Behavioral and adrenocorticoid responsiveness of squirrel monkeys to a live snake: Is flight necessarily stressful?

BEHAVIORAL AND NEURAL BIOLOGY

32, 391-405 (1981)

Behavioral and Adrenocorticoid Responsiveness of Squirrel Monkeys to a Live Snake: Is Flight Necessarily Stressful?' JERRY L .

VOGT, 2

CHRISTOPHER L .

COE, AND SEYMOUR LEVINE 3

Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Stanford, California 94305 Bolivian and Guyanese squirrel monkeys were assessed for responses to a live boa constrictor snake and to the novelty/disturbance associated with this presentation. Wild-born animals from adult groups which contained males, females, and pregnant females were tested during both individual and group conditions. The latter condition involved placing the empty or snake-occupied wire-mesh stimulus box on top of the group's home cage. In the individual conditions, an animal was removed from the social group cage and placed alone in a vertically oriented test cage with the stimulus box on top. Spatial and behavioral measures were recorded during these 30-min experimental sessions, and blood samples for plasma cortisol analysis were obtained at the end of each weekly session. Blood samples were also taken during undisturbed conditions 1 week before and after the experimental period. All monkeys revealed agitated behavior in the presence of the snake. In contrast, the cortisol levels in response to the snake and empty box were not different for any group of animals. However, the mean of the two individual tests was greater than the mean of the group tests for males, females, or pregnant females. These data support previous studies which indicate that wild-born squirrel monkeys clearly respond with behavioral distress to a live snake. Although cortisol elevations were not specifically produced by exposure to the snake, the responsiveness of the squirrel monkey pituitary-adrenal system to novelty was demonstrated, as all types of monkeys showed significant elevations to the disturbance/novelty of the individual tests.

Nonhuman primates generally exhibit fear and avoidance of snakes, and this observation has led to a number of laboratory studies on the This study was supported by MH-23645 from NIMH and HD-02881 from NICH&HD. The authors would like to thank Edna Lowe for assistance in the execution of the project, Drs. Michael B. Hennessy and Carol Gonzalez for critical comments on an earlier draft, and Helen Hu for analysis of plasma cortisol. 2 Supported by SPI78-15613 from NSF and Postdoctoral Training Grant MH-15147 from NIMH while on faculty leave from St. John's University. Present address: Department of Psychology, St. John's University, Collegeville, Minn. 56321. 3 Supported by Research Scientist Award MH-19936 from NIMH. 391 0163-1047/81/080391 - 15$02.00/0 Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.

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sensitivity to snakes or complex, snake-like stimuli (Bernstein & Mason, 1962; Green, 1965; Huebner, Lentz, Wooley, & King, 1979; Wolin, Ordy, & Dillman, 1963). When compared to the responsiveness to models of snakes, or to novel stimuli in general, the reactions of monkeys to live snakes are much more vigorous and dramatic, but only among wild-born animals (Joslin, Fletcher, & Emlen, 1964; Murray & King, 1973). A very recent study also reports the greater responsiveness of wild- rather than laboratory-born monkeys, and suggests further that the feral animals may be exhibiting more a generalization gradient of fear to snake-like stimuli than a highly specific discrimination of live snakes (Mineka, Keir, & Price, 1980). The differential reactivity of wild- and lab-reared monkeys indicates a strong learning component in monkeys' fearfulness of snakes, which are presumed to be a major predator of monkeys in their natural environment. Virtually all of the studies examining the affective responses of monkeys to snakes have reported only behavioral measures. However, presentation of a highly arousing stimulus, such as a snake, would also be expected to induce a variety of concomitant physiological changes related to the "fight or flight" syndrome (Cannon, 1915). One measure of the internal state of the animal is the pituitary-adrenal system, which in many situations has been shown to be a reliable indicator of stress or arousal (Hennessy & Levine, 1979). Several recent experiments with the squirrel monkey have demonstrated that this primate shows very dramatic elevations of plasma cortisol in response to modifications of its social environment (Coe, Mendoza, Smotherman, & Levine, 1978b; Levine, Coe, Smotherman, & Kaplan, 1978; Mendoza, Smotherman, Miner, Kaplan, & Levine, 1978b; Vogt, Coe, Lowe, & Levine, 1980) and to other physical and psychological stressors (Coe, Mendoza, Davidson, Smith, Dallman, & Levine, 1978a; Coe, Mendoza, & Levine, 1979; Manogue, Leshner, & Candland, 1975). One of these latter studies examined adrenocortical activity of adult males to a snake and to other stressors (Manogue et al., 1975). They reported that the pituitary-adrenal responsiveness was a function of social status and that dominant animals exhibited greater adrenal reactivity. However, the responses to removal from the group and to the novelty of the test situation were not evaluated, and thus adrenocorticoid elevations specific to the stress of the snake presentation were not clearly demonstrated. Indeed, the novelty of a test situation may be as powerful in eliciting pituitary-adrenal activation as is the fear stimulus itself. This effect is well established with laboratory rodents; the adrenocorticoid elevations to electric foot shock in a novel test apparatus are often no greater than the elevations to the novel test chamber alone (Ader, 1970; Ader & Friedman, 1968; Bassett, Cairncross, & King, 1973; Friedman, Ader, Grota, & Larson, 1967). In contrast, the effects of novelty upon corticosteroid values in monkeys have not been

