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Seasonal changes in plasma testosterone, thyroxine, and cortisol levels in wild rabbits (Oryctolagus cuniculus algirus) of Zembra island
GENERAL
AND
COMPARATIVE
ENDOCRINOLOGY
57, 383-388 (1%)
Seasonal Changes in Plasma Testosterone, Thyroxine, and Cortissl Levels in Wild Rabbits (CkyctoEagus cunicufus algirus) of Zembra Island MONCEF
BEN SAAD*
AND JEAN-DOMINIQUE
BAYLB?
*DPpartement de Physiologie Rnima[e, Facult& des Sciences de Tunis, Tunisia, and ilaboratoire Neuroendocrinologie B, UniversitP de Montpellier II, Montpellier F 34060, France
de
Accepted May 30, 1984 Annual variations in testosterone, thyroxrne, and cortisol concentrations were recorded in plasma samples obtained monthly from male wild rabbits living in their natural biotope. For comparison, a group of animals was held in semicaptivity close to Tunis. Zembra is an uninhabited, hardly accessible island, north of the bay of Tunis and is a part of a large, protected zone of natural reserve. Warrens of Zembra appear to subsist from a very remote past, without any contact with other strains., In both natural and captive environments, testosterone levels peak sharply in October, decline in November-December, and are low from January to September. Thyroxine titers also peak in October but a second peak occurs in spring, the magnitude of which is markedly higher in natural than in captive conditions. As to cortisol, netting in Zembra results in stress-induced high values but semicaptive specimens exhibit a clear-cut annual rhythm peaking in January. 0 t985 Academic press. IK.
Most animals usually live in environments in which daylight, temperature ) rainfall, and other variables fluctuate periodically. Survival in a seasonal environment requires that individuals adjust homeostatic and reproductive processes to changing external conditions. Such temporal adjustments of endocrine functions involve control mechanisms possibly including an endogenous oscillating machinery synchronized by circannual periodicity in environmental parameters of the biotope. Unfortunately, due to obvious practical difficulties, field investigations on seasonal endocrine changes are scarce except for the sexual cycle. Zembra island constitutes a fairly useful and valuable ecosystem and is probably unique in the west Mediterranean region. This island is quite inaccessible, is uninhabited, and is part of a large controlled zone of natural ‘reserve. Various migratory avian species (Falco eleonorae, Puffinus diomedea) stop at Zembra and sojourn for the reproductive period. A permanent
colony of warrens inhabits the island. Interestingly, it seems that wild rabbits of Zembra Island subsist from a very remote past (possibly the Phoenician or the Roman period?) without any contact with other lagomorph strains, and preserve some conspicuous peculiarities. For example Y specific allotypes of the b series were observed in the Zembra colony that were not found in Portugese or other European wild rabbits (Ben Ammar-El Gaied, 1981). They do not burrow holes but merely shelter in rock anfractuosities or under stubs and roots of shrubs. We took the opportunit,y to investigate circannual endocrine adjustments in such purebred and isolated specimens living under the influence of environmental variables of this isolated and nncontaminated biotope. We measured annual variations in plasma testosterone, thyroxine, and cortisol levels in adult male’ rabbits caught monthly in their natural babitatZembra Island. For comparison, a boup of warrens was held in semicaptivity :&se to Tunis. 383 001~6480/85 $1.50 Copyright B 1985 by Academic Press, iac. Ml rights of reproduction in any farm reserved.
384
BENSAADANDBAYLE
MATERIALS
AND METHODS
Experiments were carried out from July 1982 to July 1983. Zembra Island is located north of the bay of Tunis (latitude 36” 50’ N; longitude 10” 14’ E), approximately 6 sea-miles from Cap Bon. Relative humidity is high throughout the year (65-95%). Winters are not very cold but are distinct. Summers are moderately warm (29-30”) providing important insolation. Rainfall varies markedly from July (0.5 mm) to December (98 mm) (Fig. 1). Moncef Ben Saad got the authorization from the Government Office to stay in Zembra for 4 days at the end of every month. Nets were set in frequented passages between 0000 and 0001 and inspected between 0006 and 0007. Only adult males were kept, and 12 to 15 specimens were captured monthly. Two-milliliter blood samples were withdrawn from the marginal vein and centrifuged. Plasma vials were stocked in a liquidnitrogen container. A lot containing 25 adult rabbits of both sexes was also constituted. Animals were captured in March 1982 and transported to Tunis. They were kept close to the university campus and exposed to natural seasonal climatic variations (light, temperature, solar radiation, and rainfall). All 25 specimens lived in an enclosure. 5 x 10 m, surrounded with wire netting.
i'
/i.
