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Effects of variations in male copulatory behavior on ovulation and implantation in prairie voles, Microtus ochrogaster
HORMONES
AND
BEHAVIOR,
5, 389-396
Effects of Variations Ovulation and Implantation
(1974)
in Male Copulatory Behavior on in Prairie Voles, Micro&s ochroguster
GARY D. GRAY, MATTHEW ZERYLNICK, HARRY and DONALD A. DEWSBURY
N. DAVIS,
Department of Psychology and Center for Neurobiological Sciences, University of Florida, Gainesville, Florida 3261 I
The relationships between parameters of male copulatory behavior and ovulation and implantation were examined for Microtus ochroguster. One ejaculatory series is sufficient for induction of ovulation and implantation, and a second series does not elicit any increase in either the probability of ovulation or the number of corpora lutea and implanted embryos. Within the first ejaculatory series, the probability of ovulation increases with increased numbers of intromissions and intravaginal thrusts. Mounting without vaginal penetration is not sufficient to induce ovulation. It is suggested that Microtus species display a coadaptation of male copulatory behavior and stimulation requirements of the female reporductive system.
While it is clear that one function of mammalian copulatory behavior is the transfer of sperm from male to female, the stimulation provided by copulation also functions to elicit responses in the female reproductive system which are essential for successful reproduction, e.g., ovulation (Jochle, 1973) and implantation (Wilson, Adler, and Le Boeuf, 196.5; Adler, 1969). Research on montane voles (Microtus montanus) has shown an apparent coadaptation between male copulatory behavior and stimulation requirements of the female reproductive system (Davis, Gray, Zerylnick, and Dewsbury, 1974). Males typically attain five ejaculations before satiety, and females require more than two ejaculations in order to insure a high probability of ovulation and implantation. In order to examine the specificity of this adaptive relationship, the present study investigated the stimulation requirements for ovulation and implantation in prairie voles (Microtus ochrogaster). M. ochrogaster males rarely achieve more than two ejaculations before satiety (Gray and Dewsbury, 1973), and it is possible that the difference in male behavior between M. ochrogaster and M. montanus may be correlated with a difference in the stimulation requirements of the females. 389 Copyright All rights
@ 1974 by Academic Press, Inc. of reproduction in any form reserved.
390
GRAY
ET AL
M. ochrogaster is a small rodent (30-50 gm), native to the midwestern United States. It has been studied extensively because of its substantial population fluctuations, and because animals are capable of breeding year round (Fitch, 1957; Krebs, Keller, and Tamarin, 1969). Femalesare induced ovulators, with no discernible estrous cycle (Richmond and Conaway, 1969a). Mean litter size is 3.77, and a postpartum estrus is present (Richmond and Conaway, 1969b). Sexual maturity occurs at approximately 60 days for males and females(Richmond and Conaway, 1969b).
EXPERIMENT 1 Methods Subjects were 30 adult female M. ochrogaster (age 4-12 mo). Ten adult maleswere used asmating partners. All animalswere laboratory reared and of proven fertility (i.e., each had produced at least one litter). Animals were housed individually in clear, plastic cages(48 X 27 X 13 cm for malesand 29 X 19 X 13 cm for females), with Purina Rabbit Chow and water available at all times. Animals were maintained on a reversed 16:8 light-dark cycle (lights on 8:OOPM-12:00 noon). Femaleswere brought into estrus by being placed in proximity to adult males(Richmond and Conaway, 1969a). Home cagesof maleswere divided in half by removable, hardware cloth barriers, thereby confining the male to one side and the female to the other. The reproductive state of females was evaluated by daily vaginal smears.Mating tests were conducted on the second consecutive day of estrus(50% or more corniced cells) after introduction into the divided cage. Mating tests were conducted 2-4 hr after the beginning of the dark period. Tests were initiated by removing the hardware cloth barrier, thereby allowing the male accessto the female. If the female was judged unreceptive or the male failed to achieve an intromission within 60 min, the test was discontinued and discounted. Mating tests involved one of three experimental conditions. In the first condition, the male and female were allowed to copulate through two ejaculations before the male was removed. In the secondcondition, copulation was allowed through one ejaculation. For the third condition, the male was removed before elimination of the hardware cloth barrier, and no copulation occurred. Each female was tested in only one condition, with ten femalesin each group. Males were employed once in each condition. Femaleswere killed 8 days after the mating tests. Their ovaries and uteri were examined under a dissecting microscope. The number of corpora lutea (CL) was recorded for each ovary, and the number of implanted embryos was
OVULATION
IN PRAIRIE
391
VOLES
noted for each uterine horn. Microscopic examination of several serially sectioned ovaries was conducted in order to verify the accuracy of the CL counts. Ovaries were fixed in formalin, embedded in paraffin, sectioned at 10 pm, and stained with hematoxylin and eosin. Results The percentage of females ovulating (showing at least one CL) in each of the three experimental conditions is presented in Table 1. Copulation is required for ovulation. Thus, no females in the “no copulation” condition ovulated. However, there was no difference between the one and two ejaculation conditions in the percentage of females ovulating. One female failed to ovulate in each condition. In both cases the same male was the partner, although his behavior fell well within the normal range. Quantitative data on ovulation and implantation also are included in Table 1. There were no significant differences between the one- and twoejaculation conditions in (1) total number of CL, (2) total number of implanted embryos, (3) intrauterine mortality, as measured by the difference between CL number and the number of implanted embryos.
