Sex is an expensive business, biologically speaking. Finding a suitable mate takes time and energy. Offspring are also a huge investment of resources. But sex does offer a rewarding possibility: children who are fitter than their parents, thanks to new and “better” combinations of genes. Darwin realised that many animal species, therefore, carefully select their mates.
There is an innate biological inequality, however. Eggs are relatively few in number – a large and costly investment – while sperm are small and vastly more abundant. And embryos often need further investment in the body or outside. Since the greater investment tends to fall on females, they are often the more selective sex (while males compete to be chosen).
