When we humans feel very tired, we often say that we can fall asleep standing up, but this is just a description. In reality, it is impossible for humans to sleep standing up, but there are many animals in nature that have this such a skill that humans do not have, for example, horses can sleep standing up effortlessly.

The existence of reproductive isolation, so that these situations can not occur, even if there are some individuals of different species sometimes complete the mating, but they generally can not produce offspring, or produce offspring can not continue to reproduce

Sloth monkeys, the world's only poisonous primates