how come human females are the only things or beings on this earth that get menstrual cycles?

Answer 1

From Wikipedia:

Menstrual cycles in other mammals

A regular menstrual cycle as described here [in the Wiki article] only occurs in the great apes. Menstrual cycles vary in length from an average of 29 days in orangutans to an average of 37 days in chimpanzees.

Females of other mammalian species have estrus cycles rather than menstrual cycles. Unlike animals with menstrual cycles, females with estrus cycles are only receptive to copulation while they are in "estrus" or "heat." During these times, immenent ovulation is advertised to males with visual and behavioral cues, pheromones, or both. If no fertilisation takes place, the uterus reabsorbs the endometrium: no menstrual bleeding occurs. Significant differences exist between the estrus and the menstrual cycle. Some animals, such as domestic cats and dogs, experience small amounts of bleeding while in heat. This phase of the estrus cycle corresponds most closely to the follicular phase of the menstrual cycle and should not be confused with menstruation.

Answer 2

Actually, most mammals have menstrual cycles. It's referred to as going into heat.

Answer 3

Um, they're not. Who told you that?

Answer 4

They're not.

Answer 5

I don't know but it SUCKS!!!