The effect of rearing under a long or short photoperiod on testis growth, plasma testosterone and prolactin concentrations, and the development of sexual behaviour in rams

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Abstract
Two groups of 6 rams were maintained under constant photoperiodic conditions consisting of short days (8 h light: 16 h dark; Group S) and long days (16 h light; 8 h dark; Group L) from 4-20 mo. of age. Other rams (5) were reared under a photoperiod representative of that occurring-naturally (Group N). Testis size, plasma testosterone [T] and prolactin [PRL] concentrations were monitored weekly and sexual behavior tests were carried out at regular intervals. Over the treatment period Groups S and L did not differ in terms of testis growth or plasma T. Both groups had a phase of testis growth and increased T followed by a decline and the temporal patterns for the 2 groups were equivalent. Sexual behavior was slower to develop in Group L than in Group S, indicating that photoperiod can affect the sexual behavior development irrespective of peripheral plasma T concentrations. Plasma PL levels showed a cyclic variation in Group L and were significantly higher overall than in Group S rams. This, together with a trend towards negative correlations between PRL concentrations and sexual behavior in Group L, indicates that PRL may be involved in the photoperiod effect on sexual behavior. The presence of a cycle of testicular growth and of hormone concentrations in young animals under constant photoperiod tentatively suggests that these cycles are endogenous. The constant photoperiod did affect the animals because the cycles which occurred in Groups S and L were out of phase with those of Group N by about 4 mo.