Scandinavian Journal of Psychology, 2003, 44, 279–288
Detailed analysis of the male copulatory motor pattern in mammals:
Hormonal bases
Blackwell Publishing Ltd
GABRIELA MORALÍ, MARÍA ASUNCIÓN PÍA SOTO, JOSÉ LUIS CONTRERAS, MARCELA ARTEAGA,
MARÍA DOLORES GONZÁLEZ-VIDAL and CARLOS BEYER
Unidad de Investigación Médica en Farmacología, Centro Médico Nacional Siglo XXI, IMSS, Mexico,
División CBS, Universidad Autónoma Metropolitana, Mexico,
Centro de Investigación en Reproducción Animal, CINVESTAV-UAT, Ixtacuixtla, Tlaxcala, Mexico
Moralí, G., Soto, M. A. P., Contreras, J. L., Arteaga M., González-Vidal M. D., and Beyer C. (2003). Detailed analysis of the male copulatory
motor pattern in mammals: Hormonal bases. Scandinavian Journal of Psychology, 44, 279 –288.
Data obtained, using a polygraphic technique, on the characteristics of the motor and genital copulatory responses of male rabbits, rats, mice,
hamsters, and guinea pigs are reviewed. This methodology provided detailed information, not accessible to other analyses, on the frequency
and dynamic organization of copulatory pelvic thrusting trains of the species studied. This comparative analysis showed that: (1) The male
rat may display two types of ejaculatory responses, differing in the dynamic organization of the pelvic thrusting train, and in the duration of
the intravaginal thrusting period preceding ejaculation. (2) In the guinea pigs and small rodents, but not in rabbits, pelvic thrusting at ejaculatory
responses persists during intromission, and a period of fast intravaginal thrusting is associated with ejaculation. (3) The motor copulatory
pattern of the rabbit, but not of the rat, hamster, or guinea pig, is affected by castration and hormone treatment, suggesting that, in rabbits,
androgen acts both on motivation and on the spinal neural systems related to copulation.
Key words: Sexual behavior, polygraphic analysis, motor copulatory pattern, pelvic thrusting.
Gabriela Moralí, Unidad de Investigación Médica en Farmacología, Coord. Invest. Salud IMSS, Coahuila 5 Col. Roma, PO Box A-047, Mexico
06703 DF, Mexico. E-mail: [email protected]
Sexual behavior in mammals involves the participation of an
arousal mechanism that drives the individual to the search
for and onset of sexual interaction with a partner, and a
consummatory mechanism that allows the individual to
perform this interaction (Beach, 1967). A modulatory
mechanism may act on motivation and performance, by
inhibiting the expression of sexual behavior under some
circumstances (Beyer, 1974).
Extensive literature has been devoted to the general
description of sexual behavior patterns; data on their incidence and temporal course allow an evaluation of arousal
and performance in terms of endocrine, neural, ontogenetic,
social, and environmental factors involved in the expression
of this behavior. However, there have been few studies of the
“morphology” of the various behavioral patterns involved
in copulation.
Male copulation in mammals includes the activation of
three interacting components: a motor component, an external
genital component, and an internal genital component
(Moralí & Beyer, 1992). The motor component typically
involves those muscles that allow the male to climb, clasp,
and mount its partner, and to execute the copulatory rhythmic
pelvic thrusting movements against the female’s rump that
either induce or intensify the adoption of the receptive
posture by the female and that facilitate intravaginal penile
insertion and, eventually, ejaculation. The external genital
component involves the penile vascular and muscular
responses involved in penile erection and insertion into
the vagina. The internal genital component comprises the
contractile autonomic and somatic activities of the various
organs involved in seminal emission and ejaculation. A full
understanding of the functional significance of copulatory
behavior requires precise information on the interactions
between these three components.
Knut Larsson has been one of the pioneers in exploring
several of the factors determining the expression of masculine sexual behavior of the laboratory rat, and in describing
some general and detailed characteristics of the copulatory
behavioral patterns (Larsson, 1956, 1959, 1963, 1969, 1979;
Carlsson & Larsson, 1962). In several of his valuable
and fruitful visits to our laboratory in Mexico City in the
early 1980s, he directed his interest, among many other
areas of research, to an innovative, reliable methodology
developed and implemented by some of us (Carlos Beyer,
José Miguel Cervantes, José Luis Contreras, and Javier
Almanza), and followed by us and our students. This
methodology provides objective and detailed images of
signals generated in relation to copulatory pelvic thrusting,
and allows an analysis of some of its dynamic aspects
(rhythmicity and vigor) and its temporal relation to penile–
vaginal contacts and to seminal vesicle pressure. The application of this methodology in rabbits, rats, mice, hamsters,
and guinea pigs has been, since then, a subject of interest in
our laboratory, as have the effects on these phenomena of
some hormonal and pharmacological manipulations. The
collaborative work with Knut led to some of the results
© 2003 The Scandinavian Psychological Associations. Published by Blackwell Publishing Ltd., 9600 Garsington Road, Oxford OX4 2DQ, UK
and 350 Main Street, Malden, MA 02148, USA. ISSN 0036-5564.
