We studied the role of the prostate gland in determining the level of male sexual drive in the hermaphroditic pond snail Lymnaea stagnalis. Male sexual drive is high after a period of social isolation and decreases after copulation as a male. A positive correlation exists between the level of male sexual drive and the volume of the prostate gland. Like male sexual drive, the volume of the prostate gland increases during a period of social isolation and decreases after copulation as a male. Behavioural experiments demonstrated that animals with a lesion of the nerve that innervates the prostate gland (NP1) have a lower level of male sexual drive after social isolation than control animals. However, lesion of NP1 did not affect the increase in the volume of the prostate gland caused by social isolation. Extracellular recordings from NP1 in a semi-intact preparation show a change in firing pattern during an experimentally induced increase in prostate gland volume. The results indicate that NP1 serves as a nervous pathway for the male sexual drive. We propose a simple motivational model for male sexual behaviour in L. stagnalis in which the volume of the prostate gland sets the level of male sexual drive.

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