Why does the Orangutan Need its Mother so Long?

Did you know that orangutans have the longest immature period of all ape species? Primates have a prolonged immature phase, “childhood,” compared to most other mammals. Orangutans stay with their mothers until they reach eight years and rely on their mother’s care which coincides with the time before the birth of another offspring. Clinging to the mother is observed until the juvenile is six years old! But why does the orangutan need its mother’s care for such a long time? The Gijsbertus et al. article examines the interval of time (inter-birth) between subsequent births. Orangutans have the longest inter-birth interval suggesting a long period of dependence.

Several hypotheses suggest that predation and food availability play important roles in lengthening the immature phase. The long inter-birth phase may reduce the chance of immature death from malnutrition. An arboreal lifestyle requires more energy expansion than a terrestrial lifestyle and may lower the survival of the young orangutan. Orangutans are frugivorous mammals and fruit-eating animals tend to have longer developmental phases. However, the most interesting aspect of the development of this primate is the acquirement of necessary skills such as foraging and social interaction. It coincides with an interesting pattern of food resources which are available at irregular time intervals. Mast years denote times when there is an abundance tree fruit production. The orangutans must therefore learn to cope with unpredictable food resources.

One hypothesis suggests that the long inter-birth phase is required because a mother-orangutan is unable to care for its older independent offspring while caring for the younger offspring and she must therefore devote more time to the immature offspring to help it acquire the skills necessary for total independence. The solitary-lifestyle hypothesis accounts for the slow reproductive rate of the orangutan. The adult female no longer associates with her previous offspring when obligated to nurse and nurture a new infant.

The effects of immature age on activity are evidently expressed in orangutans. The amount of time spent resting decreases with age as more time is being spent actively feeding and foraging. However, by the age of two most orangutans travel independently. The distance from the mother and time the juvenile spends at a greater distance increases with age. The amount of time spent at a distance greater than 30 meters augments. The increase in distance is increases significantly after three years, from a range of approximately two meters distance from the mother. After seven years the orangutan’s range can be greater than 30 meters. At eleven the young orangutan can travel solitarily for days at a time.



However, the reasons for the long immature phase in the species still remain open to debate. Does it take so long for the juvenile to learn the skills necessary for survival and social interaction, or is protection against predators the reason for the prolonged phase, or food availability? Perhaps all these factors are responsible for a cumulative effect of the long immature interval in the orangutan. A detailed study of each of these factors would perhaps shed light onto the lifestyle of species. We can learn a great deal about our own species through the study of these mammals. We stay with our parents longer than any other mammalian species. The orangutan may link us to our primate ancestors and allow us to analyze our own natural heritage. Besides, the orangutan is a really interesting mammal…and loves to eat tropical fruits, my favorites!

Article:
Gijsbertus G.J., van Adrichem, Sri Suci Utami, Serge A. Wich, Jan A.R, van Hooff, and Elisabeth H. M. Sterck. The development of wild immature Sumatran orangutans (Pongo abelii) at Ketambe. Primates (2006) 47: 300-309.
--Timoteo