Sexual Selection & Investment Why do males usually evolve the ‘sexy’ traits? Why do females usually do the choosing? What do we mean by ‘investment’? Is all this relevant to human behaviour?

Investments and Returns Reproduction requires an investment of resources (energy) It is poor evolutionary sense to waste resources; there must be a ‘return’ in terms of survival and/or reproductive success The greater the amount of resources invested, the harder an organism will try to protect its investment

Anisogamy Sexually dimporphic species are typically anisogamous – they produce gametes (reproductive cells) of different sizes

Anisogamy Sperm – very small & numerous; short lived; rapidly renewed Ovum – very large & energy rich; long lived; limited in number

Anisogamy In anisogamous species, the female invests more energy in reproduction than the male Females adopt a quality strategy (selective) Males adopt a quantity strategy (indiscriminate) Consequently: Females choose from the males on offer Males compete with each other to be chosen

Women & Men Can produce offspring with many women in a short time. Low energy investment in each gamete Does not carry offspring Can produce offspring with one man at a time High energy investment in gamete Carries offspring for 9 months

Women & Men Both women & men are trying to reproduce successfully Because of the physiological differences between them they adopt different strategies So they are looking for different things in a potential reproductive partner Consequently, evolution predicts that they will find different characteristics attractive