Photoperiodic Differences during Development in the Dwarf Hamsters Phodopus sungorus and Phodopus campbelli

Abstract

The dwarf hamsters Phodopus sungorus and to a lesser extent Phodopus campbelli are widely used to study pineal physiology and seasonal reproduction because of their marked responses to photoperiod in adulthood. These species are frequently confused, partly because the common names "Siberian hamster" and "Djungarian hamster" are applied to both, often interchangeably. The current study examines whether the effects of photoperiod on reproductive maturation and prolactin secretion differ between these closely related species. Both male and female P. sungorus and P. campbelli were gestated and raised in long days (LD) of 16 hr light: 8 hr dark or in short days (SD) of 8 hr dark: 16 hr light. They were raised in pairs of the same sex after weaning at 21 days of age. Reproductive organ weights and serum prolactin concentrations were studied at 35 and 70 days of age. Whereas reproductive development proceeded rapidly in LD in both male and female P. sungorus, SD photoperiod completely inhibited growth of the gonads during this study. There were also marked differences in serum prolactin concentrations in both sexes of P. sungorus at all ages: levels were approximately 10-fold higher in LD than in SD. In P. sungorus of both sexes, SD also significantly decreased body weight relative to conspecifics raised in LD. In P. campbelli, reproductive maturation in LD occurred even more rapidly than in P. sungorus. Although significant effects of photoperiod were still apparent, the magnitude of its effect was less marked than in P. sungorus, and a substantial degree of reproductive development was observed in SD-reared males and females. There was no significant effect of photoperiod on serum prolactin concentrations at either age in P. campbelli, and the effects on body weight gain were inconsistent. Breeding rates were also compared between the two species. Litter size was significantly larger in P. campbelli and neonatal loss was less; consequently more pups survived to weaning. The interbirth interval was also shorter in P. campbelli than in P. sungorus (mode: 20 vs 24 days). The marked photoperiodic differences in reproductive development and prolactin secretion and the differences in breeding success between P. sungorus and P. campbelli clearly distinguish these species.