What is Sex?

The term "sex" refers to both the reproductive abilities of a particular organism, and also to the act of initiating reproduction by procreation. Not all organisms reproduce by the sexual behaviour familiar in mammals: many produce asexually (one sex), hermaphroditically (both sexes in a single organism), and other organisms have more than two sexes. Additionally, certain species include otherwise specific human behaviour in their sexual practices. For instance, scorpions dance before mating, and penguins are monogamous reproducers. Modern human-specific sexual practices include the female physically altering her body to attract a mate, in the case of breast augmentation and breast reconstruction, or other various types of surgery.

Humans and most other organisms have two distinct sexes, labeled "male" and "female". The term "male" is given to the sex that provides small, mobile spermatozoa which must seek and impregnate the larger, immobile ova of the "female" sex. In mammals and most other classes, the female is the sex which births and nurses the young. Thus, in unusual or borderline cases, the label "female" is given to the sex that most closely fits the description of either birthing or nursing the offspring. A noteworthy exception to this is the seahorse, in which the large ova are moved from body to body rather than the smaller spermatozoa. Thus, it is said that it is the male seahorse (provider of the spermatozoa) which gives birth.

Sexual behaviour and practice is an area of study independent of the physical structures which define sex. Usually, the male and female members of any particular species behave differently from one another. In complex societies, such as those formed in dog packs, fish schools, and human communities, the male and female members often take on respective responsibilities that are common across different species. For instance, while the female is nursing the offspring, the males of most societies hunt and retrieve food for both the mother and the offspring. Additionally, in most (but not all) societies it is the male who protects the community from predators and invaders. Territorial marking is another male-only trait, as in most species it is the male who actively seeks the female and not vice-versa.