Adaptation and Natural Selection

George Williams

The birds are another group of animals in which the expected relation between size, or other indications of mortality rate, and reproductive effort might be tested. … The lowest effort seems to be expended by large marine or predatory birds, which have very low adult mortality rates. They frequently skip nesting seasons, typically have very few young, and may show little tendency to replace lost eggs. The eggs are not only few, they are small in proportion to adult size. The mature birds seldom show striking sex dichromatism or elaborate courtship behavior. Small birds and ground living birds, however, frequently show elaborate courtship, lay relatively many and large eggs, usually replace them when they are lost, and … are more likely to be sexually dichromatic. (p. 181)

Comments: The lowest effort of parental care in human societies seems to be expended by the rich people or in rich society, which have very low adult mortality rates. They frequently skip reproductive opportunities, typically have very few young. The children are not only few, the investment on children are small in proportion to total wealth. The mature people seldom show striking sex dichromatism or elaborate courtship behavior. Poor people or people in poor societies, however, frequently show elaborate courtship, have many children, and are more likely to have different roles for different sexes.

The intensity of reproductive effort should increase with age. (p. 182)

Comments: This is also true for human beings.