Post-partum changes in hormones and sexual behaviour in captive groups of marmosets (Callithrix jacchus).

Video-tape recordings were made of behavioural interactions in eight family groups of marmosets for 24-30 days after parturition. Blood samples were collected from females for measurement of plasma luteinizing hormone (LH) and progesterone. A pre-ovulatory LH surge occurred in six females between days 10-18 (mean +/- sem, 13.8 +/- 1.3 days). In these groups, sexual interactions increased in frequency during the post-partum period and males began to initiate mounts on days 1, 2, 2, 6, 8 and 10 respectively. Mount and ejaculation frequencies increased significantly during the peri-ovulatory phase as compared to the pre-ovulatory phase and females were more proceptive and receptive than in the early post-partum period. Peri-ovulatory increases in sexual activity were more pronounced in 3 groups in which females had failed to conceive. In the remaining 3 groups, sexual interactions tended to continue at higher levels into the post-ovulatory phase when the females were pregnant. In captive marmoset groups, copulatory behaviour during the post-partum period is stimulated by changes in non-behavioural cues from the female (increased sexual attractiveness to the male) as well as by changes in her behaviour (proceptivity, receptivity) which vary in importance between individuals.