Behavioural development of twin siamangs (Hylobates syndactylus)

Multiple births are very rare among gibbons. The birth of siamang twins at the Zürich Zoo in 1992 therefore presented a valuable opportunity to observe the development of the twins and to contrast it with a survey of previous reports on the development of single offspring of siamang and gibbons of thelar group. Furthermore, the hypothesis that the presence of twins among siamang may facilitate the occurrence of helping behaviour (defined as the care of offspring by individuals other than their parents) was re-examined (Dielentheis et al., 1991). The Zürich 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 single offspring. (2) A clear difference between the twins was observed: the female twin developed more rapidly than the male. (3) Neither the twins' father nor their older sisterRama was ever observed carrying the twins. The hypothesis ofDielentheis et al. (1991) is hence not supported by the present study, although it is possible that the older sisterRama did not carry the twins because she was younger than the juvenile in that study. (4) Siamangs may have a longer maturation period than gibbons of thelar group.