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systematically evaluated and therefore the present study examined behavioral and adrenocortical responsiveness to novelty as well as to a live boa constrictor snake. The results of previous squirrel monkey research have also emphasized the importance of social factors in determining the stress response (Coe et al., 1979; Manogue et al., 1975). One way of examining the effect of social variables is to compare the response of groups that differ in their social organization. Squirrel monkeys from different parts of South America differ considerably in their size, coloration, and social relations (Mendoza, Lowe, & Levine, 1978; Rosenblum, 1968). Bolivian squirrel monkeys, which show the greatest degree of sexual dimorphism, have a sexually segregated social organization. Guyanese squirrel monkeys, in contrast, are less dimorphic and tend to have a less segregated social organization and a more integrated dominance hierarchy (Mendoza et al., 1978a). Using these two subspecies of squirrel monkeys, we therefore assessed the behavioral and physiological stress response to disturbance/novelty and to snake presentation during an individual test as well as during a group test. The individual test involved removal of a monkey from the group and placing it alone in the novel cage, while the group test involved only the placement of a wire mesh box--empty or containing the snake-on top of the group's home cage. METHOD

Animals The animals used in this study were 24 squirrel monkeys (Saimiri sciureus) which had been imported as adults from Guyana or Bolivia and had lived in the laboratory for several years. The four groups, two each of the Guyanese and Bolivian subspecies, were formed 1-2 weeks before the start of testing which was several months after the end of the mating season in our laboratory. In each group there were two males and four females, and so for each subspecies there were four males and eight females. Five of the females in each subspecies became pregnant following the formation of the groups. These animals provided the fortuitous opportunity to assess the additional effects of pregnancy upon behavioral and adrenal responsiveness. These groups were housed in identical wiremesh cages (1.83 x 1.83 x 1.22 m) with three sets of perches extending the length of the cage. Groups of the same subspecies were housed in adjacent cages with a sheet-metal divider between cages preventing direct visual contact. All cages were within a larger, temperature-controlled colony building with extensive windows and thus were on a natural light cycle. Standard monkey chow and water were continuously available, and fresh fruit and a vitamin-enriched orange drink were provided twice weekly as a dietary supplement.

