Circannual prolactin rhythm in intact dogs housed outdoors

Prolactin (PRL) and progesterone (P4) values were collected from eight intact (4 female, 4 male) mixed-breed dogs housed outdoors for a two-year span. A circannual component was significant for PRL for each dog (P less than 0.01) and the rhythm was validated for the population (P less than 0.004). Females had two estrus periods a year demonstrated by a P4 frequency of 23.5 +/- 0.47 wk. A 6-month component was statistically significant for P4 for three dogs and of borderline statistical significance for the fourth dog. Circannual acrophases yielded a statistically significant population rhythm (P = 0.012), whereas the 6-month component was of borderline statistical significance (P = 0.056). No time-macroscopic relationship between PRL cycles and P4 cycles was seen. These findings are consistent with similar PRL rhythms in non-domestic canids and emphasize the importance of considering seasonal effects when interpreting PRL data.