Two species of monkeys are captured in the Amazonian forest and placed in the zoo. The zookeeper feels safe in putting the two species together in the same enclosure, because they are not known to interbreed in the wild. But soon they find that the monkeys are interbreeding and producing hybrid offspring. The zookeeper checks his records and discovers that although these two species live in the same jungle, one species is only active at night while the other is only active during the day. Which intrinsic isolating mechanism has been keeping these two species from interbreeding?