NEONATAL WEIGHT IN GIBBONS (HYLOBATES SPP)

Neonatal and birth weights of gibbons have mostly been reported for si ngle individuals, and larger samples (n = 2-8) have apparently been pu blished for only two species of gibbons (Hylobates lar and H. syndacty lus). In addition, a critical examination of the few published neonata l weights of gibbons shows that several of them should not be used. Ne onatal weights are here defined as weights taken on infants up to seve n days old, whereas birth weights include only those taken on the day of birth. This paper presents neonatal weights for six representative species of gibbons (H. lar, H. leucogenys, H. moloch, H. muelleri, H. pileatus, H. syndactylus) and some of their hybrids. Most of our data stem from surviving animals that were subsequently hand-reared and inc lude 80 infants, thus making the previously available dataset 5 times larger. Our neonatal weights fall roughly into three different classes : neonates of the Lar group (about 390 g, n = 27), the concolor group (about 510 g, n = 7), and the siamang (about 540 g, n = 46). This grou ping corresponds not only to taxonomic units within the hylobatids, bu t also to grouping of gibbons by adult body weight. No weight differen ce between males and females is evident in our sample, and hybrids of the Lar group do not appear to differ in weight from pure species. Tru e birth weights (i.e., weights recorded on the day of birth) are avail able for only a few individuals. These weights are, on average, 7% hig her than neonatal weights, but the difference is not statistically sig nificant. Additional samples of neonatal weights suggest that infants that die on the day of birth weigh, on average, 17% less, twins weigh 29% less, and infants born by Cesarean section weigh 19% more than our reference sample of neonates. (C) 1995 Wiley-Liss, Inc.