Chapter 6: Hormones and Behavior
This chapter describes the relationship between hormones and behavior. Many readers are likely already familiar with the general idea that hormones can affect behavior. Students are generally familiar with the idea that sex-hormone concentrations increase in the blood during puberty and decrease as we age, especially after about 50 years of age. Sexual behavior shows a similar pattern. Most people also know about the relationship between aggression and anabolic steroid hormones, and they know that administration of artificial steroid hormones sometimes results in uncontrollable, violent behavior called “roid rage.” Many different hormones can influence several types of behavior, but for the purpose of this chapter, we will restrict our discussion to just a few examples of hormones and behaviors. Behavioral endocrinologists are interested in how the general physiological effects of hormones alter the development and expression of behavior and how behavior may influence the effects of hormones.
To understand the hormone-behavior relationship, it is important to describe hormones. Hormones are organic chemical messengers produced and released by specialized glands called endocrine glands. Hormones are released from these glands into the blood, where they may travel to act on target structures at some distance from their origin. Hormones are similar in function to neurotransmitters, the chemicals used by the nervous system in coordinating animals’ activities. However, hormones can operate over a greater distance and over a much longer time than neurotransmitters (Focus Topic 1). Examples of hormones that influence behavior include steroid hormones such as testosterone (a common type of androgen), estradiol (a common type of estrogens), progesterone (a common type of progestogen), and cortisol (a common type of glucocorticoid) (see Table 1, A-B). Several types of protein or peptide (small protein) hormones also influence behavior, including oxytocin, vasopressin, prolactin, and leptin.
Although neural and hormonal communication both rely on chemical signals, several prominent differences exist. Communication in the nervous system is analogous to traveling on a train. You can use the train in your travel plans as long as tracks exist between your proposed origin and destination. Likewise, neural messages can travel only to destinations along existing nerve tracts. Hormonal communication, on the other hand, is like traveling in a car. You can drive to many more destinations than train travel allows because there are many more roads than railroad tracks. Similarly, hormonal messages can travel anywhere in the body via the circulatory system; any cell receiving blood is potentially able to receive a hormonal message.
Neural and hormonal communication differ in other ways as well. To illustrate them, consider the differences between digital and analog technologies. Neural messages are digital, all-or-none events that have rapid onset and offset: neural signals can take place in milliseconds. Accordingly, the nervous system mediates changes in the body that are relatively rapid. For example, the nervous system regulates immediate food intake and directs body movement. In contrast, hormonal messages are analog, graded events that may take seconds, minutes, or even hours to occur. Hormones can mediate long-term processes, such as growth, development, reproduction, and metabolism.
Hormonal and neural messages are both chemical in nature, and they are released and received by cells in a similar manner; however, there are important differences. Neurotransmitters, the chemical messengers used by neurons, travel a distance of only 20–30 nanometers (10-9 m)—to the membrane of the postsynaptic neuron, where they bind with receptors. Hormones enter the circulatory system and may travel from 1 millimeter to more than 2 meters before arriving at a target cell, where they bind with specific receptors.
Another distinction between neural and hormonal communication is the degree of voluntary control that can be exerted over their functioning. In general, there is more voluntary control of neural than of hormonal signals. It is virtually impossible to will a change in your thyroid hormone levels, for example, whereas moving your limbs on command is easy.
Although these are significant differences, the division between the nervous system and the endocrine system is becoming more blurred as we learn more about how the nervous system regulates hormonal communication. A better understanding of the interface between the endocrine system and the nervous system, called neuroendocrinology, is likely to yield important advances in the future study of the interaction between hormones and behavior.
|Increases carbohydrate metabolism; mediates stress respones
|Uterine and other female tissue development; regulates sexual motivation and performance in females and males
|Promotes sperm production and male secondary sexual characteristics; promotes sexual motivation and behavior, typically by being converted to estradiol
|Peptides and Protein Hormones
|Stimulates milk letdown and uterine contractions during birth; Promotes social bonding
|Many actions relating to reproduction, water balance, and behavior associated with parental care
|Increases oxidation rates in tissue and affects neural development
|Increases water reabsorption in the kidney and affects learning and memory
Hormones coordinate the physiology and behavior of individuals by regulating, integrating, and controlling bodily functions. Over evolutionary time, hormones have often been co-opted by the nervous system to influence behavior to ensure reproductive success. For example, the same hormones, testosterone and estradiol, that cause gamete (egg or sperm) maturation also promote mating behavior. This dual hormonal function ensures that mating behavior occurs when animals have mature gametes available for fertilization. Another example of endocrine regulation of physiological and behavioral function is provided by pregnancy. Estrogens and progesterone concentrations are elevated during pregnancy, and these hormones are often involved in mediating maternal behavior in the mothers.
Not all cells are influenced by each and every hormone. Rather, any given hormone can directly influence only cells that have specific hormone receptors for that particular hormone. Cells that have these specific receptors are called target cells for the hormone. The interaction of a hormone with its receptor begins a series of cellular events that eventually lead to the activation of enzymatic pathways or, alternatively, turn on or turn off gene activation that regulates protein synthesis. The newly synthesized proteins may activate or deactivate other genes, causing yet another cascade of cellular events. Importantly, sufficient numbers of appropriate hormone receptors must be available for a specific hormone to produce any effects. For example, testosterone is important for male sexual behavior. If men have too little testosterone, then sexual motivation may be low, and it can be restored by testosterone treatment. However, if men have normal or even elevated levels of testosterone yet display low sexual drive, then it might be caused by a lack of receptors, so treatment with additional hormones will not be effective.
