Behringer, V., Stevens, J. M. G., Hohmann, G., Möstl, E., Selzer, D., & Deschner, T. (2014).
Testing the effect of medical positive reinforcement training on salivary cortisol levels in bonobos and orangutans. PLoS One, 9(9): e108664. doi:10.1371/journal.pone.0108664.