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Scand J Psychol 44 (2003)
presented here, as well as a close academic and personal
relationship with him.
METHODS FOR THE ANALYSIS OF THE MOTOR
AND GENITAL COMPONENTS OF MALE SEXUAL
BEHAVIOR
Several techniques, including high-speed cinematography
(Bermant, 1965; Stone & Ferguson, 1940) and videotape
recording (Sachs & Barfield, 1976) with subsequent analysis
in slow motion, have been used to quantitatively describe the
motor components of male sexual behavior. However, these
techniques do not give quantitative information on the
dynamic aspects of copulatory movements, such as their
vigor and rhythmicity.
The “accelerometric” technique developed at our laboratory has allowed the precise quantification of some motor
aspects of copulatory activity, initially of rabbits (Contreras
& Beyer, 1979; Beyer et al., 1980; Soto et al., 1984) and rats
(Beyer et al., 1981, 1982; Moralí et al., 1983, 1985), and then
of mice (Wang et al., 1989), hamsters (Arteaga & Moralí,
1997), and guinea pigs (Moralí, González-Vidal & Cervantes,
submitted). With this technique, the accurate measurement
of several parameters of the male copulatory motor pattern
is achieved by placing an acceleration transducer on the
male’s back. In relation to the rapid acceleration changes
that occur during the forward–backward displacements of
the male’s pelvis at copulation, the accelerometer generates
electrical signals that can be easily recorded on a polygraph
or on an oscilloscope (Fig. 1).
The accelerometric technique allows the precise measurement of the following parameters: (1) duration of individual
pelvic thrusts; (2) frequency of pelvic thrusting, that is,
number of pelvic thrusts per second; (3) acceleration or
vigor of pelvic movements, represented by the amplitude
of the signals; and (4) duration of mounting trains, that is,
the uninterrupted series of pelvic thrusts. When combined
with power spectrum analysis, this technique also gives
information on the periodicity or rhythmicity of pelvic
thrusting (Fig. 2). As can be seen, for a selected range of
frequencies (1–50 Hz), a spectrum is provided whose peak
and dispersion give an estimation of the periodicity of the
thrusting train.
Pierce and Nuttall (1961) and Rubin and Azrin (1967)
designed an electric circuit that can be connected by subcutaneous electrodes to a male and a female individual,
and that is closed when moist contact between the penis
and the vagina of the copulating pair is established. The use
of this device permitted these authors to determine the
precise duration of penile insertions during intromission
and ejaculatory patterns in the rat (Pierce & Nuttall, 1961;
Carlsson & Larsson, 1962) and in the rabbit (Rubin &
Azrin, 1967). This technique, however, does not provide
information about the copulatory motor activities themselves. Furthermore, this technique alone does not allow
© 2003 The Scandinavian Psychological Associations.
Fig. 1. Polygraphic records of three consecutive copulations of a
male rabbit. Upper tracings, marked T, time signal and marks
introduced by an observer when intromission occurred. Middle
tracings, marked PM, frequency and characteristics of pelvic
movements recorded with an accelerometer. Each intromission
was preceded by a variable period of mounting. Lower tracings,
marked SVP, seminal vesicle pressure. Note that pelvic thrusting is
not associated with SVP changes, and that 100 –300 ms after the
onset of intromission a gradual contraction of the seminal vesicles
occurred, which outlasted copulation. Modified from Contreras &
Beyer (1979).
a determination of the moment during insertion when
ejaculation occurs.
By using a similar device, it has been possible to combine
the accelerometric technique with the detection of genital
contacts during the copulatory activity displayed by rats
(Moralí et al., 1983; Moralí & Beyer, 1992), hamsters
(Arteaga & Moralí, 1997), and guinea pigs (Moralí,
González-Vidal & Cervantes, submitted). This methodology
has allowed us to obtain information on the precise timing
of penile insertion in relation to pelvic thrusting, to determine the occurrence of fast pelvic thrusting during penile
insertion at ejaculation, and, by showing the moment during
insertion when ejaculation occurs, to ascertain the exact
duration of the period of penile stimulation required for the
male to ejaculate (Figs. 3, 4, and 5).
There is little information on the complex interactions
between the copulatory movements and the activity of the
genital organs, controlled by autonomic organs, Information
about the activity of the internal genitalia, essential for a
precise timing of the occurrence and duration of seminal
emission and ejaculation, has been obtained by chronically
implanting a catheter in the seminal vesicles. Changes in
tone or contractions of these organs result in alterations in
pressure that are continuously recorded with a polygraph
linked to a pressure transducer (Figs. 1 and 3).
