The Case of the Sexually Arrested Orangutans

by
Susan B. Chaplin and Laura J. Baumgartner
Department of Biology
University of St. Thomas, St. Paul, MN

Part I—A Call from Anne

It was late in the work day and I was rushing around trying to get some new chimps settled in their habitat when the phone rang.

“Hi Lisa, it’s Anne and I’ve got a favor to ask. I know you have some captive male orangutans at the National Zoo and I’m hoping you’ll help me collect some growth, development, and behavior data on these animals. I’ve enlisted the cooperation of curators at 12 other zoos in the U.S. to help me assemble this database on juvenile male orangutan development.

“As you may know,” Anne continued, “when juvenile males are housed together, some begin to develop the characteristic secondary sex characteristics at age four, resulting in the massive body size, large cheek flanges and laryngeal sac, while others stay juvenile looking but grow to nearly adult size. I think the difference in development might be due to variation in the effects of captive stress, mostly from the presence of other adult males. When we separate the arrested individuals from the rest, they complete their maturation to adulthood rather quickly.”

As an Assistant Curator at the Smithsonian National Zoo in Washington D.C., I was in charge of the care and health of the primates and pandas, and thought this sounded like an interesting project.

“I think we can do that Anne. Actually, I have also noticed those developmental differences in juvenile orangutans and always wondered why they occur.”

“Good, then you’ll know what to look for. I need information on their housing situation, diet, medical history, growth, behavior patterns, and as they go through puberty, flange and laryngeal sac development, social interactions, and semen quality if you manage to collect that. I would also like you to collect fresh urine samples from each individual periodically. The lab at the Center for Reproduction of Endangered Species (CRES) at the San Diego Zoo will run hormone assays on the urine for me. I’ll send you a spreadsheet for keeping records on each animal. You can send the urine samples and spreadsheet data to CRES for analysis. I really appreciate your help.”

Questions

1. Explain the normal hormonal control of male sexual development by the hypothalamus-pituitary-gonadal axis.
2. Are there other hormones that might influence reproductive maturation? Which ones and how do they affect growth and development?
3. Make a list of hormones that might be lacking in males exhibiting arrested development of secondary sex characteristics.
4. What are the stress hormones? How do they act? Specifically, how does stress affect growth and development?
5. What sort of data might support Anne’s hypothesis that arrested sexual development of juvenile orangutan males is a result of social stress?

Date Posted: January 13, 2009.

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