[The annual cycle in the Djungarian Hamster Phodopus sungorus Pallas]

Seasonal variations in several functions were observed in a strain of Phodopus s. sungorus bred and kept in the laboratory at Erling-Andechs (47° 58'N, 11° 11'E) under natural illumination: 1. During their first winter most hamsters changed into a whitish winter coat (Figs. 2, 5, and 14). The change in fur coloration is described (Fig. 1). In most animals molt into the winter coat started in October or November, and was completed in December. Molt into the summer coat started in January or February, and was completed in March or early April. Hamsters kept at outdoor temperatures started molt into winter pelt earlier, and finished molt into summer pelt later, than animals kept indoors (Figs. 3 and 4). Winter coloration was more extreme in animals kept at outdoor temperatures. 2. Molt into the winter coat was induced in summer by exposing hamsters to short photoperiods (Fig. 6). However, these animals spontaneously changed back into summer fur while remaining under short-day conditions. 3. The animals had a marked annual cycle in body weight with maximum weight in July and August, and minimum weight in December and January, while they were in winter pelage (Figs. 7 and 8). 4. Reproduction was observed only between February and November (Fig. 9). Young were born within 18 days (2 cases) or 19 days (27 cases) after the breeding pairs were established. Mean litter size was 5 (range 1-9) (Fig. 10). Average litter size was smaller in the first litter of a ♀ than in the second, but was smaller again in subsequent litters (Fig. 11). 5. Growth curves of young hamsters were compared with data from the literature (Fig. 12). In the mean ♂ ♂ were heavier than ♀ ♀ (Table 1). 6. The majority of ♂ ♂ showed testis involution during the first winter. The weight of winter testes was about 1/9th that of summer testes (Fig. 13). The cauda epididymidis contained no spermatozoa in winter animals, and many in summer animals. 7. Daily torpor was observed in many animals, but only during the winter, and only in those animals that had changed into winter coloration. Body temperature dropped to about 20°C in hamsters kept at 6°C ambient temperature. 8. More than 10% of the animals remained in summer colour during the first winter (Figs. 2, 3, and 14). In these animals, testicular state and body weight corresponded to those of summer animals (Fig. 15), and torpor was not observed. 9. During their second winter, only a small percentage of animals changed into the winter coat, and even in these animals winter coloration was poorly expressed (Fig. 2). Testicular regression was also rare in the second winter, and less marked than in first-year animals (Fig. 16). The two individual testes in an animal often differed considerably in size and activity.-Reproduction almost ceased after the hamsters had lived through their second winter. 10. Phodopus has a midventral sebaceous gland, which is much larger in ♂ ♂ than in ♀ ♀. An annual cycle in its activity was indicated, but was not studied quantitatively. A discussion of the observations is presented which includes aspects of the situation in free-living populations.-The annual cycles of all the individual functions observed in the laboratory (body weight, testis activity, reproduction, colour change, torpidity) were strictly synchronized with each other, which suggests that these functions are regulated by a single complex of coupled factors, or even by one single factor, within the organism. The annual cycle was influenced by photoperiod, but it was not exclusively caused by external factors and their changes; an internal component of the annual cycle has to be potulated.