\ \
'i
i
\
FIG. 1. Climatic conditions of the island of Zembra (bay of Tunis: latitude 36” 50 N; longitude 10” 14’ E). P = natural photoperiod (hr day, -); T = monthly temperature (“C, -.a-); R = monthly rainfall (mm water, lliiiiiii ); rp = reproductive period.
Males were allowed to mix and mate with females during all months. These animals were fed ad libitum with a standard rabbit chow (S.I.G. Sfax; iodide 0.76 kg/g). Plasma samples were obtained from males at the same day of the month and time of day as for the Zembra specimens, from July 1982 to July 1983. Plasma testosterone, thyroxine, and cortisol concentrations were determined using radioimmunoassays. RIA kits were obtained from the CEA (Saclay, France): TESTOK-125; TETRAK-PEG; CORTCTK125. Kits were used according to the manufacturer’s instructions. The minimum detectable amounts of testosterone, thyroxine, and cortisol were 20 pgiml, 0.8 rig/ml, and 0.4 ngiml, respectively. The mean intraassay coefficients of variation ranged between 4.1 and 6.3% for all three hormones and the coefficients of variation between assays (N = 15) were 4.4% for testosterone, 68% for thyroxine, and 5.4% for cortisol. In this paper, data are presented as means r SEM. For statistical comparisons, parametric (Student’s t) or nonparametric (Wilcoxon’s IV) tests were used according to the distribution of the data. Correlations between specific parameters were evaluated using Spearman’s rank correlation coefficients.
RESULTS
Body weight of animals did not vary significantly throughout the year in either natural or semicaptive environment (Zembra: 1162 & 65 g; Tunis: 1173 2 58 g; extremes: 1090- 1280 g). As a whole, hormone concentrations were found to be higher in the plasma of captive rabbits than in that of animals living in their natural environment (Figs. 2 and 3). Annual Reproductive and Testosterone Cycles
In both natural (Zembra: Z) and semicaptive (Tunis: T) environments, plasma testosterone concentrations displayed a clearcut monophasic annual cycle. Values remained low from January to September, i.e., less than 1.7 ng ml-’ in the Z group and less than 2.4 ng ml-’ in the T group. They then rose sharply, reaching a peak in October (Z: 3.66 & 0.21; T: 6.08 + 0.22 ng ml- ‘). After that, the plasma concentrations of testosterone declined suddenly. Annual Thyroxine Cycle
In warrens caught monthly
in Zembra (Z
ANNUAL
ENDOCRINE
RHYTHMS
IN WILD RABBITS
385
Tt,
’
‘A’
’ c! ’
’ 0’
‘F’
A‘ ’
‘J’
3.
1
’
FIG. 3. Monthly determination (ng ml-6 mean i SEM of 12-15 values/point) of plasma thyroxine (T4, -), testosterone (Tt, - -), and cortisol (C:..) levels in warrens living in semicaptivity (Thnis). FIG. 2. Monthly determination (ng ml-‘: mean -+ SEM of 12- 15 values/point) of plasma thyroxine (T4, --), testosterone (Tt, - -), and cortisol (C, a**)levels in warrens living in their natural biotope (Zembra).
group), the annual variations in the concentrations of plasma thyroxine (T4) followed a biphasic cyclic pattern. Plasma T4 levels were low (between 42 and 54 ng ml-‘) in winter (December, January) and in summer (June- September). They rose abruptly from September to October (peak: 83.5 c 3. I ng ml-‘), then declined to low winter concentrations, started again to rise, and reached a plateau in February. From the end of February to the end of May, the mean thyroxine concentrations were always greater than 70 ng ml- l. Study of the graph shown in Fig. 3 shows a prominent autumnal peak of T4 levels (144.4 t 3,s ng ml-r in October) in captive rabbits, but the rate rapidly fell to lower concentrations in December- January. The spring recrudescence, however, was markedly less important in this captive T group than in Z animals (83.3 ng ml-i in March vs 42 to 70 ng ml-’ from May to August and in January). In both the T and Z series, the highest plasma thyroxine and testosterone values
were recorded ples).
