Copulation methods in origami birds. Yellow individuals are female, blue ones are male. If the differences in the forewing and hindwing sizes of the two partners are small, copulation will occur in OBS. The copulation method (a or b) depends on the shapes of the partners’ wings. We assume that if the female’s forewing is smaller than her hindwing, the male attempts to put his head through his partner’s hindwing from behind (a). Alternatively, the male attempts to put the partner’s tail through his forewing from behind if the female’s forewing is larger than her hindwing (b).