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Procedure During the 4-week experimental period, the animals were tested weekly between 1030 and 1230 hr for responses either to a live 1.5-m boa constrictor (Boa constrictor) presented inside a 1.3-cm wire-mesh box (35 x 61 x 91 cm) or to the empty box itself. All animals were exposed to the snake during only two test conditions and had not been previously exposed to snakes or snake models in our laboratory. During the Group Snake or Group Empty conditions, the box was placed on top of the group home cage for 30 min, after which blood samples for plasma cortisol analysis were obtained from all animals. During the Individual Snake and Individual Empty conditions, the box was placed on top of a vertically oriented wire-mesh cage (56 x 48 x 137 cm) which was open at the bottom and on 46-cm stilts. This vertical cage was in an adjacent but separate room that housed no animals. An animal was removed from its home cage and placed in a 46 x 46 x 46-cm wiremesh transport cage, which was positioned beneath the vertical cage. A sliding door on the top of the transport cage was then removed so that it formed with the vertical cage a 183-cm-high cage. An animal remained in this apparatus for 30 min, after which it was removed and a blood sample obtained. The order of the test conditions was counterbalanced so that snake presentations were alternated With presentations of the empty box. Group tests involved testing all animals from a group on a particular day, while the individual conditions involved the successive testing of two animals from the same group on a particular day of a week. The overall test schedule was arranged so that there were 6-8 days between two test conditions for an animal. Animals from adjacent cages were not tested on the same day. Blood samples were also taken to determine basal or resting levels of cortisol. One week prior to, and 1 week after, the 4week experimental period, blood samples were collected from all animals during undisturbed conditions at 1100 hr.

Behavioral Observations Spatial and behavioral measures were recorded by observers behind one-way glass during the four 30-min experimental conditions. For the group tests one observer used a focal animal observation procedure to record the specific behavior of each individual at 15-sec intervals during a 5.25-min segment of the 30-min test period. This approach provided a representative sample of each individual's behavior and a record of the group's behavior during the entire period. A second observer recorded spatial locations of all animals every 2 min. The group cage was divided into four levels, each 46 cm high, by the three sets of horizontal perches, and each animal's location was recorded at one of these levels. The behavioral categories which have been analyzed include: Proximity to Box (location on top perch beneath the box), Movement, and Orient

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395

to Box (visual orientation toward the box), all of which were recorded on-signal. Other patterns such as Approach (movement to within Proximity), Vocalization, displays, scratch, and olfactory behavior were recorded but occurred too infrequently to differentiate the test conditions. For the individual tests one observer noted behaviors and location during the one hundred twenty 15-sec intervals of the 30-min test. A checksheet was used to record the following measures on-signal: Movement, Orient to Box, and Location. For the latter category, the 183-cmhigh test chamber was divided into four equal 46 cm parts, as was the group cage, and an animal's location was scored from 1 (top, or Proximity to Box) to 4 (bottom, or Avoidance Level). As in the group tests, other patterns such as Approach (movement to within Proximity), Vocalization, displays, scratch, and olfactory behaviors were noted but occurred too infrequently to differentiate the test conditions. In addition, dominance relationships during undisturbed conditions were determined by observing each group for a 15-min period between 0900 and 1100 hr twice per week over the 6 weeks of the study. A group scan procedure was used in which the occurrence of two dominancerelated behavior patterns by any animal was recorded. The patterns were: (1) Displacement--one animal approaching within 15 cm of another (recipient) animal and causing it to relocate at least 15 cm away from the first animal within 3 sec, and (2) Genital Display--one animal abducting a leg so as to expose its genital region to the face of a partner. These measures were then used to rank order the males (at 1 or 2) and females (at 1, 2, 3, or 4) in separate hierarchies within each group.

Plasma Cortisol Determinations Blood was obtained in heparinized syringes via cardiac puncture while animals were anesthetized following brief exposure to ethyl ether. The blood samples of 0.6 ml were rapidly collected within 2 min of capture. For the group test and basal conditions, the order of capture within a group was randomized and all six samples were taken by two experimenters within 6 min of the start of the procedure. Analysis of the order in which the blood samples were taken revealed no relationship between cortisol values and the time elapsed since the capture of the group. The blood was then centrifuged and the plasma collected and frozen until assay. Cortisol was assayed using antiserum No. F2153 from Endocrine Sciences, Tarzana, California, as described by Klemm and Gupta (1975). Further details of blood sampling and radioimmunoassay for cortisol have been previously described in Coe et al. (1978b) and Mendoza et al. (1978b).