Therefore, through the use of the polygraphic methodology
described above, and in combination with observational
techniques, it is possible not only to quantitatively analyze
the male copulatory motor pattern and to recognize subtle
differences between and within species, but also to relate
these events to penile–vaginal interactions and reflex
responses of the male ejaculatory apparatus.
Scand J Psychol 44 (2003)
Male copulatory motor pattern in mammals 281
Fig. 2. Accelerometic records and frequency spectrum analysis graphs of pelvic movements in typical mounts displayed by intact male and
female rabbits, and by ovariectomized (ovx) rabbits receiving estradiol benzoate (EB) or testosterone propionate (TP) treatments. Upper
graphs: signals generated by the accelerometer during mounting. Lower graphs: frequency analysis (range 1–50 Hz) of the signals generated
during an 8 s period in which mounting occurred. The frequency analysis of the male mount shows a single component with a peak frequency
value (F) of 14.875 Hz, as indicated by the cursor. In contrast, that of the female mount does not show a dominant frequency but a series
of ill defined components. Note sharp, single components in the spectra of the EB-treated, ovariectomized rabbits, showing also high
thrusting frequencies (F = 21.00 and 17.625 Hz). Frequency analysis of TP-stimulated mounts reveals two patterns: one with a well defined
component (F = 12.500), and the other showing both a dominant frequency (F = 16.00) and an ill defined band of higher frequencies.
Modified from Soto et al. (1984).
DESCRIPTION OF THE MASCULINE COPULATORY
MOTOR PATTERN OF THE RABBIT, RAT,
HAMSTER, GUINEA PIG, AND MOUSE
Rabbit
Copulation in rabbits involves the display of mounts with
rhythmic, vigorous pelvic thrusts performed for several
seconds against the female’s rump, and that may or may not
culminate in intravaginal penile insertion. At intromission,
ejaculation invariably occurs. The dynamic characteristics
of pelvic thrusting have been described in detail by using
the accelerometric technique (Contreras & Beyer, 1979). The
pelvic thrusts (around 14 per second) are highly regular and
© 2003 The Scandinavian Psychological Associations.
periodic until penile insertion occurs, at which moment
they are interrupted (Fig. 1). Effective mounts, that is,
those culminating in intromission and ejaculation, tend to
be shorter than ineffective mounts and are more regular and
rhythmic than ineffective ones, as evidenced by frequency
spectrum analysis. Similarly, effective mounts show slightly
but significantly higher thrusting frequencies than ineffective
ones (Table 1). This suggests that the adequate stimulus
for inducing the lordosis posture in the doe is a rhythmic,
“high”-frequency pelvic thrusting.
During pelvic thrusting, no changes in the seminal vesicle
pressure are observed in rabbits. However, shortly after
penile insertion, a slow rise in pressure, outlasting copulation,
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Scand J Psychol 44 (2003)
Fig. 4. Polygraphic records of signals generated by the accelerometer
in relation to the pelvic thrusting (PT) movements of male hamsters
during mount, intromission, ejaculation, and long intromission
behavioral patterns, and by the intromission detection circuit indicating the occurrence and duration of genital contacts (GC) associated
with penile insertion during these responses. Penile insertion is related
to intromission responses, to the interruption of rhythmic pelvic
thrusting and, at ejaculations, to a defined period of intravaginal
thrusting. Long intromissions are characterized by prolonged periods
of penile insertion during which the male displays slow intravaginal
thrusting (2 thrusts/s). Modified from Arteaga & Moralí (1997).
Fig. 3. Polygraphic recordings of changes in seminal vesicle pressure
(SVP), of the signals generated by the accelerometer in relation to
pelvic thrusting movements (PT) of intact male rats during a typical
mount, intromission, and ejaculation behavioral patterns, and of the
occurrence and duration of the genital contacts (GC) during these
behavioral responses. Note the fusiform organization of the accelerometric record of the mounting train and the similar appearance of
the intromission and ejaculation thrusting trains until penile insertion is achieved, as indicated by the dotted line and by the plateau
signal generated by the intromission detection circuit and recorded
in the polygraph. Note a small broad rise in SVP during the mount
and the sharp increase coinciding with penile insertion. Two types
of ejaculatory patterns can be recognized: short and long ejaculations, differing in the duration and dynamic organization of the
pelvic thrusting train, and in the duration of the period of intravaginal thrusting that precedes ejaculation, and that is revealed by this
combined methodology. Note that the SVP rises during penile insertion and intravaginal thrusting in ejaculation responses, culminating
in a further steep rise associated with seminal emission and remaining above the baseline for several seconds. Modified from Beyer et
al. (1982) and Moralí & Beyer (1992).
is recorded and ejaculation occurs with a brief latency after
intromission is achieved (Contreras & Beyer, 1979) (Fig. 1).
