5 Social and Reproductive Behavior

This lecture has three aims: to understand the role of key hormones and neurotransmitters in social behaviors; to explain differences in these findings between species; and to describe how changes in hormonal systems lead to phenotypic changes (changes in an animal’s observable traits, including its behavior). It opened with a memory test on the Hebbian principles from the previous lecture (see Check yourself).

Estrogen and testosterone

Cognition

• Estrogen is essential for memory formation and maintenance. A lack of activation of estrogen receptors leads to cell death in the hippocampus.
• Testosterone deprivation leads to loss of synapses in the hippocampus, and to an increase in amyloid plaques (clumps of protein that build up between neurons, as in Alzheimer’s disease).

The slide’s figure shows stained sections of mouse hippocampus from wild-type (“wt”) and mutant (“reln”) mice at 3 and 9 months of age, with arrows marking small clusters of dark deposits.

Sex steroids and sexual behavior

• Estrogen is required for sexual behavior.
• Testosterone amplifies sexual behavior.

The slide shows a pair of sandhill cranes in a courtship dance.

Aggressive behavior

Both testosterone and estrogen are associated with aggression:

• estrogen is necessary for aggression;
• testosterone can amplify estrogen-dependent aggression.

“Aggression” can show up as any form of competition. (The slide shows a male lion roaring, with cubs at his side.)

Seasonal changes

• Testosterone increases during mating seasons.
• Testosterone increases last longer in polygynous species (where one male mates with several females) than in monogamous species (where a male and a female form a mating pair).

The lecture asked why in both cases (see Check yourself). The slide shows a male satin bowerbird decorating his bower with blue objects while a female looks on.

The winner effect

• The increase in receptors is likely increasing the capacity to respond to testosterone and estrogen during aggression.
• Winning increases resilience against traumatic stressors.

The slide features Dr. Temitayo O. Oyegbile, with a photo of mice. The next slide is a photo of a football coach shouting on the sideline: competition in human form.

Parental care

• Estrogen regulates the formation and maintenance of parental behavior.
• Testosterone decreases parental behavior.
• Monogamous species have an increase in testosterone around the time of birth. (Why? See Check yourself.)

The slide shows a cat nursing her kittens. A discussion slide followed: generally, monogamous species have an increase in testosterone after their offspring is born, but in humans we see the opposite, where males have a decrease in testosterone after a child is born. Why might this be? (See Check yourself.)

The hypothalamus–pituitary–gonadal (HPG) axis

The production of the sex steroids is controlled by a chain of three organs, the hypothalamus–pituitary–gonadal (HPG) axis:

1. The hypothalamus releases gonadotropin-releasing hormone (GnRH).
2. GnRH acts on the pituitary, which releases luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
3. LH and FSH act on the gonads (testis or ovary), which produce testosterone.
4. Aromatase converts testosterone into estrogen.

Testosterone and estrogen also act back on the hypothalamus and the pituitary (the curved arrows). What happens if the hypothalamus releases too much GnRH? More GnRH means more LH and FSH, and so more testosterone and estrogen. The pathway is self-regulating, though: if too much testosterone or estrogen binds to receptors in the hypothalamus, GnRH release decreases.

The lecture then asked what happens when parts of this loop fail (see Check yourself).

Vasopressin and oxytocin

• Vasopressin and oxytocin are made by neurons in the hypothalamus (in its paraventricular and supraoptic nuclei), whose axons carry them to the posterior pituitary, from which they are released.
• They play key roles both in the brain and in the periphery (the rest of the body).
• They are highly implicated in sociality.

The slide’s figure locates the hypothalamus deep in the brain and shows neurons of its paraventricular nucleus and supraoptic nucleus sending axons down to the posterior pituitary. It also gives the chemical structures of the two hormones: each is a chain of nine amino acids, and they differ at only two positions (isoleucine and leucine in oxytocin, phenylalanine and arginine in vasopressin).

Oxytocin

• Oxytocin depolarizes the membrane potential in muscle fibers, which allows uterine contractions.
• There is a large oxytocin release after birth, and after sex. Oxytocin is commonly referred to as the “love” hormone.
• Oxytocin increases affiliative behavior (behavior that seeks out and keeps closeness with others).

To show what “depolarize” means, the slide recalls the action potential: (1) a stimulus arrives at a membrane in its resting state (−70 mV); (2) once the membrane passes threshold (−55 mV) it depolarizes as Na ions flow in, up to a peak near +40 mV; (3) it repolarizes as K ions flow out; (4) it overshoots into hyperpolarization below rest; and (5) it returns to the resting state. Stimuli that do not reach threshold produce only small “failed initiations.”

Vasopressin

• In the periphery, vasopressin causes water retention.
• It is associated with aggression and dominance, in particular territorial aggression and mate-guarding. The lecture called vasopressin “applied” sex steroids.
• It matters for the difference between monogamous and polygynous mating systems (next section).

The slide shows two betta fish (Siamese fighting fish) facing off with their fins flared.

The importance of receptors

• Monogamous voles have more oxytocin and vasopressin receptors than polygynous voles.
• Increasing vasopressin receptors induces pair bonding (a lasting preference for one partner) and parental care; blocking vasopressin receptors blocks these behaviors.
• Monogamous voles have an extended microsatellite (a stretch of DNA made of a short sequence repeated many times) in the vasopressin receptor gene.

As the slide’s title says, receptors are what matter here: the number of receptors for a hormone shapes the social behavior it produces. (The slide shows a pair of voles with their pups, framed by hearts.)

The lecture closed with a reading on vasopressin, oxytocin and PTSD (post-traumatic stress disorder): ScienceDirect article.

Check yourself