EXPERIMENT
2
The first experiment established that one ejaculation was sufficient to induce ovulation in M. ochrogaster. Yet, normative data on copulatory behavior in this species indicate that males show a series of behaviors prior to ejaculation. In fact, a mean of 4.3 mounts (without vaginal penetration), 9.5 intromissions (mounts with vaginal penetration and intravaginal thrusting), and 42.7 thrusts are exhibited by males in the series of behavior leading to the first ejaculation (Gray and Dewsbury, 1973). The second experiment was TABLE Effects
of Number
1
of Ejaculations
on Ovulation
and Implantation
Ejaculations Item Number
0
of females
10
of females ovulating Mean number of corpora lutea Mean number of implanted Mean intrauterine mortalitya of CL minus
10
10
-
90 5.00 f 0.41
-
3.66 + 0.62 1.66 f 0.69
3.11 f 0.54 1.89 + 0.68
0
embryos
number
2
90 5.33 f 0.47
Percentage
aNumber
1
of implanted
embryos.
392
GRAY
ET AL.
designed to examine the parameters of copulatory series necessary for induction of ovulation.
behavior within
the first
Methods Seventy adult females (age 4-12 mo) were employed in the experiment. Nineteen adult males were used as mating partners. All animals were laboratory reared and of proven fertility. Estrus was elicited in females as in Experiment 1. The testing procedure differed from the first experiment only in that the mating partners were not those males housed in the divided cage. Mating tests were initiated by removing the female from the divided cage and placing her in the home cage of the male partner. During mating tests, females received a specified number of intromissions. Six experimental conditions were employed. In four of the conditions, the female was permitted 1, 3, 5, or 8 intromissions, after which the male was immediately removed from the cage. If, however, during any of the above four conditions the male achieved ejaculation before completing the specified number of intromissions, that female was reassigned to the fifth condition (intromissions + ejaculation). In the sixth condition, no intromissions were permitted, and females received only a variable number of mounts without vaginal penetration. Vaginal penetration was prevented by the use of a “vaginal mask” (Hardy, 1972). Ten females were used in the 0 intromission (mounting) condition, 14 females each in the 1, 3, 5, and 8 intromission conditions, and four females in the intromission plus ejaculation condition. No male was employed more than once in each condition; 1.5 males participated in at least three conditions. Females were killed 4 days after the mating tests. The ovaries were examined under a dissecting microscope, and the number of CL on each ovary was recorded. Several ovaries from each experimental group were prepared for histological examination (as described in Experiment 1). Results The percentage of females ovulating in each of the six experimental conditions is illustrated in Fig. 1. Results showed an increasing percentage of females ovulating as a function of increasing numbers of intromissions. Moreover, the stimulation provided by a vigorously mounting male (from 8 to 33 mounts) was not sufficient, in the absence of vaginal penetration, to induce ovulation. It might be argued that the presence of a vaginal mask may have accounted for the absence of ovulation. Therefore, as a control procedure, four females were first masked and the vaginal mask then removed just prior to copulation. All four females ovulated.
OVULATION
IN PRAIRIE
VOLES
393
100 -
n:4
INTROMISSIONS
Fig. 1. Percentage of females ovulating as a function of number of intromissions. “mount.” refers to females receiving no intromissions but a variable number of mounts without vaginal penetration. “Ejac.” refers to females receiving intromissions and ejaculation. Results of statistical analysis: x2 = 23.18; p < .Ol.
