WellCookedFetus said:
do egg laying mammals like the duck bill platypus get periods?
Only the females of "placental" mammal species have them. Marsupials (pouched mammals like kangaroos) and monotremes (egg-laying mammals, the platypus and echidna) don't prepare the uterus for a placenta because the fetus is expelled either into the pouch or within the eggshell and doesn't use a placenta. Therefore the uterus does not fill with blood that needs to be gotten rid of.
The females of all egg-laying vertebrates do not build placentas. Birds, reptiles, amphibians, and fish -- none of them have periods.
As an earlier posting explained, the period occurs in synchronization with the "estrus" (fertility) cycle. It happens several days after the female has ovulated and therefore is capable of becoming pregnant. In almost all mammals, that is the only time during the cycle that the female is physically capable of copulating.
Humans are very unusual, the female can copulate any time, even when she is not fertile. Dolphins also have that trait. Female dogs, horses, elephants, etc. cannot copulate except for the short period of time during which they are fertile. That's the time during which we say that the female is "in heat."
In most species the female, when "in heat," exudes pheromones (they are kind of like smells, except the male usually "smells" them unconsciously) which the males can sense, causing them to understand that the female can copulate and also causing their endocrine system to secrete hormones of their own into their blood stream, making them both capable of copulating and highly interested in doing so. Human and dolphin females don't do that. This makes the study of human sexuality much different from virtually all other mammals, because the desire for mating in both sexes is not psychologically linked with absolute biological precision to a short time span during which pregnancy can occur.
