As is the case in most animals, butterfly sex is determined when the egg is fertilized. Interestingly, in butterflies, it is the female gamete (the egg) that determines the sex of the offspring. In mammals, females have 2 "X" chromosomes and males have an "X" and a "Y" chromosome. If an egg is fertilized by a sperm carrying an X chromosome, it becomes a female (XX). If it is fertilized by a sperm carrying a Y chromosome, it becomes a male (XY). In butterflies (and birds), males are ZZ and females are ZW, so some eggs carry a W chromosome, and some a Z chromosome. There is no important difference between Y and W (and X and Z) chromosomes; scientists just use different letters to help differentiate species in which males and females determine the sex. Temperature and other environmental cues do not influence sex, and individual females produce both male and female offspring. There are some animals for which environmental cues determine sex (some reptiles, for example), and some that actually change sex during their development (some fish). But butterflies (including monarchs) and mammals are not in either of these groups.