Male-like sexual behavior (pseudomale behavior) may be
shown by female rabbits that includes all the components of
© 2003 The Scandinavian Psychological Associations.
male copulation, and appears to be similar to it. However,
using the accelerometric technique, clear sexual differences
in the vigor, frequency, and periodicity of pelvic thrusting
displayed by mounting male and female rabbits have been
found (Soto et al., 1984) (Fig. 2). Frequency spectrum analysis
of the signals generated by the accelerometer typically
show a single peak around the 14 Hz band, indicating the
rhythmical nature of thrusting in the male rabbit. Mounts
displayed by females are shorter than those of males, comprise
isolated pelvic movements of variable duration, and generate
weak, irregular signals of lower amplitude. As shown in
Fig. 2, frequency spectrum analysis of female pelvic thrusting
failed to reveal clear rhythmicity in female mounts.
In spite of this sexual difference, female rabbits seem to
possess the neural circuitry involved in the display of
rhythmic thrusting, but lack the adequate hormonal levels
to activate this system. Thus, as shown in Fig. 2, testosterone
propionate (TP)-treated female rabbits not only display
vigorous pelvic thrusting in many cases but also clear
rhythmicity, with frequencies similar to those of intact males.
Most interestingly, estrogen treatment (estradiol benzoate,
EB) induced highly synchronic thrusting activity, of higher
frequency than that of normal or androgen-treated males.
Laboratory rat
The polygraphic analysis of mounts, intromission responses,
and ejaculation responses of male rats reveals their fine
Scand J Psychol 44 (2003)
Male copulatory motor pattern in mammals 283
Fig. 5. Polygraphic records of the signals generated by the accelerometer in relation to the pelvic thrusting movements performed during
typical mount, intromission, and ejaculation behavioral responses of a guinea pig and an ejaculation response of a mouse, and the plateau
signals generated by the intromission detection circuit when genital contact occurred in relation to penile insertion during the behavioral
responses of the guinea pig. In both species, fast pelvic thrusting occurring before penile insertion shifts during insertion to a slow pattern
that lasts for 20 s or more in the mouse, and that is followed by a characteristic period of intravaginal fast thrusting associated with
ejaculation.
dynamic organization. As can be seen from the accelerometric tracings of Fig. 3, mounts consist of a series of 6–12
pelvic thrusts of similar duration but variable amplitude,
giving the mounting train a fusiform appearance (Beyer
et al., 1981). When using the intromission detection circuit,
it can be seen that, at mounts, no (or only occasional) brief
contacts occur between the penis and the vagina of the
receptive female (Moralí et al., 1983; Moralí & Beyer, 1992).
When seminal vesicle pressure (SVP) recordings are also
made, a slight rise in SVP may be found at mounts as a
single wave in association with pelvic thrusting (Beyer et al.,
1982).
Pelvic thrusting trains at intromission consist of only 4–7
rhythmic thrusts, being shorter than at mounts (Fig. 3;
Table 1). They are indistinguishable from mounts in their
initial part, but differ in showing a final period of irregular
broad signals coinciding with penile insertion and withdrawal.
Penile insertion results in the interruption of thrusting and
lasts for 410 ± 150 ms (mean ± SD) (Moralí et al., 1983;
Moralí & Beyer, 1992). As in mounts, a slight rise in SVP
occurs during pelvic thrusting at the intromission responses,
© 2003 The Scandinavian Psychological Associations.
but it is followed by a steep rise coinciding with penile
insertion (Beyer et al., 1982).
Pelvic thrusting trains at the ejaculatory behavioral
responses are longer than those of mounts and intromissions. The accelerometric technique has shown that rats may
exhibit two types of ejaculatory response, consisting of either
a short (0.68 s as an average) or a long (1.04 s as an average)
pelvic thrusting train (Beyer et al., 1981, 1982; Moralí &
Beyer, 1992) (Fig. 3). A period of intravaginal pelvic thrusting, which had not been previously demonstrated by other
techniques, occurs at both ejaculatory responses. Besides
their duration, long ejaculations differ from short ones in
showing clearly identifiable extra- and intravaginal thrusting
trains; in short ejaculations, extra- and intravaginal thrusting proceeds in immediate succession as a single phase of
gradually increasing amplitude, which culminates in a series
of irregular pelvic movements associated with seminal
emission and ejaculation.
