Experimental Endocrinology

The earliest report of experimental endocrinology, in the mid-nineteenth century, demonstrated that replacements of testicular tissue would maintain comb growth and sexual behavior in castrated roosters. Techniques for determining endocrine function used today include ablation and replacement.

Steps Natural Selection
Hormones trigger many kinds of animal behavior, including bird migration. (Digital Stock)

Ablation (removal) of endocrine tissue results in deficiency symptoms. The effects of ablation are not always unambiguous. If the testes and accessory tissues are not completely removed when a horse is castrated, for example, tissue capable of producing testosterone remains, and the consequence is an infertile gelding that behaves like a stallion.

Hormones produced by different glands can have similar physiological effects. Both the adrenal glands and the testes produce androgens (masculinizing hormones). Sexually experienced male cats do not lose their sex drive if castrated, and researchers do not have a satisfactory answer as to why this occurs. Perhaps the adrenal hormones are sufficient to maintain established feline male sexual behavior but not sufficient to initiate it in inexperienced cats. The ablation of the adrenal glands, however, has severe consequences in terms of electrolyte and blood glucose imbalances that are life threatening. Replacement of ablated endocrine tissue can reinstate normal function. If a male cat is castrated as a kitten, it will not develop normal male sexual behaviors. If, however, a normal testis is later transplanted to the abdominal cavity (or elsewhere), normal behavior will develop.

In the early 1960's, Janet Harker became convinced that the "biological clock" controlling the daily activity cycle of the cockroach was contained in the subesophageal ganglion, a patch of nervous tissue the size of a pin head resting just below the esophagus. When she ablated this ganglion, the cockroach became arrhythmic. Harker removed the legs from a normal roach and glued the roach on top of the arrhythmic roach, surgically uniting their body cavities so that the same body fluids circulated through both roaches. The arrhythmic roach ran about the cage with an activity rhythm dictated by the rhythm of hormones released into the body fluids by the legless roach on its back.

Most hormone and behavior studies involve nonhuman species and most involve sexual be haviors. Most behaviors are oriented toward perpetuating one's species. Only those individuals with behaviors conducive to rearing offspring will provide the genetic basis for behaviors represented in the next generation. It may seem that wild, unrestrained mating would be selected for, but that makes no real sense. Mate selection, shelter-seeking, feeding, maintenance of social position, and a host of other behaviors are critical to the success of one's progeny. Individuals that produce more offspring than they can feed or protect usually rear fewer than those who produce fewer to begin with. Many predator species (owls and wolves, for example) tend not to mate in years when prey is scarce. This ultimately maximizes reproduction by reserving energies that can best be spent later. This restraint is mediated by adjusting hormonal levels. Hormones have been called the ultimate arbiters of sexual behavior.

Pregnancy And Childbirth

Pregnancy And Childbirth

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