Influence of the opposite sex on photoperiodically induced LH and gonadal cycles in willow tit (Parus montanus).

In early January photosensitive willow tits (Parus montanus) were transferred from natural to long days (20L;4D). The study included three groups of birds: one group consisted of pairs (one male and one female kept together in a cage), a second group consisted of males, and a third group included only females. Birds from the separate groups could neither see nor hear each other. Gonadal and LH cycles were followed over a 2-month period. The reproductive systems of male and female willow tits were activated by the long day exposure. This response was, however, affected by the presence/absence of a mate. Ovarian follicles in females caged together with a male grew faster than did follicles in females kept alone in their cages. Ovarian regression started at the same time in both female groups. Contrary to females, gonads in males caged together with females grew slower and also regressed much earlier than did tests in males kept alone. LH cycles differed only slightly between the two groups of females. In both groups maximum LH levels were reached already after 3 days of long day exposure. Maximum LH level was attained earlier in paired males (Day 3) than in single male (Day 7). Males without female company however maintained high plasma levels of LH for a much longer period of time (about 4-5 weeks) than did paired males. The observed differences are discussed in relation to the ecology of the willow tit.