As in the case of mounting and intromission responses, a
smooth, gradual rise in SVP occurs during the initiation of
mounting in the ejaculatory responses, followed by a change
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Scand J Psychol 44 (2003)
Table 1. Characteristics of the motor and genital copulatory responses displayed by male rabbits, rats, hamsters, mice, and guinea pigs
(means ± SD).
Extravaginal train
Species
Mount
Duration of the pelvic thrusting train (s)
Rabbit1
3.08 ± 2.16
Rat2
0.38 ± 0.08b
Hamster3
Mouse4
Guinea pig5
1.28 ± 0.15c
0.60 to 3.60
1.18 ± 0.07b
Frequency of pelvic thrusting (movements/s)
Rabbit1
12.08 ± 0.98a
Rat2
20.95 ± 0.82
Hamster3
14.78 ± 0.28a
Mouse4
Guinea pig5
11.27 ± 0.18a
Intravaginal train
Intromission/ejaculation*
Slow
Fast
2.61 ± 1.50
0.31 ± 0.07a Short: 0.37 ± 0.09ab
Long: 0.50 ± 0.06c
0.87 ± 0.10b
–
–
–
6 to 25
up to 40
1.44 ± 0.12
–
0.31
0.54
0.45
2.00
1.14
–
–
2.31
2.24
1.50
–
22.00
16.40 ± 0.41b
22.00
12.34 ± 0.23b
0.54 ± 0.05a
13.54
21.22
15.20
22.00
11.88
± 1.11b
± 1.33
± 0.30ab
to 25.00
± 0.25ab
± 0.07a
± 0.06c
± 0.03a
± 0.05b
Ejaculation responses
Intromission responses
Duration of the genital contact (s)
Rabbit1
0.72 ± 0.27
Rat6
0.41 ± 0.15a
Hamster4
Guinea pig5
2.21 ± 0.16b
1.44 ± 0.12ª
To ejaculation
Total contact
0.72 ± 0.27
Short: 0.306 ± 0.08a
Long: 0.542 ± 0.08b
1.25 ± 0.08a
2.54 ± 0.08b
0.72 ± 0.27
1.136 ± 0.22c
1.452 ± 0.33d
3.15 ± 0.10b
3.82 ± 0.34c
* When no differences between intromission and ejaculation exist, data are pooled together.
Data taken from: 1Contreras & Beyer (1979); 2Beyer et al. (1981, 1982); 3Arteaga & Moralí (1997); 4Wang et al. (1989) and Moralí & Sachs
(unpublished results); 5Moralí, González-Vidal & Cervantes (submitted); 6Moralí & Beyer (1992) and Moralí, Contreras & Beyer
(unpublished results).
Different letters indicate significant differences ( p < 0.05) within a row, Tukey tests.
in the slope of the curve associated with penile insertion.
After a variable period of intravaginal thrusting, a further
steep rise in SVP takes place and then declines just prior
to the end of the ejaculatory pattern. This last phasic rise
associated with seminal emission lasts approximately 100 ms.
Thereafter, the SVP falls to its previous level, and remains
above the baseline for several seconds (Fig. 3).
From these recordings, it can be seen that a more abrupt
rise in SVP occurs during short than during long ejaculation
responses, leading to ejaculation after a significantly shorter
period of genital contact (0.31 vs. 0.54 s) (Table 1) (Moralí,
Contreras & Beyer, manuscript in preparation). The factors
determining the occurrence of short and long ejaculatory
patterns have not been elucidated yet. However, hormonal
conditions, age, motivational state, and phenomena related
to the sequential display of successive ejaculatory series seem
to contribute to the occurrence of a short vs. a long ejaculatory
pattern (Moralí, Contreras & Beyer, unpublished results).
Female rats display mount and intromission patterns
similar to those of males (Beach, 1942). Moreover, under
particular experimental circumstances, such as neonatal
androgenization, painful stimulation, or pharmacological
© 2003 The Scandinavian Psychological Associations.
treatments (see Baum, 1979; Meisel & Sachs, 1994, for
reviews), female rats can display the ejaculatory pattern.
Great similarity has been found in the temporal organization
and even in the vigor of the mounting and intromission
motor patterns of female and male rats (Moralí et al., 1985).
Power spectrum analysis of the frequency of the signals
generated during mount and intromission responses of
intact female and male rats showed similar values (19 –22 Hz)
in both sexes, as well as similar rhythmicity and periodicity.
The only significant difference between the sexes was a
longer duration of females’ mounts. The ejaculatory
responses of neonatally androgenized females recorded at
adulthood, without any additional hormonal treatment,
were similar to the long ejaculatory pattern of male rats, with
the only difference being that the phase of low-amplitude
thrusts, which normally coincides with penile insertion in
males, was not evident in females.
