As scientists study different animals in their natural environment, they frequently learn things that complicate accepted pictures of animal history. A recent series of studies of nursing orangutans in Borneo and Sumatra has shown that weaning doesn’t occur in these animals until they are more than eight years old. That is a record lactation period for any wild animal.
Determining the length of nursing for orangutans has been difficult for various reasons. For one thing, the nursing takes place high in the trees, so scientists have difficulty observing. Add to that the fact that these animals are hairy, so it becomes difficult to see what is going on. Some have suggested that the babies are just cuddling.
Researchers finally settled the question by studying orangutan teeth. Studies of the layers of material containing barium in the teeth of the babies indicate that they really are feeding. Museums have collections of orangutan teeth from years ago when these animals were not considered to be endangered and were often killed by collectors. Calcium and barium from the mother’s body go into building the nursing infant’s teeth. Primate teeth are built up with a microscopic layer each day. By examining the layers, scientists can tell when the infant orangutan stopped nursing because there is a greater concentration of barium in the nursing layers. Examining teeth from orangutans at different ages, the scientists determined that nursing can continue through age eight.
Some evolutionary theories have difficulty with these very long lactating periods. Not only is the female not receptive to males during that eight years, but she must continue to produce milk. In milk production, the mother is literally dissolving some of her body to provide nourishment for the young. These two factors seem to be counter to evolutionary models for survival. These new discoveries concerning nursing orangutans may cause some rethinking concerning theories of primate evolution.
–John N. Clayton and Roland Earnst