in autumn
(October
sam-
Annual Cortisol Cycle
The adrenocortical activity cycle analyzed by the variation in plasma cortisol levels shows a distinct annual pattern in specimens kept at Tunis in semicaptivity. Plasma cortisol titers were rather low from the end of March (21.0 i 2.3 ng ml-i) to the end of December (21.2 + 1.7 ng ml-r) and values never exceeded 30 ng ml-” during the summertime. The adrenocortical gland resumed its activity at the beginning of winter (40.0 + 2.7 ng ml-” in December), reached a maximum in January (57.1 t 3.2 ng ml-‘), and therrbegan to decline (44.3 & 2.4 ng ml-l) in February. No annual variation in cortisol concentrations could be distinguished in plasma obtained from warrens captured monthly in Zembra, probably due to the highly stressful conditions evoked by the capture process. Indeed, captive and acclimated animals exposed to ether stress by ,the end of July (basal cortisol level: 25.‘6 ng ml-‘) exhibited quite higher cortisolemia 15 min
386
BENSAADANDBAYLk
after stress application ml-t).
(68.2
i
2.3 ng
DISCUSSION
Under natural conditions (Zembra), females became pregnant by the end of December and young rabbits weighing less than 300-400 g were captured until the end of April. As the mean duration of gestation is 30 days, and weaning occurs 40 days after birth, we can therefore deduce that the mating period took place approximately through December and January. In contrast, captive specimens never succeeded in reproducing. Among wild mammals there are numerous examples in which a seasonal cycle in testicular activity occurs, and generally a high correlation was found to exist between testis size and plasma testosterone concentrations during the annual reproductive cycle (for review, see Lincoln, 1981) and a prebreeding peak of testosterone is recorded in various species. The present study shows that the period of full endocrine testicular activity occurs in October, i.e., approximately 1 month before the mating period. Interestingly, the autumnal peak in testosterone concentrations is not only preserved but also amplified in captive as compared to free warrens. One can suggest that measured plasma values obtained monthly at Zembra could be disturbed by stressing the animal during trapping. The same comment applies of course to thyroxine and especially to cortisol concentrations. However, it was suggested (Jallageas and Assenmacher, 1983) that as far as testosterone and thyroxine levels are concerned, prolonged captivity of dormice (more than 1 year) results in decreased amplitude of the annual cycles, indicating that early captivity (first year) may lead to transient higher values. Circannual fluctuations in testicular, thyroid, and adrenocortical activities and their interactions have been studied mainly in hibernating mammals. Evidence for marked
circannual variations in thyroid function has been supported by radioisotopic techniques in the garden dormouse (Lachiver, 1964) and more recently by determinations of circulating hormones. In a variety of species, thyroid activity is reduced in autumn and winter, i.e., before and during the first part of hibernation (hedgehog: Saboureau, 1979; woodchuck: Young et al., 1979; edible dormouse: Jallageas and Assenmacher, 1983), and resumes around the end of the hibernating period to reach maximal T, titers in spring and early summer. However, Demeneix and Henderson (1978) indicate that plasma T4 levels of the Richardson’s ground squirrel rise sharply on entry into dormancy, remain high throughout hibernation, and decline in spring. In two hibernating species, Glis glis (Jallageas and Assenmacher, 1983) and Erinaceus ewopareus (Saboureau, 1979), plasma thyroxine and testosterone concentrations display monophasic annual cycles, closely correlated to each other. In this respect, the Zembra warren appears to follow a similar pattern although its plasma thyroxine and testosterone stay at basal levels from December to September and then rise to peak levels in October. As to the seasonal variations in plasma corticosteroids, to the author’s knowledge, the only wild animal studied at the present is the hedgehog (Saboureau et al., 1979) and the increase in adrenocortical activity which occurs in autumn, at the beginning of hibernation, cannot be explained simply as a reaction to stress induced by low outer temperature and scarcity of available food (Saboureau and Boissin, 1983). In our study, semicaptive warrens exhibit a clear-cut adrenocortical stimulation in winter likely to respond to low outer temperatures. However, our interpretation merits revision since only cortisol and no corticosterone was assayed, and there is some controversy as to whether F or B compound is the major corticosteroid in rabbits. It has been claimed (Bush, 1953; Daniel, 1975) that in domestic
