They have both an alpha male and an alpha female. There doesn't seem to be any reason (to us) why a particular villager becomes the alpha, although as laurence says, it is often one of the elders. There is a distinct 'pecking order' that can be seen particularly in the female villagers. The men will first of all look for the alpha female to kiss but if she is nursing a baby they'll go for the next in line; if she's also nursing, they'll try the next, and so on ...