Final answer:
The male rat is easily identified by his large testes, which are significantly prominent as compared to the female's reproductive organs.
Step-by-step explanation:
The male rat can be easily identified by his large testes. In comparison to the size of their body, rats have notably large testes which are part of the male reproductive system and are primarily responsible for producing sperm and the male hormone testosterone. This distinctive feature helps distinguish males from females in rodent species, as the testes are typically much more prominent than the female's ovaries. Rats' incisors, tail, and vibrissae (whiskers) do not provide a reliable indicator of gender as they are present in both males and females and do not differ significantly in appearance.