ANNUAL
ENDOCRINE
RHYTHMS
rabbits corticosterone is more abundant than cortisol, whereas cortisol was found to predominate (Gala and Westphal, 1967; Malinowska et aZ., 1972). Moreover, the F/ B ratio was increased after chronic administration of ACTH (Kass et al., 1954; Krum and Glenn, 1965). One can suggest that our stressed rabbits were subjected to exaggerated levels of endogenous ACTH, therefore favoring cortisol secretion. In any case, we plan to measure both corticosteroids in future investigations. In nonhibernating wild mammals, rather different temporal relationships appear to exist between thyroid and testosterone cycles: there are negative interactions in the badger (Maurel et al., 1977; Maurel and Boissin, 1979), the fox (Maurel, 1981; Maurel and Boissin, 1981), and the mink (Boissin-Agasse et al., 1981). Cycles of the Zembra rabbit appear more like those of hibernating (hedgehog, dormouse) than nonhibernating (badger, fox, mink) wild mammals, since simultaneous phase relationships of testicular and thyroid functions are recorded during the greater part of the year and adrenocortical activity resumes in winter. However, there are at least two main differences. First, highest thyroxine and testosterone concentrations are observed in October instead of late winter, and second, the recrudescence in T4 titers that occurs from February to May is reminiscent of the biphasic pattern of the badger. REFERENCES Ben Ammar-El Gaied, A. (1981). “Polymorphisme isotypique et allotypique de la chaine leg&e K du lapin.” These Sciences. Universite de Paris VI, 217 pp. Bush, I. E. (1953). Species differences in adrenocortical secretion. J. Endocrinol. 9, 95-101. Boissin-Agasse L., Maurel, D., and Boissin, J. (1981). Seasonal variations in thyroxine and testosterone levels in relation to the molt in the adult male mink (MusteEa vison, Peale and Beauvoir). Canad. J. Zool. 59, 1062-1066. Daniel, J. Y. (1975). “Physiologie de la fonction corticosurrenalienne du canard comparee a celle du
IN WILD
387
RABBITS
rat et du lapin. Sa regulation par les facteurs internes et externes.” These Sciences. Universite de Montpellier II, 220 pp. Demeneix, B. A., and Henderson, N. E. (1978). Serum T, and T, in active and torpid ground squirrels, Sperrnophilus richardsoni. Gen. Comp. Endocrinol.
35, 71-85.
Gala, R. R., and Westphal, U. (1967). Corticosteroid binding activity in serum of mouse, rabbit a.nd guinea pig during pregnancy and lactation: Possible involvement in the initiation of lactation. Acta Endocrinol. 55, 47-61. Jallageas, M., and Assenmacher, I. (1983). Annual plasma testosterone and thyroxine cycles in relation to hibernation in the edible dormouse Glis glis.
Gen.
Comp.
Endocrinol.
50, 452-462.
Kass, E. M., Hechter, O., Macci, I. A., and Mou. T. W. (1954). Changes in patterns of secretion of corticosteroids in rabbits after prolonged treatment with ACTH. Proc. Sot. Exp. Biol. Med. 85, 583-587. Krum, A. A., and Glenn, R. E. (1965). Adrenal steroid secretion in rabbits following prolonged ACTH administration. hoc. Sot. Exp. Biol. Med. 118, 225-258. Lachiver, F. (1964). Thyroid activity in the garden dormouse (Eiiomys quercinus L.) studied from June to November. Ann. Acad. Sci. Fenn. Ser. A 4, 285-294. Lincoln, G. A. (1981). Seasonal aspects of testicular function. In “The Testis” (H. Burger and D. de Kretser, eds.), pp. 255-302. Raven Press, New York. Malinowska, K. W., Hardy, R. N., and Nathanieisz, P. W. (1972). Neonatal adrenocortical function and its possible relation to the uptake of macromolecules by the small intestine of the guinea-pig and rabbit. .I. Endocrinol. 55, 397-404. Maurel, D. (1981). Variations saisonnieres des fonctions testiculaire et thyro’idienne en relation avec l’utilisation de l’espace et du temps chez le blaireau europten (Meles meles L.) et le renard roux (Vulpes vulpes L.). These Sciences, Universite de Montpellier, pp. 302. Maurel, D., and Boissin, J. (1979). Seasonal variations of thyroid activity in the adult male badger (1Meles meles L.). Gen. Comp. Endocrinol. 38, 207-214. Maurel, D., and Boissin, J. (1981). Plasma thyroxine and testosterone levels in the red fox (Vu&es vulpes L.) during the annual cycle. Gen. Camp. Endocrinol.
