Dal Pra, G. & Geissmann, T, 1994: Behavioural development of twin siamangs (Hylobates syndactylus). Primates 35: 325-342.
Behavioural development of twin siamangs (Hylobates syndactylus)
Geraldine Dal Pra1 &
T. Geissmann2
1Department of Biology, Swiss Federal Institute of Technology, ETH Zürich,
Switzerland
2Anthropological Institute, University Zürich-Irchel, Switzerland
Key words: Behavioural development; paternal
care; helping behaviour; multiple offspring; Hylobates syndactylus; siamang;
gibbon; twins.
Abstract: 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 the lar
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 hours 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 sister Rama was ever observed carrying
the twins. The hypothesis of Dielentheis et al. (1991) is hence not supported by
the present study, although it is possible that the older sister Rama 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 the lar group.
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