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Reproduction: key points
- Sexual reproduction is not universal and the favoured hypothesis is that it evolved as a strategy to address the threat of parasites.
- There is a broad range of reproductive strategies across species, which have evolved to maximise fitness for each species and each sex according to the physical, biotic and social environments.
- The investment of males and females in reproduction is very different. Each species has evolved strategies in which an equilibrium of interests between the sexes is reached. This is reflected in patterns of parental investment, mate choice and social structure.
- The mammalian reproductive strategy relies on placentation and lactation to provide nutritional support and immune and physical protection to the offspring.
- The human reproductive strategy includes a delayed puberty and few generally singleton pregnancies with a high parental investment in the offspring.
- The timing of puberty is influenced by developmental influences, and there is a potential disconnection between the age of biological puberty and acceptance as an adult; this is reflected in the problems of adolescence.
- The menopause is a distinctly human characteristic with several potential evolutionary explanations.
- Sexual dimorphism in body shape, behaviour and physiology is reflected in the higher risks of morbidity and shorter life span of human males.
- There is a growing mismatch between the evolved rate of ovarian aging and the sociological trend for women to delay the age of first pregnancy
Risk of death by age and sex in a high-income country
At all ages, the risk of death is higher in men than in women; note in particular the divergence of the curves, disfavouring men, during the reproductive years of 15 to 44 years. In developed countries, two thirds of those over the age of 80 years are female. In part this difference is due to males being more vulnerable to extrinsic causes of death. Men are more likely to die of violence, and in the past from diseases related to smoking or alcohol consumption. The physiological reasons why males are more vulnerable to disease are complex and some non-human primates share this trend, but an evolutionary explanation is possible. Assuming humans evolved with a mildly polygynous mating system, then the female fitness strategy depends on a longer, healthier life, whereas males depend on a period of sexual dominance, which may be relatively transient, to maximise fitness.