Statistical Evaluations For both the Bolivian and Guyanese subspecies the behavioral and cortisol results are presented for three kinds of animals: males, females,

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and pregnant females. The statistical tests were organized so as to allow initial comparisons between the snake and empty stimulus conditions. For the behavioral analyses, the values were converted to the percentage of total possible occurrence. Each of eight behavioral measures (for the individual test: Move, Orient, Proximity, and Avoidance Level; for the group test: Move, Orient, Proximity, and Maps-Top Perch) was then analyzed separately in a Sex (male, female, pregnant female) x Subspecies (Guyanese, Bolivian) x Stimulus (empty, snake) repeated measures of analysis of variance. Post hoc tests in all ANOVAs used the Newman-Keuls procedure (Winer, 1971). For the plasma cortisol analyses, the males, females, and pregnant females were each evaluated independently, since the adrenocorticoid values from each of those groups formed relatively nonoverlapping samples. The effects of the snake presence during the individual tests were also analyzed separately from the effects during the group tests in Subspecies (Guyanese, Bolivian) x Stimulus (empty, snake) ANOVAs. The two basal values, taken before and after the experimental conditions, were compared by paired t tests to assess a possible time effect and then averaged for a mean basal value for each individual animal. Dominance interactions within each group of four females and two males were tabulated in same-sex hierarchies; these hierarchies were linear and stable over the course of the study. The relationship between dominance rank and responsiveness was evaluated independently for males and females (pregnant and nonpregnant combined). Each of the behavioral measures was analyzed in a Subspecies (Guyanese, Bolivian) x Social Rank (1-2 for males, 1-4 for females) x Stimulus (empty, snake) ANOVA. The plasma cortisol values of each animal for each of the four experimental conditions were converted to percentage change from that animal's mean basal value. In Subspecies (Guyanese, Bolivian) x Social Rank (1-2 for males, 1-4 for females) x Stimulus (empty, snake) ANOVAs, the two values for the individual tests were evaluated separately from the two values for the group tests. RESULTS Behavior

The behavioral responses of all types of squirrel monkeys clearly indicated agitated and "fearful" activity in the presence of the snake. For each of the eight behavioral measures there was a highly significant main effect of stimulus condition. The mean percentages and the F and p values are summarized in Table 1. In the presence of the snake, the animals were much more likely to be active, to orient toward the box, and to maintain a maximal avoidance distance from the box, and were far less likely to be on the top perch or in proximity to the box.

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TABLE 1 Behavioral Responses of Squirrel Monkeys during the Four Experimental Conditions Percentage of intervals Test

F(1, 18) =

p <

27.8 28.3 1.3 56.0

12.40 41.40 ~ 51.61 36.78

.005 .001 .001 .001

29.8 39.2 0.8 3.1

17.94 102.7 14.76 33.17

.001 .001 .005 .001

Empty

Snake

Individual Move Orient to box Proximity to box Avoidance level

16.8 10.7 30.8 19.9

Group Move Orient to box Proximity to box Maps-Top perch

15.0 9.3 20.5 36.3

There were some behavioral differences between the types of animals, indicating that monkeys of the Guyanese, rather than the Bolivian variety, and that females, rather than males, were more responsive to the presence of the snake. During the group tests, there was a Stimulus x Subspecies interaction [F(1, 18) = 6.83, p < .05] for Orient, and the post hoc tests indicated that during the Snake condition the Orient values of Guyanese monkeys were greater than those of Bolivian monkeys (p < .01), while during the Empty condition the Orient scores for the two subspecies were not different (Fig. 1A). During the individual tests, there was an overall main effect for Subspecies [F(1, 18) = 7.35, p < .01], showing that the Guyanese (X = 24.3%) oriented more to the box, whether empty or containing the snake, than did the Bolivians (.,Y = 14.7%). There was also a Subspecies difference for the spatial data during the group tests. Post hoc comparisons on the Stimulus x Subspecies interaction [F(1, 18) = 4.66, p < .05] revealed that both Guyanese and Bolivian monkeys were unlikely to be on the top perch in the presence of the snake. But this change in location was a relatively greater one for the Guyanese monkeys as they were much more frequently observed on the top perch during the Empty condition (p < .01, Fig. 1B). Sex differences were also evident in the monkeys' spatial dispersion during the stimulus conditions. During the group tests there was a Stimulus x Sex interaction [F(2, 18) = 3.64, p < .05] for the Top Perch measure. All animals were very unlikely to be on the top perch during the Snake condition; however, for both pregnant and nonpregnant females this represented a relatively greater change in location as females were more likely than males to be on the top perch during the Empty condition (both p < .05, Fig. 2A). A comparable change in spatial location was observed during the individual test. A Stimulus x Sex interaction

