Physiology & Behavior, Vol. 21, pp. 623--627. Pergamon Press and Brain Research Publ., 1978. Printed in the U.S.A.
Social and Seasonal Influences on Testosterone Secretion in the Male Rhesus Monkey T H O M A S P. G O R D O N
Yerkes Regional Primate Research Center, Atlanta, Georgia I R W I N S. B E R N S T E I N
Department of Psychology, University of Georgia, Athens, Georgia AND R O B E R T M. R O S E
Department of Psychiatry and Behavioral Sciences, University of Texas Medical Branch, Galveston, Texas (Received 30 July 1977) GORDON, T. P., I. S. BERNSTEIN AND R. M. ROSE. Social and seasonal influences on testosterone secretion in the male rhesus monkey. PHYSIOL. BEHAV. 21(4) 623-627, 1978.--Adult male rhesus monkeys, maintained in all-male social groups housed such that only distal contact with female rhesus was possible, exhibited a seasonal testosterone rhythm indistinguishable from the pattern previously reported for male rhesus housed with females. In contrast, males maintained in social groups isolated from all contact with females failed to exhibit the typical seasonal pattern. In the latter instance, testosterone levels during the mating season were intermediate between the typical birth season low and mating season peak. These data suggest that the annual testosterone rhythm in male rhesus monkeys is mediated by stimuli from females, and confirm that these stimuli may be distal in nature, since sexual access to the females is not required. The intermediate increase exhibited by males isolated from females suggests that two different factors may play a role in regulating the annual testosterone cycle.
Sexual behavior
Testosterone
Seasonalrhythm
STUDIES conducted in a variety of settings in the past 15 years have established that the rhesus monkey (Macaca rnulatta) exhibits a distinct seasonal mating rhythm when maintained outside the laboratory environment [1, 7, 17]. Indeed, a number of published reports demonstrate seasonal influences on sexual behavior even in environmentally controlled settings [10, 11, 12, 13], and the degree of influence may relate to the length of time a subject has been in the laboratory [19]. The existence of such a powerful seasonal rhythm in the rhesus, and several other primate species [8], has stimulated interest in the mechanisms which control this seasonal behavior. Investigators have focused attention on efforts to determine those factors which regulate seasonal mating patterns, and in attempts to discover the endocrine mechanisms which insure breeding synchrony between male and female members of the species. An early report suggested a link between rainfall pattern and resultant vegetation changes and the annual onset of mating activity [20]. However, subsequent analysis suggested that these factors may be less important than other possible environmental triggers, particularly daylength [19]. It has been established that males will respond sexually to females artificially brought into estrus outside the mating season [18], and that males given access to intact females
show a significant increase in testosterone concentration both in the mating [14] and nonmating seasons [3]. These studies demonstrate that in rhesus macaques, females stimulate an increase in plasma testosterone in adult males, with or without a parallel increase in sexual behavior. It is therefore possible that seasonal environmental stimuli may act upon the females, who in turn may function as the primary stimulus for the males, thus insuring breeding synchrony. This hypothesis was supported by data demonstrating that males housed in sight of females showed evidence of sexual arousal, whereas males completely isolated from contact with females failed to show arousal [6]. More recently, it has been established that sexually mature males living in large outdoor compounds with adult females in a stable social group exhibited a seasonal rhythm in testosterone concentration which parallels seasonal mating activity [7]. In order to test the hypothesis that the crucial stimuli in triggering this annual testosterone increase are female mediated, and to determine whether this increase is observed in males permitted only distal access to females, we manipulated the degree of contact between all-male study groups and females during three sequential mating seasons. The study utilized groups composed exclusively of males, but initiation and termination dates of various phases of the study, and the timing of data collection sequences were de-
Copyright © 1978 Brain Research Publications Inc.--0031-9384/78/100623-05502.00/0
624
GORDON, BERNSTEIN AND ROSE
termined by simultaneous observations of sexual behavior in a large heterosexual group maintained in the same facility (see [7]). METHOD
