Although many people use the expression “the birds and the bees” to politely refer to sexual behavior, it took a team of scientists from Johns Hopkins University to truly drive this phrase home. Investigating the ways in which testosterone influences mating rituals among canaries, they discovered that a boost of the male hormone causes the male to sing more songs to attract a female.

The male bird’s ability to sing the kind of sweet tune that is appealing to a lady bird, however, depended largely on which region of the brain was dosed with the hormone. “The hormones in these birds are identical to those in humans, and they can regulate brain changes in a similar manner,” Gregory F. Ball, professor in the Department of Psychological and Brain Sciences, Johns Hopkins University, told Forbes. Ultimately, then, the researchers’ findings suggest that the ways in which testosterone influences human sexual behavior — and possibly the success of sexual conquest — depends a great deal on how and where it stimulates the brain.

Birds and Men

To begin their experiment, the researchers used light and heating systems to simulate the spring season and then recorded the mating rituals that took place among canaries. In particular, they noted how the male birds used songs to attract female birds, but these rituals were much more nuanced than they originally expected: tuneless if persistent chirping failed to attract female birds, who seemed to find male birds singing more artful tunes much more appealing. Understanding that hormones regulate distinct aspects of social behaviors, the researchers suspected that testosterone (T) might be the deciding factor when it came to how the male birds sang, so they divided their male canaries into two groups and injected one group with testosterone in the medial preoptic nerve (abbreviated as POM), which is known to regulate the sex drive. For the second group of birds, the researchers bathed their entire brain with T.

“Testosterone has multiple effects on learned courtship song in that it regulates both the motivation to sing in a particular social context as well as the quality of song produced,” wrote the authors in PNAS. “We show here that T implants in the [POM] increase song rate but do not enhance acoustic features. … When presented with a female canary, POM-T birds copulated at a rate comparable to birds receiving systemic T but produced fewer calls and songs in her presence.” Mating, though, is not a simple dance as most of us well know. The data from this experiment suggests that testosterone needs to act not just on one primary area but throughout the brain in order to best regulate how a male bird carries off his part of the complex choreography of attracting a mate.

“It appears that, like in so many other species, testosterone in the POM can regulate an animal’s motivation, in this case, the motivation to sing,” Beau Alward, lead author and professor at Johns Hopkins and University of Liege, told Forbes. “However, singing and courting a female is more than just motivation. There is the quality of the song that is required to successfully attract a mate and then the process of attending to the female, or singing to her, when she is there, which requires the coordination of multiple brain regions.” In short, the same may be true of testosterone’s effect on human brains and if that is the case, the implications may be far-reaching.

Testosterone as Therapy

Produced primarily in a man's testicles — though women also generate a small amount in their ovaries — testosterone is not just about the sex drive as it also helps maintain:

  • bone density
  • fat distribution
  • muscle strength and mass
  • red blood cell production
  • sperm production

Testosterone peaks in adolescence and early adulthood, but then after a man reaches the age of 30, levels gradually decline by about one percent a year (typically). Yet according to the American Urological Association, as many as four in 10 men over the age of 45 have lower than normal levels of testosterone — beyond the usual amount that occurs during normal aging. To treat this condition, which is referred to variously as andropause, testosterone deficiency syndrome (TDS), and hypogonadism, doctors often prescribe testosterone therapy, a practice that has become increasingly common. In fact, prescription sales of testosterone increased by 500 percent in the United States between 1993 and 2000 and currently the number of prescriptions written each year is between seven and eight million.

Despite the growing numbers, some doctors believe the risks of T therapy outweigh the benefits. It is believed that testosterone supplements may increase various significant and less important risks, including heart attacks, faster-growing prostate cancer, reduced fertility, and breast enlargement. Another issue with prescribing testosterone is that the dose required varies considerably among men suffering from TDS. When done properly, a doctor should be routinely monitoring, by way of a comprehensive serum analysis and physical exam, the patient’s symptoms and reaction. In other words, hormone replacement therapy is by no means an easy fix, and just like the canaries described above, the outcome may depend on many factors, including how thoroughly it is introduced to an aging man's brain.

Sources: Alward BA, Balthazart J, Ball GF. Differential effects of global versus local testosterone on singing behavior and its underlying neural substrate. PNAS. 2013.