The sex ratio is the ratio of males to females in a population. In most sexually reproducing species, the ratio tends to be close to 1:1. Reasons for the tendency are discussed in Fisher's principle. For various reasons however many species deviate from anything like an even sex ratio, either periodically or permanently. Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps such as Polistes fuscatus and Polistes exclamans, bees, ants, and termites.