Golden hamster
Male golden hamsters display four types of copulatory behavioral responses: mounts, intromissions, ejaculations, and
Scand J Psychol 44 (2003)
long intromissions. These responses have been described in
detail by Bunnell et al. (1976), who also assessed the duration of the genital contacts at these responses. The polygraphic analysis of the copulatory responses of the male
golden hamster has provided a precise quantitative estimation of the duration of the thrusting trains, and of the
frequency, rhythmicity and vigor of pelvic thrusting, in temporal correlation with the genital contacts at copulation
(Arteaga & Moralí, 1997) (Fig. 4).
Series of rhythmic, synchronic extravaginal pelvic thrusts,
with frequencies between 14 and 15.5 thrusts/s are displayed
during the four types of copulatory responses. As in rabbits,
mounting trains are usually longer and have lower thrusting
frequencies than those at intromission and ejaculation
responses. The thrusting train at the intromission behavioral
responses ends when penile insertion occurs. Penile insertion
at the intromission responses lasts for 2.2 s, on average. The
pelvic thrusting train at ejaculation ceases in relation to
penile insertion; however, after a brief period (50 –100 ms), a
short train of intravaginal pelvic thrusting, generating signals
of lower amplitude, and higher frequency (16 thrusts/s) than
those of the extravaginal train, is resumed. This intravaginal
fast thrusting train, like that of rats, seems to be associated
with ejaculation. The duration of this period (0.45 s), varies
very little both between and within the individuals, and
occurs at a precise moment after the onset of penile insertion.
After several ejaculatory series, when a male is approaching
sexual satiety, it presents “long” intromissions, in which
penile insertion is held for 30 s or more, while displaying
intravaginal pelvic thrusting of slow frequency (around
2 thrusts/s) (Bunnell et al., 1976). Extravaginal fast pelvic
thrusting trains at long intromissions are of similar frequency, rhythmicity and vigor as those of intromissions and
ejaculations, and, in response to penile insertion, shifts to
slow intravaginal thrusting generating low-amplitude signals
(Arteaga & Moralí, 1997) (Fig. 4).
Guinea pig
The copulatory activity of the male guinea pig may involve
a variable number of mounts and intromission responses
preceding ejaculation, but, as with rabbits, guinea pigs are
capable of ejaculating on a single insertion on at least some
occasions (Dewsbury, 1979). The polygraphic analysis of
copulation of adult, Hartley albino male guinea pigs has
shown that, before penile insertion, intact animals show
rhythmic pelvic thrusting trains with frequencies of 11–12
thrusts/s (Moralí, González-Vidal and Cervantes, submitted).
If penile insertion is achieved, fast pelvic thrusting ceases
and shifts to a slow pattern of about 1.5 thrusts may last
for several seconds. At the ejaculation behavioral responses,
after a brief period of slow thrusting performed during
insertion, a well defined train of intravaginal fast pelvic
thrusting, generating signals of lower amplitude and slightly
higher frequency than those of preinsertion thrusting,
© 2003 The Scandinavian Psychological Associations.
Male copulatory motor pattern in mammals 285
occurs (Fig. 5) (Table 1). This fast pelvic thrusting period is
associated with ejaculation and usually occurs at a precise
moment (2.5 s) after the onset of penile insertion. Thus, as
in hamsters and rats, the tactile stimulus required to trigger
ejaculation is a period of intravaginal fast thrusting, whose
characteristics seem to be highly predictable in this species.
On the other hand, the duration of the genital contact after
ejaculation may vary widely.
As in rabbits and hamsters, effective mounts of guinea
pigs (those in which males achieve intromission) are shorter
than ineffective mounts and show higher thrusting frequencies than ineffective ones.
House mouse
Similar to rats and hamsters, house mice display several
mounts and intromissions preceding ejaculation and, depending on the strain, may show multiple ejaculations or only
one on a mating session (for a review, see Dewsbury, 1979).
Data on their motor copulatory pattern have been
obtained from a small number of animals (Wang et al., 1989).
Although these data do not yet allow a complete quantitative description of all parameters, clear accelerometric images
have been obtained and pelvic thrusting frequencies have
been determined (Fig. 5, Table 1). Mounts, characterized
by the display of fast (22–25 thrusts/s) rhythmic thrusting
trains, may last for variable periods. If the male achieves
penile insertion, fast thrusting shifts to a slow thrusting
pattern (2/s), as occurs in guinea pigs and in long intromissions of hamsters. If the mouse loses penile insertion, fast
thrusting is resumed until insertion is achieved again. After
variable periods of intravaginal thrusting, lasting for several
seconds, males may either dismount or proceed to ejaculation.
