Survival: sizing up the sexes.

Among mammals, more males than females die as juveniles, especially when food is scarce. A widely accepted explanation holds the mother responsible, arguing that under certain circumstances she will make a larger investment in her daughter than in her son. Now zoologists at the University of Cambridge in England propose an alternative explanation that does not insist on parental bias. They suggest in the Jan. 10 NATURE that male juveniles, because they grow faster and for a longer period than do females, are more susceptible to food deprivation.

Red deer on an island off Scotland were observed by T. H. Clutton-Brock, S. D. Albon and F. E. Guinness. They report as much as 20 percent greater mortality in male deer than in females during the first two years of life. The scientists also surveyed published reports on the sex ratio among juvenile birds and mamals. They found that the discrepancy between male and female survival is greatest in species in which there is the greatest difference between male and female adult size.

Several lines of evidence support the new theory of differential survival, say Clutton-brock and colleagues. Sex differences in mortally extend throughout an animal's period of growth, long after weaning -- for instance, among many hooved animals the greatest sex differences in mortality occur during the first and second winters. In addition, experiments with young rats and pigs on inadequate diets slow that, even in the absence of parents, males are more likely to die than are females.

Some aspects of the sex ratio remain unexplained by the nutrition theory, the scientists admit. These incluse the greater tendency of males to die during the early stages of gesttion and at birth or hatching, even in species with little size difference between male and female adults. Furthermore, when the adult female is larger than the male, as in the European sparrowhawk, there is no evidence that the juvenile female has a lower survival rate than the male.

Whichever theory is correct, the scientists suggest an "evolutionary explanation": that sexual selection favoring large adult size in males has increased the male growth rate to the point at which this growth is balanced by its cost to juveniles.