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399

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for Avoidance Level [F(2, 18) = 5.02, p < .05] indicated that all animals were observed infrequently at this bottom level during the Empty condition, while during the Snake condition both pregnant and nonpregnant females were much more likely than males to be located at this Avoidance Level (both p's < .05, Fig. 2B). However, males, females, and pregnant females were all more frequently at the Avoidance Level in the presence of the snake than when exposed to only the empty stimulus box (all p's < .05). Cortisol

In contrast to the clear behavioral responses to the snake, no group of animals showed selective elevations in pituitary-adrenal activity that could be attributed to the snake. There were no significant main effects and no significant interactions in any of the Subspecies x Stimulus ANOVAs. During the individual tests, the Empty vs Snake stimulus condition means did not differ for males (176 vs 186 txg/100 ml), females (269 vs 238 Ixg/100 ml), or pregnant females (758 vs 888 txg/100 ml). Likewise during the group tests, the Empty vs Snake condition comparisons were similar for males (108 vs 127 pog/100 ml), females (189 vs 183 Ixg/100 ml), and pregnant females (690 vs 680 ixg/100 ml). Moreover, the analysis of possible time effects upon cortisol values indicated no clear changes over the course of the experiment. The basal samples from the beginning and end of the study differed only for pregnant females (t = 3.85, p < .005) but not for the males (t = 1.00) or females (t = 0.61), and the differences for the pregnant females were very likely due to the endocrine changes during gestation. The differences in the disturbance/novelty aspects of the two test

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400

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situations were, however, reflected in the pituitary-adrenal activity of all groups of monkeys, as overall corticosteroid values for the individual tests were greater than those for the group tests. Comparisons between the means of the two individual tests and the means of the two group tests indicated significant differences for all groups of animals: males (180 vs 118 ixg/100 ml, t = 3.72, p < .005); females (253 vs 187 ixg/100 ml, t = 2.58, p < .025); and pregnant females (823 vs 685 p~g/100 ml, t = 2.41, p < .025) (Fig. 3). These differences between the individual and group conditions were examined further for each animal type by comparing each overall mean with the mean basal by a Dunnett t test (Winer, 1971, p. 201). For the males, the individual (t = 4.39, p < .005) but not the group (t = 0.46) cortisol values were greater than the basal values (Fig. 3). Likewise for the pregnant females the individual (t = 2.53, p < .025) but not the group (t = 0.43) mean was significantly greater than the basal mean (Fig. 3). The means for the nonpregnant females also showed this individual-basal difference. However, their variability and small n impaired statistical evaluations which indicated no differences for the individual (t = 1.37) or the group (t = 0.29) mean when compared to basal (Fig. 3). The ANOVAs on dominance rank and responsiveness did not indicate any relationship between social status and behavioral or pituitary-adrenal activity in males or females, Guyanese or Bolivians, during the individual or group tests. Since systematic changes in the cortisol levels were observed only during the individual tests, the percentage change from mean basal of the mean individual and the mean group values were used in separate Rank x Condition ANOVAs for males and females. Again, no relationship was observed between social status and adrenocorticoid activity.

