Selection on maternal and neonate size at birth.

Neonatal size is an important factor in determining the survivorship of infants within the first month of life. Because maternal size has an influence on neonatal size, selection should operate on those characters correlated with birth weight and gestational age, including maternal prepregnancy weight, height, and age. In the present study we use a path-analysis approach to examine the operation of selection on both neonatal and maternal size. We found that neonatal survivorship depends not only on the size of the infant at birth but also on a negative allometric relationship between the size of the neonate and the size of the mother. Thus, although the size of the mother has no direct effect on neonatal mortality, the mothers of surviving infants tend to be smaller relative to the size of their neonate. This may provide a mechanism whereby selection maintains a balance between maternal size and neonate size. Over the past 40 years numerous studies have examined the relationship between birth weight and neonatal mortality in humans. Such studies have provided anthropologists and human biologists with a classic example of selection on a quantitative trait in humans (Cavalli-Sforza and Bodmer 1971) and have aided in determining those conditions under which infants experience problems during their growth and development. Since the study on neonatal mortality by Karn and Penrose (1951), a number of investigators have estimated the amount of selection operating on birth weight using changes in the moments of the birth-weight distribution [see, for example, the studies by Van Valen and Mellin (1967), Promboon et al. (1983), Terrenato (1983), and Rajanikumari and Rao (1984)]. However, unlike the study of Karn and Penrose (1951), these other studies involved only a single trait, birth weight. This is unfortunate, as only limited insight is gained into the phenotypic effects of neonatal mortality because the effects of selection on birthweight correlates are ignored. Furthermore, because selection acts on many characters si1 Department of Anthropology and Center for Human Growth and Development, University of Michigan, Ann Arbor, MI 48109. 2 Visiting Researcher, Department of Anatomy and Anthropology, Sackler School of Medicine, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel. Human Biology, August 1993, v. 65, no. 4, pp. 579-591. Copyright © 1993 Wayne State University Press, Detroit, Michigan 48202