At ejaculation responses, as in guinea pigs, after a preinsertion fast thrusting train and an intravaginal slow thrusting
period, which in mice may last for 20 s or more, a remarkably
characteristic, new period of fast thrusting of similar
frequency to that of extravaginal thrusting, and presumably
associated with ejaculation, is recorded (Fig. 5). After this
period, the male may remain inserted for several seconds
and then withdraw (Moralí & Sachs, unpublished results).
Interspecies comparisons
Comparisons of the motor copulatory patterns of the species
studied show that the duration and frequency of pelvic
thrusting trains, as well as their dynamic organization, are
particular to each species. In rabbits, ejaculation occurs
with a brief latency after the onset of intromission, in the
absence of pelvic thrusting. In the other species, whether
they display intravaginal slow thrusting (guinea pigs and
mice) or not (rats and hamsters), a remarkable period of
intravaginal fast pelvic thrusting, of similar frequency to
that of extravaginal thrusting, is performed in association
with ejaculation.
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EFFECT OF HORMONES ON THE MASCULINE
COPULATORY MOTOR PATTERN OF THE RABBIT,
RAT, HAMSTER AND GUINEA PIG
Rabbit
Rabbits may display mounts for several months after
castration, but only a low proportion of these mounts
culminate in ejaculation, mainly because of a decrease in
the effectiveness of the mount to stimulate lordosis in the
female (Stone, 1932; Beyer et al., 1980). The accelerometric
recordings of these mounts show periods of low-amplitude
movements, and interthrust intervals that cause a decrease
in thrusting frequency and break the rhythm of thrusting
(Beyer et al., 1980) (Fig. 6). At a later stage in the disorganization of the mounting pattern, pelvic thrusts disappear
and the mounts are characterized by weak, tremor-like
pelvic movements. These mounts never stimulate lordosis
in the receptive females. The changes in the copulatory
motor pattern following castration occurred earlier than
the decrease in the number of mounts per test, suggesting
that the motor parameters (thrusting frequency and
rhythm) are more sensitive to androgen deprivation than
is sexual motivation. Androgen administration (TP, 10 mg/
day for 15 days) to castrated rabbits gradually restores the
incidence of mounting but not of intromission to the
intact level. Androgen administration also significantly
increases thrusting frequency and strength of pelvic movements until the normal morphology of pelvic thrusting is
restored.
Fig. 6. Accelerometric records of the first four mounts performed
by a New Zealand white rabbit when intact (I), and mounts performed
30 days after castration (C), and 15 days after the initiation of
testosterone propionate administration (T). Upper tracings in each
record carry a time mark (1 s) and a signal operated by the observer
when detecting intromission. Note the rhythmic characteristics of
mounts of intact rabbits, of which the first three ended in intromission. Castration resulted in a diminution of the amplitude of the
signals and in the appearance of trains of small signals interspersed
with periods of more vigorous activity. Note that testosterone
administration tended to restore the normal mounting pattern in at
least some mounts (C and D). From Beyer et al. (1980), reproduced
with permission.
© 2003 The Scandinavian Psychological Associations.
Scand J Psychol 44 (2003)
Rat
Neither the thrusting frequency nor the amplitude or
rhythmicity of pelvic thrusts in the copulatory responses
shown by male rats several weeks after surgery are significantly
affected by castration (Beyer et al., 1981). Only the duration
of the mounting trains is somewhat longer in castrated rats
(mean ± SD: 536 ± 20 ms vs. 380 ± 80 ms in intact rats). This
effect may result from some failure either of penile erection
or in the orientation of the penis to the vaginal region, as
has also been described in cats, along with a lengthening of
mounts, after castration (Rosenblatt & Aronson, 1958).
Administration of TP reestablishes full copulatory behavior in castrated rats, with identical motor characteristics
to those of intact rats (Beyer et al., 1981). Administration
of estrogen (EB) to castrated rats stimulates the display of
mount and intromission responses, and only few ejaculations
corresponding to the short pattern (Fig. 7). Pelvic thrusting
was highly rhythmic in estrogen-treated male rats, and they
showed intromission responses with thrusting frequencies
significantly higher (mean ± SD: 22.15 ± 1.29 thrusts/s) than
those of intact rats (19.43 ± 1.57 thrusts/s).
The lack of effects of castration on the frequency, rhythmicity and vigor of pelvic thrusting in male rats could indicate that sex steroids do not modulate the copulatory motor
pattern of the rat. However, it is also possible that rats stop
copulating before alterations in the copulatory motor pattern
are detected. The hormonal stimulation of the initiation of
male sexual behavior in the rat seems to be exerted in the
medial preoptic area (mPOA) and anterior hypothalamic
area. The copulatory motor pattern of long-term castrated
rats whose sexual behavior was restored by local implants of
TP in the mPOA was compared with that shown by the
same individuals when intact (Moralí et al., 1985). The
vigor, frequency, and temporal organization of the copulatory pelvic thrusting shown by the castrated, TP-implanted
rats were similar to those previously shown by the rats when
intact, even when, as revealed by the atrophy of the sexual
accessory organs, it could be inferred that the spinal circuits
involved in pelvic thrusting were also not exposed to
significant amounts of circulating androgen. These data may
indicate that the spinal neurons related to the organization
of pelvic thrusting during copulation in the male rat can
function without direct androgenic stimulation.