DISCUSSION The data from this study indicate that wild-born squirrel monkeys show clear and vigorous behavioral responses to a live snake. All monkeys exhibited increased levels of agitated activity and vigilance and almost total avoidance of the stimulus box when the snake was present. These results are consistent with the studies of Murray and King (1973) and Joslin et al. (1964), which reported clear discrimination and avoidance of live snakes by wild-born monkeys. There were some sex and subspecies differences in the behavioral responses to the snake, which indicated a greater fearfulness of females as compared to males and of Guyanese as compared to Bolivian squirrel monkeys. The presence of the snake, while producing clear behavioral responses, did not elicit a pituitary-adrenal response that was elevated over that obtained under the novel condition. It is important to realize that the novel condition was by no means innocuous. It involved capture, re-

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moving the animal from the social group, and keeping the monkey isolated in a small unfamiliar environment. The intensity of the cortisol response under these conditions may have been sufficiently vigorous to preclude a further increment due to the visual presence of the snake. Data obtained on rodents (Hennessy, Heybach, Vernikos, & Levine, 1979; Hennessy & Levine, 1978) has indicated that under low levels of stimulus change the pituitary-adrenal system does respond with gradations that are sensitive to different levels of stimulus change. However, beyond a certain level the addition of more intense noxious stimuli (i.e., electric shock) does not result in a further increase in cortisol excretion (Friedman et al., 1967). Further, since only one time point (30 min) was used in this study, one cannot preclude the possibility that if the period of exposure to the snake had been extended, discriminations in the cortisol response may have been observed. The stress response of an animal in an aversive or threatening situation can be greatly reduced or eliminated by means of several factors, including the social environment and control over the aversive stimulation. The important role of control has been previously demonstrated with both rodents and primates. For example, rats able to press a lever or turn a wheel to avoid shock showed less severe physiological disturbances than yoked subjects with no control, even though both groups received the same amount of shock (Weiss, 1968, 1971). Similarly, rhesus monkeys with control over high-intensity noise revealed plasma cortisol levels similar to those of animals exposed to no noise at all, while monkeys with no control or which lost control exhibited significantly elevated adrenocorticoid values (Hanson, Larson & Snowdon, 1976). In the present study, the monkeys were allowed a certain degree of control over the threatening stimulus, since they were not forced to approach the snake. To the extent that a monkey's fearfulness was related to the enforced proximity of the snake, an animal was able to exert some control over the arousal level of the test situation and thus prevent the stress response. Besides control over aversive stimulation, another important factor in the stress response is the influence of social relationships. Specifically, the presence of familiar social partners during the group tests may have played a primary role in preventing adrenocorticoid elevations to the snake. Other primate studies have demonstrated that social relationships can reduce the response to stressful situations. In adult rhesus monkeys, the presence of social companions can decrease the behavioral manifestations of stress (Rowell & Hinde, 1963). Further, studies on squirrel monkeys have indicated that the physical interactions between mother and infant after a stressful event can substantially reduce or totally suppress the cortisol elevations that each member of the dyad would otherwise reveal (Levine et al. 1978; Mendoza et al., 1978b).

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The test condition of this experiment which did induce clear and significant cortisol elevations was the individual test. The greater corticosteroid values of this condition probably reflect the combined effect of a number of factors, including disturbance, capture, handling, removal from the home cage, isolation from the social group, and exposure to the novel test cage. It thus appears, in squirrel monkeys as well as rodents (Ader, 1970; Bassett et al., 1973), that quite dramatic elevations in pituitary-adrenal activity can be induced by exposure to novelty. This disturbance/novelty set of factors may partly account for the results of the only other investigation of pituitary-adrenal responsiveness of squirrel monkeys to snakes (Manogue et al., 1975). Cortisol elevations were reported for the higher-ranking males when removed from the group and exposed individually to a live garter snake. However, a clear and unequivocal response to the snake per se was not demonstrated, as Manogue et al. did not evaluate adrenocorticoid activity to the test apparatus alone. Moreover, their baseline cortisol values for comparison were obtained from the animals when housed together in the home cage, and our present study indicated that cortisol elevations were produced by removal from the group and individual exposure to a novel test cage. The disparate nature of the physiological and behavioral responses to the snake may have a close parallel in the human literature. Curtis and colleagues (Curtis, Buxton, Lippman, Nesse, & Wright, 1976; Curtis, Nesse, Buxton, & Lippman, 1978) have reported a series of studies in which they presented a fear object to a person with an extreme phobia to that specific object. They found maximal subjective and behavioral anxiety and yet only slight, if any, pituitary-adrenal changes. Moreover, there were significant plasma cortisol elevations by several subjects to the novelty-adaptation sessions. This observed dissociation between subjective-behavioral and physiological aspects of responses to a highly fearful or anxious situation was attributed by Curtis et al. to a "desynchrony of fear" (Rachman & Hodgson, 1974) hypothesis, in that the different components of fear may strengthen or diminish at different rates and therefore may not always appear together. This desynchrony of fear hypothesis was also used by Mineka et al. (1980) to explain the behavioral responses of rhesus monkeys to repeated snake exposures; these investigators noted that behavioral disturbance declined within, but not across, the several test sessions. An interesting and unexpected finding concerns the very high cortisol values obtained from the pregnant females. All blood samples from these animals were taken during the first half of pregnancy. Within this period the mean cortisol values of the pregnant females were about three times those of the nonpregnant females, and several animals revealed values during undisturbed basal conditions of over 1000 p~g/100 ml, including one individual at 1404 p~g/100 ml. Changes in pituitary-adrenal activity