43, 402-404.
Maurel, D., Joffre, J., and Boissin, J. (1977). Cycles annuels de la testosteronemie et de la. thyroxinemie chez le blaireau europeen. C. R. Acad. Sci. Ser. D 284, 1577-1580. Saboureau, M. (1979). “Cycle annuel du fonctionnement testiculaire du,herisson (Erinaceus europaeus L.). Sa regulation par les factenrs externes
388
BEN SAAD AND BAYLE
et internes.” These Sciences, Universite de Tours, 284 pp. Saboureau, M., and Boissin, J. (1983). Endocrine cycles and hibernation in the hedgehog: Mechanisms of adaptation to natural variations in the environment. In “Plant, Animal and Microbial Adaptations to Terrestrial Environment” (N. S. Margaris, M. Arianoutsou-Faraggitaki, and R. J. Reiter, eds.), pp. 203-217. Plenum, New York. Saboureau, M., Laurent, G., and Boissin, J. (1977).
Daily and seasonal rhythms of locomotor activity and adrenal function in male hedgehog (Erinaceus europaeus L.). J. Znterdiscip. Cycle Res. 10, 245266.
Young, R. A., Danforth, E., Vagenokis, A. G., Krupp, P. P, Frink, R., and Sims, E. A. H. (1979). Seasonal variations and the influence of body temperature on plasma concentrations and binding of thyroxine, and triiodothyronine in the woodchuck. Endocrinology 104, 996-999.
AND
COMPARATIVE
ENDOCRINOLOGY
57, 383-388 (1%)
Seasonal Changes in Plasma Testosterone, Thyroxine, and Cortissl Levels in Wild Rabbits (CkyctoEagus cunicufus algirus) of Zembra Island MONCEF
BEN SAAD*
AND JEAN-DOMINIQUE
BAYLB?
*DPpartement de Physiologie Rnima[e, Facult& des Sciences de Tunis, Tunisia, and ilaboratoire Neuroendocrinologie B, UniversitP de Montpellier II, Montpellier F 34060, France
de
Accepted May 30, 1984 Annual variations in testosterone, thyroxrne, and cortisol concentrations were recorded in plasma samples obtained monthly from male wild rabbits living in their natural biotope. For comparison, a group of animals was held in semicaptivity close to Tunis. Zembra is an uninhabited, hardly accessible island, north of the bay of Tunis and is a part of a large, protected zone of natural reserve. Warrens of Zembra appear to subsist from a very remote past, without any contact with other strains., In both natural and captive environments, testosterone levels peak sharply in October, decline in November-December, and are low from January to September. Thyroxine titers also peak in October but a second peak occurs in spring, the magnitude of which is markedly higher in natural than in captive conditions. As to cortisol, netting in Zembra results in stress-induced high values but semicaptive specimens exhibit a clear-cut annual rhythm peaking in January. 0 t985 Academic press. IK.
Most animals usually live in environments in which daylight, temperature ) rainfall, and other variables fluctuate periodically. Survival in a seasonal environment requires that individuals adjust homeostatic and reproductive processes to changing external conditions. Such temporal adjustments of endocrine functions involve control mechanisms possibly including an endogenous oscillating machinery synchronized by circannual periodicity in environmental parameters of the biotope. Unfortunately, due to obvious practical difficulties, field investigations on seasonal endocrine changes are scarce except for the sexual cycle. Zembra island constitutes a fairly useful and valuable ecosystem and is probably unique in the west Mediterranean region. This island is quite inaccessible, is uninhabited, and is part of a large controlled zone of natural ‘reserve. Various migratory avian species (Falco eleonorae, Puffinus diomedea) stop at Zembra and sojourn for the reproductive period. A permanent
colony of warrens inhabits the island. Interestingly, it seems that wild rabbits of Zembra Island subsist from a very remote past (possibly the Phoenician or the Roman period?) without any contact with other lagomorph strains, and preserve some conspicuous peculiarities. For example Y specific allotypes of the b series were observed in the Zembra colony that were not found in Portugese or other European wild rabbits (Ben Ammar-El Gaied, 1981). They do not burrow holes but merely shelter in rock anfractuosities or under stubs and roots of shrubs. We took the opportunit,y to investigate circannual endocrine adjustments in such purebred and isolated specimens living under the influence of environmental variables of this isolated and nncontaminated biotope. We measured annual variations in plasma testosterone, thyroxine, and cortisol levels in adult male’ rabbits caught monthly in their natural babitatZembra Island. For comparison, a boup of warrens was held in semicaptivity :&se to Tunis. 383 001~6480/85 $1.50 Copyright B 1985 by Academic Press, iac. Ml rights of reproduction in any farm reserved.