To investigate the possible role of early postnatal androgen
in the organization of the copulatory motor pattern of the
rat, the copulatory responses of neonatally androgenized
female rats and neonatally castrated male rats were compared
with those of control male and female rats before and after
treatment with either TP or EB (Moralí et al., 1985). The
copulatory motor patterns of control and neonatally androgenized females, as well as those of neonatally castrated
male rats, were indistinguishable from those of control
male rats (Moralí et al., 1985) (Fig. 7). Neonatally castrated
rats treated with EB when adults never showed ejaculatory
Scand J Psychol 44 (2003)
Male copulatory motor pattern in mammals 287
Fig. 7. Accelerometric records of mount (M), intromission (I), and ejaculation (E) behavioral responses performed by control (postpubertally
castrated), and neonatally castrated male rats, and by ovariectomized control, and neonatally androgenized female rats, under estradiol
benzoate (EB) or testosterone propionate (TP) treatment. Note the similarity in the organization, frequency, and rhythmicity of pelvic
thrusting trains in the four groups of rats under either treatment. Neonatally castrated rats did not ejaculate when treated with EB. Those
receiving TP generally performed the short ejaculatory pattern, with similar characteristics to those of short ejaculations of control males.
Control females did not show ejaculatory behavior. Neonatally androgenized females under EB treatment often showed the short-type
ejaculatory pattern, whereas those under TP treatment generally displayed the long ejaculatory pattern. Modified from Moralí et al. (1985).
behavior. Surprisingly, EB, which in castrated males significantly increased thrusting frequencies, lacked this effect in
ovariectomized rats.
The results show that the organization of the movements
involved in the masculine sexual behavior in rats are identical in both sexes, thus suggesting that the neural circuits
controlling these behaviors could be identical. Neonatal or
postpubertal androgen in the rat influences the incidence
of male-like responses, but not their temporal or dynamic
characteristics.
Golden hamster and guinea pig
Castration of hamsters results in a progressive decline of
their sexual activity, so that, two weeks after castration,
ejaculatory responses are no longer displayed and the
duration of penile insertions is significantly reduced
(Arteaga & Moralí, unpublished results). As described for
rat mounting trains, the duration of the extravaginal pelvic
thrusting trains of castrated hamsters was longer in all
copulatory responses in relation to that of intact animals.
The frequency, vigor and periodicity of thrusting were
similar to those of intact subjects. Treatment of castrated
© 2003 The Scandinavian Psychological Associations.
animals with androgen gradually restored the occurrence of
copulatory responses, with similar motor characteristics to
those of intact animals. These data suggest that the neural
mechanisms responsible for the presentation of the copulatory behaviors (motivation), as well as those involved in the
execution of genital responses during both intromissive and
ejaculatory behaviors, require androgen for their functioning.
In contrast, the neuronal circuits involved in the expression
of the motor components of the copulatory behavior of the
hamster seem to be independent of androgen action.
Similarly, mounts recorded in guinea pigs up to nine
weeks after castration show pelvic thrusting trains with
similar frequency and rhythmicity to those of intact animals
(Moralí, González-Vidal & Cervantes, submitted). Under
TP treatment, castrated guinea pigs showed copulatory
responses with similar motor and genital characteristics to
those of intact animals.
Interspecies comparisons
Overall, the effects of castration and hormone treatment
upon the masculine copulatory motor pattern differs between
rabbits, rats, hamsters, and guinea pigs. Two mechanisms
288
G. Moralí et al.
have been proposed to explain the hormonal regulation of
the male motor copulatory pattern: first, a single-site model
in which androgen sensitizes supraspinal command neurons
that trigger copulation and, in turn, act on spinal, hormoneindependent neural nets related to the patterning of temporal
characteristics of copulatory movements; and second, a
multiple-site model in which androgen or its metabolites,
besides acting on command neurons, also regulate the
activity of the patterning neural nets and spinal motor neurons
themselves, in the final expression of copulatory movements
(Beyer & González-Mariscal, 1991). From the results
described above, rats, hamsters, and guinea pigs seem to
belong to the single-site model, so that once the command
neurons are stimulated by androgen, the copulatory movements appear with the characteristics typical of the species.
On the other hand, rabbits seem to correspond to the
multiple-site model, androgen both acting on motivation
and regulating the activity of the neural systems related to
the motor aspects of copulation.
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