*
100
-
80
-
n=3
n=4
n=4
26-30
‘30
Ejoc
to
number
n=7 E I: 3
60
-
40
-
20
-
& L !$
1-5
thrusts.
6-10
Fig. 2. Percentage of females Results of statistical analysis:
II-15 THRUST
1X.3 21-25 FREQUENCY
ovulating in relation x2 = 31.40; p < .Ol.
of
intravaginal
Given ovulation, there was no indication that the number of CL varied as a function of intromission frequency. In M. ochrogaster, the motor pattern of intromission consists of a variable number of distinct, intravaginal thrusts. Thus, a concomitant variable to intromission frequency in any quantification of vaginal stimulation is the total number of thrusts. Figure 2 presents an analysis of the percentage of
394
GRAY
ET AL.
females ovulating as a function of thrust frequency. It is apparent that a high percentage of females ovulating was associated with high thrust frequencies. A further indication that thrust frequency is a relevant variable is evident when one examines ovulation and associated thrust frequencies within experimental groups. For example, in the 8 intromission condition, the nine females ovulating received a mean of 28.8 thrusts, a significantly higher number of thrusts than the mean of 17.2 thrusts received by the five females not ovulating (P< .05: randomization test; Siegel, 1956). In the 3 intromission condition, ovulating females received a mean of 11.6 thrusts as opposed to 7.1 for those not ovulating (P < .lO: randomization test; Siegel, 1956). Females in the intromission + ejaculation condition received a mean of 49.0 thrusts and showed a 100% ovulation rate. Intromissions are necessarily distributed in time, and accordingly, several temporal measures of mating performance were analyzed in relation to ovulation. Neither Intromission Latency (time from introduction of the female into the male’s cage to the first intromission) nor Copulation Time (time from the first intromission of a test to removal of the male) appeared to affect the percentage of females ovulating. However, Total Test Time (Intromission Latency + Copulation Time) did exhibit some relationship to ovulation. Considering females in the 1, 3, 5, and 8 intromission groups reveals that, up to 800 set, Total Test Time was positively correlated with a greater incidence of ovulation (Y = 0.72; p < .Ol), with all females having Total Test Times of 600-800 set ovulating. This correlation is not surprising since Total Test Time is clearly related to intromission frequency. However, none of the seven females with Total Test Times greater than 800 set ovulated, irrespective of number of intromissions or thrusts.
DISCUSSION These experiments were designed to evaluate functional relationships between parameters of male copulatory behavior and ovulation and implantation in M. ochrogasfev. Within the first series, the probability of ovulation increases in response to increased vaginal stimulation provided by intromissions and intravaginal thrusting. No discrete threshold for ovulation was evident in the data. Some females receiving eight intromissions and IS-24 thrusts failed to ovulate while other females receiving only three intromissions and 8-11 thrusts ovulated. A number of factors may account for this lack of a distinct threshold, e.g., interaction effects between thrusts and intromissions, individual differences among males, individual differences among females. The importance of Total Test Time was apparent, in that long Total Test Times were associated with a failure of ovulation. Long Total Test Times may be related to a low degree of female receptivity, and the failure of ovulation may
OVULATION
IN PRAIRIE
395
VOLES
be a function of decreased responsiveness on the part of females to vaginal stimulation. Although only females showing estrous vaginal smears and a lordotic response to mounting by males were included in the experiment, gross observation did indicate variations in receptivity among animals. However, the lack of direct, quantitative data on the quality of female receptivity precludes any definitive conclusions on the role of this variable in modulating the probability of ovulation. The lordosis postures in this species is so minimal that quantification is impractical. Quantitative aspects of ovulation and implantation do not appear to be functionally related to parameters of copulatory behavior. Thus, given ovulation, the number of CL and implanted embryos is independent of the number of ejaculations or intromissions received by the female. Similar results were found with M. montunus (Davis et al., 1974). It might be noted that total number of implanted embryos from the first experiment are in close agreement with litter sizes from our laboratory and from published reports of other laboratories (Richmond and Conaway, 1969b). Consideration of the species-typical copulatory behavior patterns of M. ochrogaster and M. montanus reveals a congruence between male behavior pattern and response characteristics of the female reproductive system. M. ochrogaster males show a low ejaculation frequency, rarely attaining more than two ejaculations before satiety (Gray and Dewsbury, 1973), and females require only a single ejaculation for maximal ovulation and implantation. Within the first ejaculatory series, males exhibit a mean of 9.5 intromissions and 42.7 thrusts (Gray and Dewsbury, 1973). Concomitantly, a high probability of ovulation requires vaginal stimulation equal to more than eight intromissions and 25 thrusts. In contrast, M. montanus males typically exhibit five ejaculations before satiety (Dewsbury, 1973) and females require more than two ejaculations for a high probability of ovulation (Davis et al,, 1974). Such congruence would seem to indicate a coadaptation of male copulatory behavior and female reproductive physiology, with the adaptive significance of the species-typical male pattern interlocked with the stimulation requirements of the female reproductive system. ACKNOWLEDGMENTS The authors thank Barbara McCube and the Center for Neurobiological sciences for assistance in the histological preparation of ovaries. G.D.G. and H.N.D. were supported by Training Grant NIH-MH-10320 to the Center for Neurobiological Sciences, University of Florida. This research was supported by Grant GB-33837X from the National Science Foundation. REFERENCES Adler,
