Hamsters have many different colorations of fur, many resulting from different mutations. Some of the colors which have helped hamsters blend into their environment. In addition to different coat colorations, there are differences in marking around the body.
The female may have their first estrus at 35 days, and the duration of the estrous cycle is 4 days. Estrus and mating usually occur in the evenings. The average gestation period is 16 days, with an average litter size of 5-9. Hamsters are born weighing 2-3 grams and are hairless, and have closed eyelids and ears. The young are then weaned at 21 days, cycling resumes after 2-18 days after weaning. This can result in litter intervals occurring every 25 to 40 days.
In females gestation enhances the levels of aggression found, it is found that aggression during pregnancy was higher than during the estrous cycle or pseudopregnancy. Over the last four days of prepartum, females showed a decrease in digging and an increase in nest building. In times where the female is nearing the time of receptivity, they obtain a “prelordotic posture” where the back is arched, the tail is raised and the animal remains immobile for brief periods or time. This posture is also shown by submissive females, in female pairs and males defeated by females.