Hermes, R., Olson, D., Goritz, F., Brown, Janine L., Schmitt, D. L., Hagan, D., Peterson, J. S., Fritsch, G., and Hildebrandt, Thomas Bernd. 2000. "
Ultrasonography of the estrous cycle in female African elephants (Loxodonta africana)."
Zoo biology, 19 369–382.