N. T. (1969). Effects of the male’s copulatory behavior the female rat. J. Comp. Physic4 Psychol. 69, 613-622.
on successful
pregnancy
of
GRAY ET AL.
396
Davis, H. N., Gray, G. D., Zerylnick, M., and Dewsbury, D. A. (1974). Ovulation and implantation in montane voles, Microtus montanus, as a function of varying amounts of copulatory stimulation. Horm. Behav., 5, 383-388. Dewsbury, D. A. (1973). Copulatory behavior of montane voles (Microtus montanus). Behaviour 44, 186-202. Fitch, H. S. (1957). Aspects of reproduction and development in the prairie vole (Microtus ochrogaster). Univ. Kansas Mus. Nat. Hist. Pub/. 10, 129-161. Gray, G. D., and Dewsbury, D. A. (1973). A quantitative description of copulatory behavior in prairie voles, Microtus ochrogaster. Brain Behav. Evol. 8, 437-452. Hardy, D. F. (1972). Sexual behavior in continuously cycling rats. Behaviour 41, 288-297. Jochle, W. (1973). Coitus-induced ovulation. Contraception 7, 523-564. Krebs, C. J., Keller, B. L., and Tamarin, R. H. (1969). Microtus population biology: Demographic changes in fluctuating populations of M. ochrogaster and M. pennsylvanicus in southern Indiana. Ecology 50, 587-607. Richmond, M., and Conaway, C. H. (1969a). Induced ovulation and oestrus in Microtus ochrogaster.
J. Reprod.
Fert.,
Suppl.
6, 357-376.
Richmond, M., and Conaway, C. H. (9169b). Management, breeding, and reproductive performance of the vole, Microtus ochrogaster, in a laboratory colony. Lab. Anim. Care 19, 80-87. Siegel, S. (1956). Nonparametric Statistics For the Behavioral Sciences. McGraw-Hill, New York. Wilson, J. R., Adler, N. T., and Le Boeuf, B. (1965). The effects of intromission frequency on successful pregnancy in the female rat. Proc. Nat. Acad. Sci. USA 53, 1392-1395.
AND
BEHAVIOR,
5, 389-396
Effects of Variations Ovulation and Implantation
(1974)
in Male Copulatory Behavior on in Prairie Voles, Micro&s ochroguster
GARY D. GRAY, MATTHEW ZERYLNICK, HARRY and DONALD A. DEWSBURY
N. DAVIS,
Department of Psychology and Center for Neurobiological Sciences, University of Florida, Gainesville, Florida 3261 I
The relationships between parameters of male copulatory behavior and ovulation and implantation were examined for Microtus ochroguster. One ejaculatory series is sufficient for induction of ovulation and implantation, and a second series does not elicit any increase in either the probability of ovulation or the number of corpora lutea and implanted embryos. Within the first ejaculatory series, the probability of ovulation increases with increased numbers of intromissions and intravaginal thrusts. Mounting without vaginal penetration is not sufficient to induce ovulation. It is suggested that Microtus species display a coadaptation of male copulatory behavior and stimulation requirements of the female reporductive system.
