EFFECT OF PHOTOPERIOD ON THE BREEDING-SEASON OF THE MARSUPIAL POSSUM TRICHOSURUS-VULPECULA

The production of young by three groups of eight possums, housed in a natural, a short-day (10 h light:14 h dark) and a long-day (14 h light :10 h dark) photoperiod was monitored for 24 months to determine the r ole of photoperiod on the occurrence and duration of the breeding seas on. Possums were housed in each of the three light regimens on 22 Nove mber. The possum has a gestation duration of 17.5 days. Removal of the newborn young leads to ovulation approximately 9 days later and birth 26 days later. Repeated removal of pouch young was used to determine the duration of the breeding season. Possums in a natural photoperiod showed two breeding seasons for the 2 year observation, from March to October of each year. Over the same 2 year period, there were three br eeding periods in possums in short-day and long-day photoperiod. A com parison of the timing of the first breeding season showed that short-d ay photoperiod brought the breeding season forward by 2 months and lon g-day photoperiod delayed it by 2.5 months. The total number of months in which births occurred was the same for natural and short-day photo period (16 out of 24 months) and less for long-day photoperiod (11 mon ths). Fertility in the three groups was also examined. Within each gro up, there was no change in fertility with consecutive seasons. However , fertility, whether expressed as births per possum per month for each breeding period or births per possum for the total observation period , was higher (P < 0.05) in possums in natural photoperiod (total of 9. 0 +/- 1.2 births per possum) than in short-day (6.5 +/- 1.9 births per possum) or long-day (3.8 +/- 1.2 births per possum) photoperiod. It w as concluded that photoperiod is important in the timing of breeding s eason, in the fertility of breeding possums and, possibly, in the dura tion of breeding seasons, but that it does not determine the occurrenc e of the breeding season.