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during pregnancy have not been reported previously in the squirrel monkey. Though this several-fold increase in adrenocorticoid output is comparable to increases in human pregnancy plasma cortisol values, which increase two- to threefold by midterm (Cohen, Stiefel, Reddy, & Laidlaw, 1958; Gemzell, 1953; Martin & Mills, 1958), it contrasts with rhesus monkey plasma corticosteroid levels, which do not change significantly during gestation (Challis, Davies, Benirschke, Hendrickx, & Ryan, 1975; Wolf & Bowman, 1966). These very high resting titers in squirrel monkeys did not, however, preclude further increases as significant elevations from basal values for the pregnant females were observed and values of up to 1627 tzg/100 ml were obtained following disturbance. There was no statistical relationship obtained between dominance levels and any of the behavioral or adrenocorticoid measures. All these data were obtained approximately 1-2 months after group formation and so on the basis of previous studies (Coe et al., 1979; Mendoza, Coe, Lowe, & Levine, 1979) one might have expected the more dominant animals, at least among the males, to exhibit higher basal corticosteroid values and smaller stress increments. However, both the Coe et al. and Mendoza et al. studies were performed during the laboratory mating season. In contrast, the present experiment and one other recent study (Vogt et al., 1980) failing to replicate this finding were done several months out of the mating season. Taken together, these data may indicate the important influence of squirrel monkey annual cycles on pituitary-adrenal activity during basal and stress conditions.

REFERENCES Ader, R. (1970). The effects of early experience on the adrenocortical response to different magnitudes of stimulation. Physiology and Behavior, 5, 837-839. Ader, R., & Friedman, S. B. (1968). Plasma corticosterone response to environmental stimulation: Effects of duration of stimulation and the 24-hour adrenocortical rhythm. Neuroendocrinology, 3, 378-386. Bassett, J. R., Cairncross, K. D., & King, M. G. (1973). Parameters of novelty, shock predictability and response contingency in cortieosterone release in the rat. Physiology and Behavior, 10, 901-907. Bemstein, S., & Mason, W. A. (1962). The effects of age and stimulus conditions on the emotional responses of rhesus monkeys: Responses to complex stimuli. Journal of Genetic Psychology, 101, 279-298. Cannon, W. B. (1915). Bodily Changes in Pain, Hunger, Fear and Rage. New York: Appleton. Challis, J. R. G., Davies, I. J., Benirschke, K., Hendrickx, A. G., & Ryan, K. J. (1975). The effects of dexamethasone on the peripheral plasma concentrations of androstenedione, testosterone and cortisol in the pregnant rhesus monkey. Endocrinology. 96, 185-192. Coe, C. L., Mendoza, S. P., Davidson, J. M., Smith, E. R., Dallman, M. F., & Levine, S. (1978a). Hormonal response to stress in the squirrel monkey (Saimiri sciureus). Neuroendocrinology, 26, 367-377. Coe, C. L., Mendoza, S. P., & Levine, S. (1979). Social status constrains the stress response in the squirrel monkey. Physiology and Behavior, 23, 633-638.

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