While it is clear that one function of mammalian copulatory behavior is the transfer of sperm from male to female, the stimulation provided by copulation also functions to elicit responses in the female reproductive system which are essential for successful reproduction, e.g., ovulation (Jochle, 1973) and implantation (Wilson, Adler, and Le Boeuf, 196.5; Adler, 1969). Research on montane voles (Microtus montanus) has shown an apparent coadaptation between male copulatory behavior and stimulation requirements of the female reproductive system (Davis, Gray, Zerylnick, and Dewsbury, 1974). Males typically attain five ejaculations before satiety, and females require more than two ejaculations in order to insure a high probability of ovulation and implantation. In order to examine the specificity of this adaptive relationship, the present study investigated the stimulation requirements for ovulation and implantation in prairie voles (Microtus ochrogaster). M. ochrogaster males rarely achieve more than two ejaculations before satiety (Gray and Dewsbury, 1973), and it is possible that the difference in male behavior between M. ochrogaster and M. montanus may be correlated with a difference in the stimulation requirements of the females. 389 Copyright All rights
@ 1974 by Academic Press, Inc. of reproduction in any form reserved.
390
GRAY
ET AL
M. ochrogaster is a small rodent (30-50 gm), native to the midwestern United States. It has been studied extensively because of its substantial population fluctuations, and because animals are capable of breeding year round (Fitch, 1957; Krebs, Keller, and Tamarin, 1969). Femalesare induced ovulators, with no discernible estrous cycle (Richmond and Conaway, 1969a). Mean litter size is 3.77, and a postpartum estrus is present (Richmond and Conaway, 1969b). Sexual maturity occurs at approximately 60 days for males and females(Richmond and Conaway, 1969b).
EXPERIMENT 1 Methods Subjects were 30 adult female M. ochrogaster (age 4-12 mo). Ten adult maleswere used asmating partners. All animalswere laboratory reared and of proven fertility (i.e., each had produced at least one litter). Animals were housed individually in clear, plastic cages(48 X 27 X 13 cm for malesand 29 X 19 X 13 cm for females), with Purina Rabbit Chow and water available at all times. Animals were maintained on a reversed 16:8 light-dark cycle (lights on 8:OOPM-12:00 noon). Femaleswere brought into estrus by being placed in proximity to adult males(Richmond and Conaway, 1969a). Home cagesof maleswere divided in half by removable, hardware cloth barriers, thereby confining the male to one side and the female to the other. The reproductive state of females was evaluated by daily vaginal smears.Mating tests were conducted on the second consecutive day of estrus(50% or more corniced cells) after introduction into the divided cage. Mating tests were conducted 2-4 hr after the beginning of the dark period. Tests were initiated by removing the hardware cloth barrier, thereby allowing the male accessto the female. If the female was judged unreceptive or the male failed to achieve an intromission within 60 min, the test was discontinued and discounted. Mating tests involved one of three experimental conditions. In the first condition, the male and female were allowed to copulate through two ejaculations before the male was removed. In the secondcondition, copulation was allowed through one ejaculation. For the third condition, the male was removed before elimination of the hardware cloth barrier, and no copulation occurred. Each female was tested in only one condition, with ten femalesin each group. Males were employed once in each condition. Femaleswere killed 8 days after the mating tests. Their ovaries and uteri were examined under a dissecting microscope. The number of corpora lutea (CL) was recorded for each ovary, and the number of implanted embryos was
OVULATION
IN PRAIRIE
391
VOLES
noted for each uterine horn. Microscopic examination of several serially sectioned ovaries was conducted in order to verify the accuracy of the CL counts. Ovaries were fixed in formalin, embedded in paraffin, sectioned at 10 pm, and stained with hematoxylin and eosin. Results The percentage of females ovulating (showing at least one CL) in each of the three experimental conditions is presented in Table 1. Copulation is required for ovulation. Thus, no females in the “no copulation” condition ovulated. However, there was no difference between the one and two ejaculation conditions in the percentage of females ovulating. One female failed to ovulate in each condition. In both cases the same male was the partner, although his behavior fell well within the normal range. Quantitative data on ovulation and implantation also are included in Table 1. There were no significant differences between the one- and twoejaculation conditions in (1) total number of CL, (2) total number of implanted embryos, (3) intrauterine mortality, as measured by the difference between CL number and the number of implanted embryos.
