Ultrasonic Courtship Vocalizations of Adult Male Mice: A Laboratory Exercise Illustrating Comparable Activation by either Estradiol or Testosterone

The importance of testosterone (T) in maintaining full expression of male mammal reproductive behavior is well documented and widely acknowledged. However, the role of estradiol (E2) in the regulation of male reproductive behaviors is much less widely recognized or appreciated. Indeed, T can be enzymatically converted to E2 via aromatase. Many regions within the brain of males and females are rich in aromatase, thereby offering the possibility that many of T's actions in the brain require conversion to E2. According to this aromatization hypothesis, full expression of male reproductive behaviors is mediated via the action of E2 in the brain. This paper describes a laboratory exercise that illustrates this phenomenon as it relates to 70 kHz ultrasonic courtship vocalizations by male mice in response to either a female conspecific or the odor of female urine. The results indicate that while castration reduces male vocalizations to either female stimulus, subcutaneous implants of estradiol benzoate (EB) or testosterone propionate (TP) are equivalent in maintaining high levels of vocalization. I find this exercise extremely useful in illustrating to students why the characterization of hormones as either masculine or feminine is erroneous. Background information is presented that will help guide class discussions. Modifications to the laboratory exercise are described that do not require surgical castration or the use of controlled substances.