SEXUAL BEHAVIOUR IN MALE RATS

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.