EXPERIMENT
2
The first experiment established that one ejaculation was sufficient to induce ovulation in M. ochrogaster. Yet, normative data on copulatory behavior in this species indicate that males show a series of behaviors prior to ejaculation. In fact, a mean of 4.3 mounts (without vaginal penetration), 9.5 intromissions (mounts with vaginal penetration and intravaginal thrusting), and 42.7 thrusts are exhibited by males in the series of behavior leading to the first ejaculation (Gray and Dewsbury, 1973). The second experiment was TABLE Effects
of Number
1
of Ejaculations
on Ovulation
and Implantation
Ejaculations Item Number
0
of females
10
of females ovulating Mean number of corpora lutea Mean number of implanted Mean intrauterine mortalitya of CL minus
10
10
-
90 5.00 f 0.41
-
3.66 + 0.62 1.66 f 0.69
3.11 f 0.54 1.89 + 0.68
0
embryos
number
2
90 5.33 f 0.47
Percentage
aNumber
1
of implanted
embryos.
392
GRAY
ET AL.
designed to examine the parameters of copulatory series necessary for induction of ovulation.
behavior within
the first
Methods Seventy adult females (age 4-12 mo) were employed in the experiment. Nineteen adult males were used as mating partners. All animals were laboratory reared and of proven fertility. Estrus was elicited in females as in Experiment 1. The testing procedure differed from the first experiment only in that the mating partners were not those males housed in the divided cage. Mating tests were initiated by removing the female from the divided cage and placing her in the home cage of the male partner. During mating tests, females received a specified number of intromissions. Six experimental conditions were employed. In four of the conditions, the female was permitted 1, 3, 5, or 8 intromissions, after which the male was immediately removed from the cage. If, however, during any of the above four conditions the male achieved ejaculation before completing the specified number of intromissions, that female was reassigned to the fifth condition (intromissions + ejaculation). In the sixth condition, no intromissions were permitted, and females received only a variable number of mounts without vaginal penetration. Vaginal penetration was prevented by the use of a “vaginal mask” (Hardy, 1972). Ten females were used in the 0 intromission (mounting) condition, 14 females each in the 1, 3, 5, and 8 intromission conditions, and four females in the intromission plus ejaculation condition. No male was employed more than once in each condition; 1.5 males participated in at least three conditions. Females were killed 4 days after the mating tests. The ovaries were examined under a dissecting microscope, and the number of CL on each ovary was recorded. Several ovaries from each experimental group were prepared for histological examination (as described in Experiment 1). Results The percentage of females ovulating in each of the six experimental conditions is illustrated in Fig. 1. Results showed an increasing percentage of females ovulating as a function of increasing numbers of intromissions. Moreover, the stimulation provided by a vigorously mounting male (from 8 to 33 mounts) was not sufficient, in the absence of vaginal penetration, to induce ovulation. It might be argued that the presence of a vaginal mask may have accounted for the absence of ovulation. Therefore, as a control procedure, four females were first masked and the vaginal mask then removed just prior to copulation. All four females ovulated.
OVULATION
IN PRAIRIE
VOLES
393
100 -
n:4
INTROMISSIONS
Fig. 1. Percentage of females ovulating as a function of number of intromissions. “mount.” refers to females receiving no intromissions but a variable number of mounts without vaginal penetration. “Ejac.” refers to females receiving intromissions and ejaculation. Results of statistical analysis: x2 = 23.18; p < .Ol.
*
100
-
80
-
n=3
n=4
n=4
26-30
‘30
Ejoc
to
number
n=7 E I: 3
60
-
40
-
20
-
& L !$
1-5
thrusts.
6-10
Fig. 2. Percentage of females Results of statistical analysis:
II-15 THRUST
1X.3 21-25 FREQUENCY
ovulating in relation x2 = 31.40; p < .Ol.
of
intravaginal
Given ovulation, there was no indication that the number of CL varied as a function of intromission frequency. In M. ochrogaster, the motor pattern of intromission consists of a variable number of distinct, intravaginal thrusts. Thus, a concomitant variable to intromission frequency in any quantification of vaginal stimulation is the total number of thrusts. Figure 2 presents an analysis of the percentage of
394
GRAY
ET AL.