The data reported herein are derived from behavioral observation of and assay of blood samples obtained from groups of male rhesus monkeys maintained at the Yerkes Regional Primate Research Center Field Station in Lawrenceville, Georgia, during a thirty-month period which encompassed three mating seasons. Two all-male social groups of rhesus monkeys were studied. The first group (R-5) was composed of 20 adult males, of whom seven representing high, intermediate, and low positions in the dominance hierarchy were selected as focal animals (RCb, RHc, RJa, RLc, RMc, RPb, RWb). The second study group (R-11) consisted of seven males, one of whom had also been in the first study group (RAb, RCb, RCc, REc, RFc, RHa, RKc). The exact ages of the animals were not known, but all were at least eight years old at the beginning of the study. Both groups were maintained initially in a 38.1 m 2outdoor enclosure with an attached indoor shelter. This enclosure is part of a facility containing four such compounds, the layout of which has been described previously [6]. In brief, the side walls of the compound consist of 1.8 m high chain link fencing topped by 2.7 m of sheet metal. The compounds center on a block building, with one pair on each side separated by a 6.7 m wide access way. Thus, the animals in either of the paired compounds are readily in the view of the animals in the other. In addition, other forms of distal contact (auditory, olfactory) may be possible. In June, several months prior to the normal onset of the mating season in October, the first study group (R-5) was placed in a compound providing distal contact with a heterosexual rhesus group (R-9) which included seven adult males and 40 adult females. Blood samples were obtained from the seven focal animals once or twice per month during the following six-month span, a period which included the months of peak mating activity in the heterosexual group (October and November). Systematic observations of the behavior of the seven animals also were made during the six-month study period. This phase of the study was terminated in late November when two of the animals sustained severe fight wounds and had to be removed for medical treatment. The second study group (R-11) was formed the following June and also housed initially in the compound which provided distal contact with the breeding group. Systematic data collection (monthly blood samples and dally behavioral observation) was begun in September and continued through the following two mating seasons (15 months). In December, after the peak mating months of the initial mating season, this group was moved to a 15.4 m 2 compound which is part of a complex located 171.5 m from the first. The animals were maintained in this compound for a 12 month period through the subsequent mating season in the breeding group (R-9). No contact with rhesus females was possible in this enclosure. As an added control, all rhesus females and all other females of the genus Macaca were removed from the facility to minimize the possibility of female stimuli arousing the males. During this phase, the only monkey forms in the view of the animals were members of the new world genera Ateles and Cebus. During all phases of the study, blood samples were obtained between 0900 hrs and 1100 hrs to control for possible
diurnal effects. All the animals had experienced the capture and handling associated with blood draw prior to this study, and all were well acclimated to the procedure. Samples were obtained from animals maintained in the outdoor enclosure by driving them into the indoor area which was equipped with a capture partition. Individual animals were then placed in a carrying cage, transferred to a restraining cage, and a 5 to 7 mi sample drawn from the saphenous vein, after which the animal was returned to the compound. Samples were centrifuged and the serum fraction drawn off and stored in the frozen state. Testosterone assay was via radioimmunoassay, the method employed being a modification of the technique of Furuyama, Mayes, and Nugent [5], which has been described in detail previously [7]. The sensitivity of the method was found to be 5 pq. Blank values were not significantly different from zero. Intra-assay reproducibility assessed by the coefficient of variation (c.v.) of duplicates averaged 2.18%, and inter-assay c.v. averaged 6.04%. Behavioral data were recorded during twenty l-hr sessions each month. The observers recorded who did what to whom, using a standard vocabulary of responses [1,2] which were later classified into: contact aggression, noncontact aggression, submission, grooming, play, and sexual responses including mounts. For each of these behavioral categories, monthly calculations were made of the rate (responses per hour) at which it was displayed by (does) or directed to (receives) each animal. The dominance hierarchy was determined by analysis of the outcome of dyadic agonistic encounters which were ordered monthly in a matrix. For the purpose of analysis, the seasonal breeding pattern of the heterosexual group, which was studied concommittantly [7], was used to define data blocks in this study. The annual mating peak occurred in October and November (Mating Season), and the majority of births were recorded in April and May, with a few in June and July (Birth Season). RESULTS
Analysis of the mean monthly testosterone concentrations for the seven subjects in the first study group (R-5) revealed a marked seasonal increase between June and November. The mean testosterone concentration was under 250 ng/100 ml in June, July, and August, then rose in September (567 ng/100 ml), October (919 ng/100 ml), and November (1119 ng/100 ml). Testosterone values were significantly higher in October and November than during June and July (T dep=6.91; p~ LIJ ' ~
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sS
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JULY
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AUG.
