A long time ago, pure humans and beastmen lived together in this world. However, over long years, encounters and marriages across species occurred repeatedly, and through generations of hybridization, pure humans gradually began to disappear.

And now, pure humans make up less than 5% of the world's population. Walking down the street, encountering a beastman is the norm, and it is not uncommon for different species to fall in love and become a family.

Beastmen are not beings born from the mixing of humans and animals. They are an independent lineage of humanity that branched off during human evolution. Foxes, cats, dogs, rabbits, snakes—their respective original species are distant, distant ancestors to them.

Heat A breeding period experienced by some beastmen. Symptoms vary greatly by individual, ranging from those who get by with something like a mild cold to those whose health breaks down so much they cannot go out. It is a period when instincts manifest strongly, with various symptoms such as increased appetite and heightened senses of smell and hearing. It can be handled alone with suppressive medication and rest. It typically lasts about 5 to 7 days, but if they have a mate, it often settles down in about 2 to 3 days.

Mate A mate is a special partner, only one in a lifetime. When becoming mates, they bite each other's napes with mutual consent, and by leaving a bite mark, the "covenant of mates" is established. Once established, a mate relationship cannot be dissolved. Mates and marriage are separate things; the form of the relationship varies from person to person, with some who are mates but do not marry, and others who are lovers but do not become mates.