How might hormones affect behavior? In terms of behavior, one can think of humans and other animals as composed of three interacting components: (1) input systems (sensory systems), (2) integrators (the central nervous system), and (3) output systems or effectors (e.g., muscles). Hormones do not cause behavioral changes. Rather, hormones influence these three systems so that specific stimuli are more likely to elicit certain responses in the appropriate behavioral or social context. In other words, hormones change the probability that a particular behavior will be emitted in the appropriate situation (Nelson, 2011). This is a critical distinction that can affect how we think of hormone-behavior relationships.
We can apply this three-component behavioral scheme to a simple behavior, singing in zebra finches. Only male zebra finches sing, and they sing to attract mates or ward off potential competitors from their territories. If the testes of adult male finches are removed, then the birds reduce singing, but castrated finches resume singing if the testes are reimplanted or if they are treated with either testosterone or estradiol. Although people often consider androgens to be “male” hormones and estrogens to be “female” hormones, testosterone is commonly converted to estradiol (Figure 1). Thus, many male-like behaviors are associated with the actions of estrogens! Indeed, all estrogens must first be converted from androgens because of the typical biochemical synthesis process. If the converting enzyme is low or missing, then it is possible for females to produce excessive androgens and subsequently develop associated male traits. Again, singing behavior is most frequent when blood testosterone or estrogen concentrations are high.
Estrogens are somehow involved in singing, but how might the three-component framework help us formulate hypotheses to explore the role of estrogen in this behavior? Estrogens could affect birdsong by influencing the sensory capabilities, central processing system, or effector organs of a bird. By examining input systems, we could determine whether estrogens alter the birds’ sensory capabilities, making the environmental cues that normally elicit singing (like females or competitors) more salient. Estrogens also could influence the central nervous system, for example, by altering neuronal architecture or the speed of neural processing. Finally, the effector organs, like the muscles in a songbird’s vocal apparatus, could be affected by the presence of estrogens. We do not understand completely how estrogens, derived from testosterone, influence birdsong, but in most cases, hormones can be considered to affect behavior by influencing one, two, or all three of these components, and this three-part framework can aid in the design of hypotheses and experiments to explore these issues.
How might behaviors affect hormones? The birdsong example demonstrates how hormones can affect behavior, but as noted, the reciprocal relation also occurs—behavior can affect hormone concentrations. For example, the sight of a territorial intruder may elevate testosterone concentrations in resident male birds and thereby stimulate singing or fighting behavior. Similarly, male mice or rhesus monkeys that lose a fight decrease circulating testosterone concentrations for several days or even weeks afterward. Comparable results have also been reported in humans. Testosterone concentrations are affected not only in humans involved in physical combat but also in those involved in simulated battles. For example, testosterone concentrations were elevated in winners and reduced in losers of regional chess tournaments.
People do not have to be directly involved in a contest to have their hormones affected by the outcome of the contest. Male fans of the Brazilian and Italian soccer teams were recruited to provide saliva samples to be assayed for testosterone before and after the final game of the World Cup soccer match in 1994. Brazil and Italy were tied, but Brazil won on a penalty kick at the last moment. The Brazilian fans were elated, and the Italian fans were crestfallen. When the samples were assayed, 11 of 12 Brazilian fans who were sampled had increased testosterone concentrations, and 9 of 9 Italian fans had decreased testosterone concentrations, compared with pre-game baseline values (Dabbs, 2000).
In some cases, hormones can be affected by anticipation of behavior. For example, testosterone concentrations also influence sexual motivation and behavior in women. In one study, the interaction between sexual intercourse and testosterone was compared with other activities (cuddling or exercise) in women (van Anders et al., 2007). On three separate occasions, women provided a saliva sample from pre-activity, post-activity, and the next morning, analyses showed that the women’s testosterone was elevated prior to intercourse as compared to other times. Thus, an anticipatory relationship exists between sexual behavior and testosterone. Testosterone values were higher post-intercourse compared to exercise, suggesting that engaging in sexual behavior may also influence hormone concentrations in women.
An organic chemical messenger released from endocrine cells that travels through the blood to interact with target cells at some distance to cause a biological response.
A ductless gland from which hormones are released into the blood system in response to specific biological signals.
Chemical substance released by the presynaptic terminal button that acts on the postsynaptic cell.
The primary androgen secreted by the testes of most vertebrate animals, including men.
A primary progestin that is involved in pregnancy and mating behaviors.
A peptide hormone secreted by the pituitary gland to trigger lactation, as well as social bonding.
A protein hormone that is highly conserved throughout the animal kingdom. It has many biological functions associated with reproduction and synergistic actions with steroid hormones.
Parental behavior performed by the mother or other female.
A chemical structure on the cell surface or inside of a cell that has an affinity for a specific chemical configuration of a hormone, neurotransmitter, or other compound.
A cell that has receptors for a specific chemical messenger (hormone or neurotransmitter).