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Most animals reproduce sexually, nevertheless in a various way. Vertebrates have one of the following three reproductive strategies: ovipary, ovovivipary, or vivipary (Schmidt, 1996). The development of an embryo within an egg but outside the mother’s body is called ovipary. This strategy mostly practiced by reptiles, amphibians and in all aves. Some fish species and reptiles have ovovivipary strategy that allowing embryo to develop in female body until it hatches. Meanwhile, vivipary is the development and sustenance of an embryo within the mother’s body. A period of parental care of the offspring may be exercised after the animal are giving birth. This reproductive strategy practiced by almost all mammals (Schmidt, 1996).

In mammals, there are two types of reproductive cycle which are menstrual cycle and estrous cycle. Menstrual cycle is a reproductive cycle known for primates, apes and human while estrous cycle is a main reproductive cycle of females of other non-primate mammals. For instances, rats, mice and horses. There are also a variety of different estrous forms which are monoestrous, polyestrous and seasonality polyestrous. Monoestrous animals have one estrous cycle per year such as dog, foxes and bear. Seasonality polyestrous categorized the animals which have multiple estrous cycle only during certain periods of the year such as sheep and horses. Animals that cycle throughout the year is categorized into the polyestrous animals such as rats, mouse, cattle and pigs.

Rat (genus Rattus) generally grouped the members of several rodent families that having bodies longer than about 12 cm. Rodents are mammals that are part of the Rodentia order under the Mammalia class. They are known for reproducing rapidly because they cycle throughout the year and breed year- round. Female rodents are polyestric which show spontaneous ovulation and have regular and consecutive estrous cycle (Paccola et al., 2013). Annually, 5- 10 litters are produced by a single female of rat. At approximately 30 days of age, each litter that consists of 5-6 young reach their sexual maturity and are able to reproduce. As they produce a great number of young, they also have high mortality rate. That is because most small mammals weighed under 1 kg are characterized by rapid reproduction and high mortality which means for animals of the same size with higher birth rates must have correspondingly higher death rates (Sibly & Brown, 2009).

Estrous cycle is a physiological change in female individuals that associate with reproduction including rats. It has four stages categorized as proestrus, estrus, metestrus and diestrus (Marc