384
BENSAADANDBAYLE
MATERIALS
AND METHODS
Experiments were carried out from July 1982 to July 1983. Zembra Island is located north of the bay of Tunis (latitude 36” 50’ N; longitude 10” 14’ E), approximately 6 sea-miles from Cap Bon. Relative humidity is high throughout the year (65-95%). Winters are not very cold but are distinct. Summers are moderately warm (29-30”) providing important insolation. Rainfall varies markedly from July (0.5 mm) to December (98 mm) (Fig. 1). Moncef Ben Saad got the authorization from the Government Office to stay in Zembra for 4 days at the end of every month. Nets were set in frequented passages between 0000 and 0001 and inspected between 0006 and 0007. Only adult males were kept, and 12 to 15 specimens were captured monthly. Two-milliliter blood samples were withdrawn from the marginal vein and centrifuged. Plasma vials were stocked in a liquidnitrogen container. A lot containing 25 adult rabbits of both sexes was also constituted. Animals were captured in March 1982 and transported to Tunis. They were kept close to the university campus and exposed to natural seasonal climatic variations (light, temperature, solar radiation, and rainfall). All 25 specimens lived in an enclosure. 5 x 10 m, surrounded with wire netting.
i'
/i.
\ \
'i
i
\
FIG. 1. Climatic conditions of the island of Zembra (bay of Tunis: latitude 36” 50 N; longitude 10” 14’ E). P = natural photoperiod (hr day, -); T = monthly temperature (“C, -.a-); R = monthly rainfall (mm water, lliiiiiii ); rp = reproductive period.
Males were allowed to mix and mate with females during all months. These animals were fed ad libitum with a standard rabbit chow (S.I.G. Sfax; iodide 0.76 kg/g). Plasma samples were obtained from males at the same day of the month and time of day as for the Zembra specimens, from July 1982 to July 1983. Plasma testosterone, thyroxine, and cortisol concentrations were determined using radioimmunoassays. RIA kits were obtained from the CEA (Saclay, France): TESTOK-125; TETRAK-PEG; CORTCTK125. Kits were used according to the manufacturer’s instructions. The minimum detectable amounts of testosterone, thyroxine, and cortisol were 20 pgiml, 0.8 rig/ml, and 0.4 ngiml, respectively. The mean intraassay coefficients of variation ranged between 4.1 and 6.3% for all three hormones and the coefficients of variation between assays (N = 15) were 4.4% for testosterone, 68% for thyroxine, and 5.4% for cortisol. In this paper, data are presented as means r SEM. For statistical comparisons, parametric (Student’s t) or nonparametric (Wilcoxon’s IV) tests were used according to the distribution of the data. Correlations between specific parameters were evaluated using Spearman’s rank correlation coefficients.
RESULTS
Body weight of animals did not vary significantly throughout the year in either natural or semicaptive environment (Zembra: 1162 & 65 g; Tunis: 1173 2 58 g; extremes: 1090- 1280 g). As a whole, hormone concentrations were found to be higher in the plasma of captive rabbits than in that of animals living in their natural environment (Figs. 2 and 3). Annual Reproductive and Testosterone Cycles
In both natural (Zembra: Z) and semicaptive (Tunis: T) environments, plasma testosterone concentrations displayed a clearcut monophasic annual cycle. Values remained low from January to September, i.e., less than 1.7 ng ml-’ in the Z group and less than 2.4 ng ml-’ in the T group. They then rose sharply, reaching a peak in October (Z: 3.66 & 0.21; T: 6.08 + 0.22 ng ml- ‘). After that, the plasma concentrations of testosterone declined suddenly. Annual Thyroxine Cycle
In warrens caught monthly
in Zembra (Z
ANNUAL
ENDOCRINE
RHYTHMS
IN WILD RABBITS
385
Tt,
’
‘A’
’ c! ’
’ 0’
‘F’
A‘ ’
‘J’
3.
