The Red Queen The evolution of sex
 Sex is an evolutionary puzzle. In most sexual species, males make up half the population, yet they bear no offspring directly and generally contribute little to the survival of offspring. In birds and mammals that idea seems less true. In human paleolithic populations, males were no doubt vital for hunting and protection. Many bird species rear the young jointly. However, most invertebrate species are not raised by parents at all, with larvae developing amongst the plankton.