Effects of hypothalamic lesions upon the sexual and social behaviour of the male common marmoset (Callithrix jacchus).

The sexual and associated behaviour of 10 adult male marmosets was recorded during pair tests with ovariectomized females, before and after bilateral thermal lesions of the hypothalamus. Four sham-lesioned males served as controls. Lesions varied in volume from 1.49 to 3.28 mm3 and extended from the ventromedial hypothalamus to the diagonal band of Broca. Precopulatory behaviours (anticipatory erections, tongue flicking and anogenital investigations of the female) as well as frequencies of mounting, intromission and ejaculation decreased in lesioned males. The greatest suppression of sexual behaviour occurred after lesioning the anterior hypothalamus (AH), beneath the anterior commissure, or at the junction of the AH with the preoptic area (POA). Lesions confined to the POA had less profound behavioural effects. Treating ovariectomized females with estradiol stimulated their proceptivity but had no consistent effects upon the males' behaviour. Lesioned males did not exhibit signs of social withdrawal and frequencies of allogrooming or grooming invitations increased post-operatively. Preliminary studies on intermale aggression indicated that lesions which had the greatest effect upon sexual behaviour also tended to decrease aggressive interactions with other males. Hypothalamic lesions did not affect plasma testosterone levels, except in one male and only one animal showed signs of ill health (weight loss and hypothermia) post-operatively. These results show that damage to the AH or AH-POA junction in male marmosets causes a profound suppression of sexual 'arousal' and copulatory behaviour and that such affects are not due to androgen insufficiency or other, non-specific, side effects of neural damage.