The mammal whose testis come into scrotal sacs only during breeding time is

Testes are the organs that produce the germ cells and scrotum is the pouch that contains the testes. The evolution of the scrotal sacs in mammals has been mysterious and there exist many hypotheses to explain the probable function of the scrotum. Primarily they provide a space, that isolate and serves to provide a temperature, lower than the body; essential for the production, maturation, and storage of sperm cells. Bats in this respect are unique and exhibit testicular migration between abdomen and scrotum, while the cauda epididymis is permanently in the scrotal sacs.