Estrus in bats - some bloody discharge while in the fertile part of their cycle. Only great apes have menstrual cycles (shedding unused uterine lining at the end of a cycle, NOT fertile when discharging blood).

Primaries can have so many candidates the median voter is never going to learn about all of them. A primary is a reasonable way to down-select to a candidate pool where they all have a chance to make their case to voters without being seen as noise.

Partisan primaries tend to produce more extreme candidates. The hope is switching to a combined primary will result in moving candidates of more general appeal on to the general election.