Size reduction associated with brachymesophalangia-5: a possible selective advantage.

Subjects with brachymesophalangia-5 (the short middle segment trait of the fifth digit) are systematically reduced in the length of the second metacarpal and in stature, by approximately 0.5 standard deviations, in contrast with unaffected individuals or unaffected siblings of affected propositi. There is conclusive evidence for axial and appendicular length reduction in affected individuals, and the smaller body size associated with brachymesophalangia-5 may confer a selective advantage during growth under conditions of chronic caloric or protein-calorie malnutrition. Brachymesophalangia-5, the short middle segment of the fifth digit, is a skeletal variant of considerable population interest (Garn, Fels and Israel, '67; Hertzog, '67; Garn, '71). It is often associated with clinodactyly-5 (the crooked little finger syndrome), camptodactyly-5 (the bent little finger syndrome) and cone-epiphysis-5 in younger subjects (Garn, Poznanski, Nagy and McCann, '72). Affected individuals may also show reduction of the proximal and distal segments of the fifth digit or reduction of the middle segments of the fourth and third digits (Garn, Fels and Israel, '67) indicating a "field' of digital reduction centering on mid-5. There is also some suspicion that individuals with brachymesophalangia-5 may also be reduced in long-bone length and in stature, suggesting a generalized size reduction associated with the trait. To explore this possibility we have identified 248 individuals with brachymesophalangia-5 among the participants in the Ten-State Nutrition Survey of 1968-1970. Such individuals were characterized by marked reduction in the length of mid-5, by as much as 6 S.D. in adults of European ancestry, and averaging 3 . 3 S.D. in them, Expressed as a ratio, reduction of mid-5 was 4.6 S.D. relative to distal 5, and 4.8 S.D. relative to proximal-5, conAM. 1. P H Y S . ANTHROP., 37. 267-270 firming differential size reduction of the middle segment of the fifth digit. In the first step of the data analysis, the possibility of metacarpal reduction in affected individuals was examined by the simple sign test. To do this, the length of the second metacarpal (measured as diaphysis plus epiphysis) was individually compared with the norm or standard for sex, age and ancestry as ascertained in the ten-state survey. So compared, 229 individuals with brachymesophalangia-5 proved significantly shorter in the length of the second metacarpal than the appropriate individual average for age, sex and ancestry. To be exact, 54 of the 229 subjects were above average in length of the second metacarpal and 175 were below expectancy. The obtained proportions (54: 175) were clearly different from the chance proportions (1 14.5: 114.5). Reduction of the second metacarpal was particularly evident in affected females (26: 110, chi-square = 51.9) as compared with affected males (28:65, chi-square = 14.7). In both sexes, however, the trend toward metacarpal reduction in brachymesophalangia-5 was highly significant, the value of p being less than 0.001. In the second step of the data analysis, the sign test was separately applied to stature (i.e., standing height) in 190 af-