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SEPT
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OCT
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FIG. 3. Mean monthly testosterone concentration and mean hourly rate of sexual behavior per month for seven adult male rhesus monkeys maintained in an all-male social group in an outdoor compound adjacent to a rhesus breeding group. The dominance hierarchy was stable during the June through November study period, with no changes in the relative ranks of the seven males being recorded. There was not a significant correlation between rank in the social dominance hierarchy and mean testosterone concentration either for the entire study period (rho=-0.32; N.S.) or mean testosterone value for October and November (rho=-0.39; N.S.). Similarly, rank was not significantly related to the observed frequency of sexual behavior (rho=.357; N.S.). The behavior exhibited by the second study group (R-11) was similar to that observed in the first. Observed sexual behavior for the seven animals as a group did show a seasonal trend, and was correlated significantly with mean testosterone concentrations over the 15 month study period (r=.870; p
Social and Seasonal Influences on Testosterone Secretion in the Male Rhesus Monkey T H O M A S P. G O R D O N
Yerkes Regional Primate Research Center, Atlanta, Georgia I R W I N S. B E R N S T E I N
Department of Psychology, University of Georgia, Athens, Georgia AND R O B E R T M. R O S E
Department of Psychiatry and Behavioral Sciences, University of Texas Medical Branch, Galveston, Texas (Received 30 July 1977) GORDON, T. P., I. S. BERNSTEIN AND R. M. ROSE. Social and seasonal influences on testosterone secretion in the male rhesus monkey. PHYSIOL. BEHAV. 21(4) 623-627, 1978.--Adult male rhesus monkeys, maintained in all-male social groups housed such that only distal contact with female rhesus was possible, exhibited a seasonal testosterone rhythm indistinguishable from the pattern previously reported for male rhesus housed with females. In contrast, males maintained in social groups isolated from all contact with females failed to exhibit the typical seasonal pattern. In the latter instance, testosterone levels during the mating season were intermediate between the typical birth season low and mating season peak. These data suggest that the annual testosterone rhythm in male rhesus monkeys is mediated by stimuli from females, and confirm that these stimuli may be distal in nature, since sexual access to the females is not required. The intermediate increase exhibited by males isolated from females suggests that two different factors may play a role in regulating the annual testosterone cycle.
Sexual behavior
Testosterone
Seasonalrhythm
STUDIES conducted in a variety of settings in the past 15 years have established that the rhesus monkey (Macaca rnulatta) exhibits a distinct seasonal mating rhythm when maintained outside the laboratory environment [1, 7, 17]. Indeed, a number of published reports demonstrate seasonal influences on sexual behavior even in environmentally controlled settings [10, 11, 12, 13], and the degree of influence may relate to the length of time a subject has been in the laboratory [19]. The existence of such a powerful seasonal rhythm in the rhesus, and several other primate species [8], has stimulated interest in the mechanisms which control this seasonal behavior. Investigators have focused attention on efforts to determine those factors which regulate seasonal mating patterns, and in attempts to discover the endocrine mechanisms which insure breeding synchrony between male and female members of the species. An early report suggested a link between rainfall pattern and resultant vegetation changes and the annual onset of mating activity [20]. However, subsequent analysis suggested that these factors may be less important than other possible environmental triggers, particularly daylength [19]. It has been established that males will respond sexually to females artificially brought into estrus outside the mating season [18], and that males given access to intact females
show a significant increase in testosterone concentration both in the mating [14] and nonmating seasons [3]. These studies demonstrate that in rhesus macaques, females stimulate an increase in plasma testosterone in adult males, with or without a parallel increase in sexual behavior. It is therefore possible that seasonal environmental stimuli may act upon the females, who in turn may function as the primary stimulus for the males, thus insuring breeding synchrony. This hypothesis was supported by data demonstrating that males housed in sight of females showed evidence of sexual arousal, whereas males completely isolated from contact with females failed to show arousal [6]. More recently, it has been established that sexually mature males living in large outdoor compounds with adult females in a stable social group exhibited a seasonal rhythm in testosterone concentration which parallels seasonal mating activity [7]. In order to test the hypothesis that the crucial stimuli in triggering this annual testosterone increase are female mediated, and to determine whether this increase is observed in males permitted only distal access to females, we manipulated the degree of contact between all-male study groups and females during three sequential mating seasons. The study utilized groups composed exclusively of males, but initiation and termination dates of various phases of the study, and the timing of data collection sequences were de-
Copyright © 1978 Brain Research Publications Inc.--0031-9384/78/100623-05502.00/0
624
GORDON, BERNSTEIN AND ROSE
termined by simultaneous observations of sexual behavior in a large heterosexual group maintained in the same facility (see [7]). METHOD
