Jensen1968; Scott 1978). While fish-fed alligators did haveplasma vitamin E levels that were significantly lowerthan both nutria-fed and wild alligators, these levelswould not be considered deficient by mammalianstandards (Fabianek et al. 1968). However, it is likelythat a deficiency in vitamin E was responsible for the lowfertility of the eggs laid by the fish-fed alligators. Ananimal that produces a clutch of 30-40 eggs, eachweighing approximately 60 g, in a single night, wouldhave to mobilize a considerable amount of vitamin E toensure embryonic survival of the entire clutch. VitaminE levels that are therefore only marginally lower withrespect to normally reproducing females could be fatalfor the offspring. Although none of the fish-fed femaleshad overt symptoms of a deficiency, severe vitamin Edeficiency is fairly common in crocodilians held inzoological collections. Two pairs of Morelet' s crocodile(Crocodylus moreleti) held at the Atlanta Zoo failed toproduce viable eggs until taken off a diet of marine fish(Hunt 1980), and a number of crocodilians maintainedin the St. Louis Zoo and fed a diet of mackerel developed

symptoms of severe vitamin E deficiency. Six of theanimals died from what was diagnosed as steatitis(Wallach and Hoessle 1968).Selenium concentrations in the plasma of wild female

alligators were not significantly different from those ofmales and did not show any variation throughout thereproductive cycle. The range of values encountered aresimilar to those seen in healthy domestic and wildmammals and birds (Underwood 1977; Van Vleet 1980;Lannek and Lindberg 1975). It has been shown thatreproductive failure in selenium and vitamin E deficientrats and chickens does not respond to selenium supple-mentation, but responds readily to small amounts ofvitamin E. However, severe selenium deficiency in thepresence of adequate amounts of vitamin E can some-times lead to reproductive failure (Scott 1978; Tappel1974). The selenium status of the alligators sampled inthe present study was clearly adequate, and unlikely tohave been a factor in reproductive failure in the captiveanimals. Fish-fed alligators which produced eggs of thepoorest quality had plasma selenium concentrationssignificantly higher than those of the wild or nutria-fedalligators (Fig. 4). Moreover, there was no indication ofa negative correlation between vitamin E and selenium.The higher levels of selenium in fish-fed animals wassimply a consequence of being fed a diet high inselenium. Atlantic croaker were reported to have selen-ium levels of 2.8 /-lg/g dry weight (Ganapathy et al.1978), whereas nutria meat has a selenium concentra-tion of only 0.04 /-lg/g wet weight (Elsey and V. Lance,unpublished). Nutria also constitute 70% of the diet ofwild alligators (McNease and loanen 1977).

AcknowledgementsThe authors would like to thank Dave Richard and

Mark Shirley of the Rockefeller Wildlife Refuge fortheir help in collecting blood samples. We would alsolike to thank Deborah Wilson for her help with theselenium analysis and lane Thompson for her help withstatistics.

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