testes displayed all of the characteristics of normal cockerels (i.e., they crowed, had sexual urges, were belligerent, and developed both combs and wattles). Furthermore, the 3 implanted birds maintained these traits even though the transplanted testes had lost their original innervation. From this evidence, Berthold hypothesized that the testes exert their effect on the bloodstream. This effect is then biochemically transferred throughout the entire organism by means of the circulation. We now know that the specific agent in question is the hormone, testosterone. Berthold's experiment was the first formal analysis on how hormones influence an animal's behavior. Indeed, it served to initiate the field of behavioral endocrinology (4:1). In the subsequent decades, physiologists in several different countries began to investigate the interactions between sex gland functioning and the mating behaviors of different animals. Eventually, they decided that internal secretions mediated the behaviors through their action on the nervous system (3:393 433). Whether or not the hormonal influences were direct or indirect, however, remained a topic of controversy for quite some time.
One of the earliest investigators was the Russian physiologist, J. R. Tarchanoff. He investigated the copulatory behaviors of male frogs.
Another early researcher, Austrian physiologist, Eugen Steinach, also conducted a variety of important investigations. Steinach was greatly influenced by both Berthold's and Tarchanoff's work. For example, he was one of the first to study the behavioral and physiological effects of removing and reimplanting gonads in species other than roosters. In addition to birds, he performed the procedure on amphibians and rodents. Other hormonal studi
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