**Acknowledgements**

We would like to thank the staff members of the numerous zoos for permission to study the gibbons in their care. We also would like to thank Kim J.J.M. Nouwen for her contribution in the data collection for our sample of pileated gibbons (*Hylobates pileatus*). We are grateful to Linda Burman-Hall for supporting this publication and for helpful comments on an earlier version of this manuscript.