females ovulating as a function of thrust frequency. It is apparent that a high percentage of females ovulating was associated with high thrust frequencies. A further indication that thrust frequency is a relevant variable is evident when one examines ovulation and associated thrust frequencies within experimental groups. For example, in the 8 intromission condition, the nine females ovulating received a mean of 28.8 thrusts, a significantly higher number of thrusts than the mean of 17.2 thrusts received by the five females not ovulating (P< .05: randomization test; Siegel, 1956). In the 3 intromission condition, ovulating females received a mean of 11.6 thrusts as opposed to 7.1 for those not ovulating (P < .lO: randomization test; Siegel, 1956). Females in the intromission + ejaculation condition received a mean of 49.0 thrusts and showed a 100% ovulation rate. Intromissions are necessarily distributed in time, and accordingly, several temporal measures of mating performance were analyzed in relation to ovulation. Neither Intromission Latency (time from introduction of the female into the male’s cage to the first intromission) nor Copulation Time (time from the first intromission of a test to removal of the male) appeared to affect the percentage of females ovulating. However, Total Test Time (Intromission Latency + Copulation Time) did exhibit some relationship to ovulation. Considering females in the 1, 3, 5, and 8 intromission groups reveals that, up to 800 set, Total Test Time was positively correlated with a greater incidence of ovulation (Y = 0.72; p < .Ol), with all females having Total Test Times of 600-800 set ovulating. This correlation is not surprising since Total Test Time is clearly related to intromission frequency. However, none of the seven females with Total Test Times greater than 800 set ovulated, irrespective of number of intromissions or thrusts.
DISCUSSION These experiments were designed to evaluate functional relationships between parameters of male copulatory behavior and ovulation and implantation in M. ochrogasfev. Within the first series, the probability of ovulation increases in response to increased vaginal stimulation provided by intromissions and intravaginal thrusting. No discrete threshold for ovulation was evident in the data. Some females receiving eight intromissions and IS-24 thrusts failed to ovulate while other females receiving only three intromissions and 8-11 thrusts ovulated. A number of factors may account for this lack of a distinct threshold, e.g., interaction effects between thrusts and intromissions, individual differences among males, individual differences among females. The importance of Total Test Time was apparent, in that long Total Test Times were associated with a failure of ovulation. Long Total Test Times may be related to a low degree of female receptivity, and the failure of ovulation may
OVULATION
IN PRAIRIE
395
VOLES
be a function of decreased responsiveness on the part of females to vaginal stimulation. Although only females showing estrous vaginal smears and a lordotic response to mounting by males were included in the experiment, gross observation did indicate variations in receptivity among animals. However, the lack of direct, quantitative data on the quality of female receptivity precludes any definitive conclusions on the role of this variable in modulating the probability of ovulation. The lordosis postures in this species is so minimal that quantification is impractical. Quantitative aspects of ovulation and implantation do not appear to be functionally related to parameters of copulatory behavior. Thus, given ovulation, the number of CL and implanted embryos is independent of the number of ejaculations or intromissions received by the female. Similar results were found with M. montunus (Davis et al., 1974). It might be noted that total number of implanted embryos from the first experiment are in close agreement with litter sizes from our laboratory and from published reports of other laboratories (Richmond and Conaway, 1969b). Consideration of the species-typical copulatory behavior patterns of M. ochrogaster and M. montanus reveals a congruence between male behavior pattern and response characteristics of the female reproductive system. M. ochrogaster males show a low ejaculation frequency, rarely attaining more than two ejaculations before satiety (Gray and Dewsbury, 1973), and females require only a single ejaculation for maximal ovulation and implantation. Within the first ejaculatory series, males exhibit a mean of 9.5 intromissions and 42.7 thrusts (Gray and Dewsbury, 1973). Concomitantly, a high probability of ovulation requires vaginal stimulation equal to more than eight intromissions and 25 thrusts. In contrast, M. montanus males typically exhibit five ejaculations before satiety (Dewsbury, 1973) and females require more than two ejaculations for a high probability of ovulation (Davis et al,, 1974). Such congruence would seem to indicate a coadaptation of male copulatory behavior and female reproductive physiology, with the adaptive significance of the species-typical male pattern interlocked with the stimulation requirements of the female reproductive system. ACKNOWLEDGMENTS The authors thank Barbara McCube and the Center for Neurobiological sciences for assistance in the histological preparation of ovaries. G.D.G. and H.N.D. were supported by Training Grant NIH-MH-10320 to the Center for Neurobiological Sciences, University of Florida. This research was supported by Grant GB-33837X from the National Science Foundation. REFERENCES Adler,
N. T. (1969). Effects of the male’s copulatory behavior the female rat. J. Comp. Physic4 Psychol. 69, 613-622.
on successful
pregnancy
of
GRAY ET AL.
396
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