1
’
FIG. 3. Monthly determination (ng ml-6 mean i SEM of 12-15 values/point) of plasma thyroxine (T4, -), testosterone (Tt, - -), and cortisol (C:..) levels in warrens living in semicaptivity (Thnis). FIG. 2. Monthly determination (ng ml-‘: mean -+ SEM of 12- 15 values/point) of plasma thyroxine (T4, --), testosterone (Tt, - -), and cortisol (C, a**)levels in warrens living in their natural biotope (Zembra).
group), the annual variations in the concentrations of plasma thyroxine (T4) followed a biphasic cyclic pattern. Plasma T4 levels were low (between 42 and 54 ng ml-‘) in winter (December, January) and in summer (June- September). They rose abruptly from September to October (peak: 83.5 c 3. I ng ml-‘), then declined to low winter concentrations, started again to rise, and reached a plateau in February. From the end of February to the end of May, the mean thyroxine concentrations were always greater than 70 ng ml- l. Study of the graph shown in Fig. 3 shows a prominent autumnal peak of T4 levels (144.4 t 3,s ng ml-r in October) in captive rabbits, but the rate rapidly fell to lower concentrations in December- January. The spring recrudescence, however, was markedly less important in this captive T group than in Z animals (83.3 ng ml-i in March vs 42 to 70 ng ml-’ from May to August and in January). In both the T and Z series, the highest plasma thyroxine and testosterone values
were recorded ples).
in autumn
(October
sam-
Annual Cortisol Cycle
The adrenocortical activity cycle analyzed by the variation in plasma cortisol levels shows a distinct annual pattern in specimens kept at Tunis in semicaptivity. Plasma cortisol titers were rather low from the end of March (21.0 i 2.3 ng ml-i) to the end of December (21.2 + 1.7 ng ml-r) and values never exceeded 30 ng ml-” during the summertime. The adrenocortical gland resumed its activity at the beginning of winter (40.0 + 2.7 ng ml-” in December), reached a maximum in January (57.1 t 3.2 ng ml-‘), and therrbegan to decline (44.3 & 2.4 ng ml-l) in February. No annual variation in cortisol concentrations could be distinguished in plasma obtained from warrens captured monthly in Zembra, probably due to the highly stressful conditions evoked by the capture process. Indeed, captive and acclimated animals exposed to ether stress by ,the end of July (basal cortisol level: 25.‘6 ng ml-‘) exhibited quite higher cortisolemia 15 min
386
BENSAADANDBAYLk
after stress application ml-t).
(68.2
i
2.3 ng
DISCUSSION
Under natural conditions (Zembra), females became pregnant by the end of December and young rabbits weighing less than 300-400 g were captured until the end of April. As the mean duration of gestation is 30 days, and weaning occurs 40 days after birth, we can therefore deduce that the mating period took place approximately through December and January. In contrast, captive specimens never succeeded in reproducing. Among wild mammals there are numerous examples in which a seasonal cycle in testicular activity occurs, and generally a high correlation was found to exist between testis size and plasma testosterone concentrations during the annual reproductive cycle (for review, see Lincoln, 1981) and a prebreeding peak of testosterone is recorded in various species. The present study shows that the period of full endocrine testicular activity occurs in October, i.e., approximately 1 month before the mating period. Interestingly, the autumnal peak in testosterone concentrations is not only preserved but also amplified in captive as compared to free warrens. One can suggest that measured plasma values obtained monthly at Zembra could be disturbed by stressing the animal during trapping. The same comment applies of course to thyroxine and especially to cortisol concentrations. However, it was suggested (Jallageas and Assenmacher, 1983) that as far as testosterone and thyroxine levels are concerned, prolonged captivity of dormice (more than 1 year) results in decreased amplitude of the annual cycles, indicating that early captivity (first year) may lead to transient higher values. Circannual fluctuations in testicular, thyroid, and adrenocortical activities and their interactions have been studied mainly in hibernating mammals. Evidence for marked
