Animal sex cells

Most species have two gametes, which we term male and female. Gametes have two fundamental jobs: to carry resources to nurture the resulting offspring, and to find gametes of the same species and fuse with them. To be good at both is a challenge, because the two jobs create opposing drives across a size-number trade-off,” explains Matthew Gage from the University of East Anglia, UK. One optimum is to produce gametes that maximise offspring survival, and so evolution makes large, nutritive gametes, which we call eggs produced by females. The other optimum is to produce gametes that maximise fusions, and so evolution makes millions of gametes we call sperm, made by males.