So what categorizes an animal as a male or a female? The most basic biological definition for each sex comes down to the gametes, or sex cells, that an animal produces. Males produce small, mobile gametes in large quantities (sperm), while females produce smaller numbers of large, stationary gametes that are nutrient-rich (eggs). The exception here, of course, is infertile individuals. However, these are the best universal definitions of “maleness” and “femaleness” that science can come up with.