circannual variations in thyroid function has been supported by radioisotopic techniques in the garden dormouse (Lachiver, 1964) and more recently by determinations of circulating hormones. In a variety of species, thyroid activity is reduced in autumn and winter, i.e., before and during the first part of hibernation (hedgehog: Saboureau, 1979; woodchuck: Young et al., 1979; edible dormouse: Jallageas and Assenmacher, 1983), and resumes around the end of the hibernating period to reach maximal T, titers in spring and early summer. However, Demeneix and Henderson (1978) indicate that plasma T4 levels of the Richardson’s ground squirrel rise sharply on entry into dormancy, remain high throughout hibernation, and decline in spring. In two hibernating species, Glis glis (Jallageas and Assenmacher, 1983) and Erinaceus ewopareus (Saboureau, 1979), plasma thyroxine and testosterone concentrations display monophasic annual cycles, closely correlated to each other. In this respect, the Zembra warren appears to follow a similar pattern although its plasma thyroxine and testosterone stay at basal levels from December to September and then rise to peak levels in October. As to the seasonal variations in plasma corticosteroids, to the author’s knowledge, the only wild animal studied at the present is the hedgehog (Saboureau et al., 1979) and the increase in adrenocortical activity which occurs in autumn, at the beginning of hibernation, cannot be explained simply as a reaction to stress induced by low outer temperature and scarcity of available food (Saboureau and Boissin, 1983). In our study, semicaptive warrens exhibit a clear-cut adrenocortical stimulation in winter likely to respond to low outer temperatures. However, our interpretation merits revision since only cortisol and no corticosterone was assayed, and there is some controversy as to whether F or B compound is the major corticosteroid in rabbits. It has been claimed (Bush, 1953; Daniel, 1975) that in domestic
ANNUAL
ENDOCRINE
RHYTHMS
rabbits corticosterone is more abundant than cortisol, whereas cortisol was found to predominate (Gala and Westphal, 1967; Malinowska et aZ., 1972). Moreover, the F/ B ratio was increased after chronic administration of ACTH (Kass et al., 1954; Krum and Glenn, 1965). One can suggest that our stressed rabbits were subjected to exaggerated levels of endogenous ACTH, therefore favoring cortisol secretion. In any case, we plan to measure both corticosteroids in future investigations. In nonhibernating wild mammals, rather different temporal relationships appear to exist between thyroid and testosterone cycles: there are negative interactions in the badger (Maurel et al., 1977; Maurel and Boissin, 1979), the fox (Maurel, 1981; Maurel and Boissin, 1981), and the mink (Boissin-Agasse et al., 1981). Cycles of the Zembra rabbit appear more like those of hibernating (hedgehog, dormouse) than nonhibernating (badger, fox, mink) wild mammals, since simultaneous phase relationships of testicular and thyroid functions are recorded during the greater part of the year and adrenocortical activity resumes in winter. However, there are at least two main differences. First, highest thyroxine and testosterone concentrations are observed in October instead of late winter, and second, the recrudescence in T4 titers that occurs from February to May is reminiscent of the biphasic pattern of the badger. REFERENCES Ben Ammar-El Gaied, A. (1981). “Polymorphisme isotypique et allotypique de la chaine leg&e K du lapin.” These Sciences. Universite de Paris VI, 217 pp. Bush, I. E. (1953). Species differences in adrenocortical secretion. J. Endocrinol. 9, 95-101. Boissin-Agasse L., Maurel, D., and Boissin, J. (1981). Seasonal variations in thyroxine and testosterone levels in relation to the molt in the adult male mink (MusteEa vison, Peale and Beauvoir). Canad. J. Zool. 59, 1062-1066. Daniel, J. Y. (1975). “Physiologie de la fonction corticosurrenalienne du canard comparee a celle du
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rat et du lapin. Sa regulation par les facteurs internes et externes.” These Sciences. Universite de Montpellier II, 220 pp. Demeneix, B. A., and Henderson, N. E. (1978). Serum T, and T, in active and torpid ground squirrels, Sperrnophilus richardsoni. Gen. Comp. Endocrinol.
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Gala, R. R., and Westphal, U. (1967). Corticosteroid binding activity in serum of mouse, rabbit a.nd guinea pig during pregnancy and lactation: Possible involvement in the initiation of lactation. Acta Endocrinol. 55, 47-61. Jallageas, M., and Assenmacher, I. (1983). Annual plasma testosterone and thyroxine cycles in relation to hibernation in the edible dormouse Glis glis.
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