BEHAVIORAL-DEVELOPMENT OF TWIN SIAMANGS (HYLOBATES-SYNDACTYLUS)

Multiple births are very rare among gibbons. The birth of siamang twin s at the Zurich Zoo in 1992 therefore presented a valuable opportunity to observe the development of the twins and to contrast it with a sur vey of previous reports on the development of single offspring of siam ang and gibbons of the lar group. Furthermore, the hypothesis that the presence of twins among siamang may facilitate the occurrence of help ing behaviour (defined as the care of offspring by individuals other t han their parents) was re-examined (DIELENTHEIS et al., 1991). The Zur ich twins (one male and one female) were observed for a total of 74 hr during their first year of life. The results show that: (1) The twins exhibited more rapid behavioural development than that reported for s ingle offspring. (2) A clear difference between the twins was observed : the female twin developed more rapidly than the male. (3) Neither th e twins' father nor their older sister Rama was ever observed carrying the twins. The hypothesis of DIELENTHEIS et al. (1991) is hence not s upported by the present study, although it is possible that the older sister Rama did not carry the twins because she was younger than the j uvenile in that study. (4) Siamangs may have a longer maturation perio d than gibbons of the lar group.