Sexual
behaviour in animal is defined as a series of action exhibited by male rats
when introduced to a receptive female rat. In rats as described by (Carlson,
2001) sexual behaviour is divided into stages:
a)
Mounts
b)
Intromission
c)
Ejaculation.
Figure
2.2: Experimental
Wistar rats showing mounting frequency, (Doty, 1984)
Mount: the animal
assumes the copulatory position on top of the female and grasps her flanks, but
does not insert his penis into the female's vagina.
Intromission: the male
mounts the female and briefly (200-300 milliseconds) inserts his penis. Semen
is not released during intromissions or mounts. After 10 to 12 intromissions
spaced 20-30 seconds apart, the rat ejaculates semen.
It
can be very difficult - even for a trained observer - to distinguish between
these behaviours. There are subtle differences in the style of dismounts that
help distinguish between mounts and intromissions. After an ejaculation, the
male tends to pause longer before dismounting. Every ejaculation is preceded by
a number of mounts and intromissions. Typically rats have multiple ejaculations
before they reach exhaustion (satiation). After each ejaculation there is a
refractory period lasting several minutes.
Sexual
encounters are divided into a number of 'series' depending upon the number of
ejaculations. A series starts with a mount or intromission, and ends with an
ejaculation. Numerous measures of the relationships between mounts,
intromissions and ejaculations are recorded to measure sexual motivation and
performance. For example:
Mount Latency: is the time
that elapses between introducing the male and female to the test apparatus
before the male mounts the female.
Intermount Interval: is defined as the
time between successive mounts and Inter Intromission Interval: is the time
between successive intromissions, while Post ejaculatory Interval: sis the time
elapsing between an ejaculation and next mount or intromission, i.e. the
beginning of the next copulatory series.
Tests
of sexual behavior are either: Time limited: the experiment ends after an
arbitrary number of minutes. Series limited: the test ends after an arbitrary
number of ejaculations.
Satiation
criterion: the test ends after the passage of an arbitrary period of time
without a mount, intromission or ejaculation.