The data reported herein are derived from behavioral observation of and assay of blood samples obtained from groups of male rhesus monkeys maintained at the Yerkes Regional Primate Research Center Field Station in Lawrenceville, Georgia, during a thirty-month period which encompassed three mating seasons. Two all-male social groups of rhesus monkeys were studied. The first group (R-5) was composed of 20 adult males, of whom seven representing high, intermediate, and low positions in the dominance hierarchy were selected as focal animals (RCb, RHc, RJa, RLc, RMc, RPb, RWb). The second study group (R-11) consisted of seven males, one of whom had also been in the first study group (RAb, RCb, RCc, REc, RFc, RHa, RKc). The exact ages of the animals were not known, but all were at least eight years old at the beginning of the study. Both groups were maintained initially in a 38.1 m 2outdoor enclosure with an attached indoor shelter. This enclosure is part of a facility containing four such compounds, the layout of which has been described previously [6]. In brief, the side walls of the compound consist of 1.8 m high chain link fencing topped by 2.7 m of sheet metal. The compounds center on a block building, with one pair on each side separated by a 6.7 m wide access way. Thus, the animals in either of the paired compounds are readily in the view of the animals in the other. In addition, other forms of distal contact (auditory, olfactory) may be possible. In June, several months prior to the normal onset of the mating season in October, the first study group (R-5) was placed in a compound providing distal contact with a heterosexual rhesus group (R-9) which included seven adult males and 40 adult females. Blood samples were obtained from the seven focal animals once or twice per month during the following six-month span, a period which included the months of peak mating activity in the heterosexual group (October and November). Systematic observations of the behavior of the seven animals also were made during the six-month study period. This phase of the study was terminated in late November when two of the animals sustained severe fight wounds and had to be removed for medical treatment. The second study group (R-11) was formed the following June and also housed initially in the compound which provided distal contact with the breeding group. Systematic data collection (monthly blood samples and dally behavioral observation) was begun in September and continued through the following two mating seasons (15 months). In December, after the peak mating months of the initial mating season, this group was moved to a 15.4 m 2 compound which is part of a complex located 171.5 m from the first. The animals were maintained in this compound for a 12 month period through the subsequent mating season in the breeding group (R-9). No contact with rhesus females was possible in this enclosure. As an added control, all rhesus females and all other females of the genus Macaca were removed from the facility to minimize the possibility of female stimuli arousing the males. During this phase, the only monkey forms in the view of the animals were members of the new world genera Ateles and Cebus. During all phases of the study, blood samples were obtained between 0900 hrs and 1100 hrs to control for possible
diurnal effects. All the animals had experienced the capture and handling associated with blood draw prior to this study, and all were well acclimated to the procedure. Samples were obtained from animals maintained in the outdoor enclosure by driving them into the indoor area which was equipped with a capture partition. Individual animals were then placed in a carrying cage, transferred to a restraining cage, and a 5 to 7 mi sample drawn from the saphenous vein, after which the animal was returned to the compound. Samples were centrifuged and the serum fraction drawn off and stored in the frozen state. Testosterone assay was via radioimmunoassay, the method employed being a modification of the technique of Furuyama, Mayes, and Nugent [5], which has been described in detail previously [7]. The sensitivity of the method was found to be 5 pq. Blank values were not significantly different from zero. Intra-assay reproducibility assessed by the coefficient of variation (c.v.) of duplicates averaged 2.18%, and inter-assay c.v. averaged 6.04%. Behavioral data were recorded during twenty l-hr sessions each month. The observers recorded who did what to whom, using a standard vocabulary of responses [1,2] which were later classified into: contact aggression, noncontact aggression, submission, grooming, play, and sexual responses including mounts. For each of these behavioral categories, monthly calculations were made of the rate (responses per hour) at which it was displayed by (does) or directed to (receives) each animal. The dominance hierarchy was determined by analysis of the outcome of dyadic agonistic encounters which were ordered monthly in a matrix. For the purpose of analysis, the seasonal breeding pattern of the heterosexual group, which was studied concommittantly [7], was used to define data blocks in this study. The annual mating peak occurred in October and November (Mating Season), and the majority of births were recorded in April and May, with a few in June and July (Birth Season). RESULTS
Analysis of the mean monthly testosterone concentrations for the seven subjects in the first study group (R-5) revealed a marked seasonal increase between June and November. The mean testosterone concentration was under 250 ng/100 ml in June, July, and August, then rose in September (567 ng/100 ml), October (919 ng/100 ml), and November (1119 ng/100 ml). Testosterone values were significantly higher in October and November than during June and July (T dep=6.91; p~ LIJ ' ~
-40
m o= j--r-
sS
-20
200-
I
JUNE
I
JULY
I
AUG.
l
SEPT
I
OCT
03v
I
NOV
DEC.
FIG. 3. Mean monthly testosterone concentration and mean hourly rate of sexual behavior per month for seven adult male rhesus monkeys maintained in an all-male social group in an outdoor compound adjacent to a rhesus breeding group. The dominance hierarchy was stable during the June through November study period, with no changes in the relative ranks of the seven males being recorded. There was not a significant correlation between rank in the social dominance hierarchy and mean testosterone concentration either for the entire study period (rho=-0.32; N.S.) or mean testosterone value for October and November (rho=-0.39; N.S.). Similarly, rank was not significantly related to the observed frequency of sexual behavior (rho=.357; N.S.). The behavior exhibited by the second study group (R-11) was similar to that observed in the first. Observed sexual behavior for the seven animals as a group did show a seasonal trend, and was correlated significantly with mean testosterone concentrations over the 15 month study period (r=.870; p
