Neuropeptides and human social behaviours

What are the hormonal influences on sexual behaviour

• Organizational effects: Hormones (like testosterone) during early development shape the structure of the brain and future behavior.

• Activational effects: Hormones during adulthood influence the expression of behaviors.

• Examples: Castration and hormone treatment in rats demonstrate how testosterone organizes male-typical behaviors and brain structures.

Castration example

Male rats castrated at birth show female receptive behaviour as adults if primed with estrogen and progesterone

Hormonal treatment example

Female rats treated with testosterone around birth will NOT show female receptive behaviour as adults regardless of the hormonal milieu, but will mount other females and even achieve intromission and a kind of ejaculation

What is Sexual Dimorphism in the Brain

• Structures like the Sexually Dimorphic Nucleus in the hypothalamus differ between males and females.

• Human conditions like Guevedoces (genetic males with underdeveloped genitalia due to hormonal mutations) show how puberty-related hormone changes affect sexual identity and behavior

What is monogamy adaptive

• It take 2 to raise a family

– Resources

– Predation

• Its hard to find a good partner

– Low population density

Monogamy in the deep sea

• In deep-sea environments where it’s incredibly difficult to find a mate due to low population density and scarce resources, some species (like deep-sea anglerfish) have evolved permanent monogamy.

Example:

• Male anglerfish are tiny compared to females.

• When a male finds a female, he physically attaches to her body, fusing with her and sharing her blood supply.

• He becomes essentially a reproductive organ for the female, giving up his independence to ensure reproduction.

Why it’s adaptive:

• In such extreme environments, finding a partner even once is rare.

• So once found, staying bonded permanently maximizes reproductive chances — no need to search again.

• This example demonstrates how monogamy can evolve not for emotional or social reasons, but as a practical solution to environmental constraints.

What is Pair Bonding and Monogamy

• Oxytocin (OT) and vasopressin (AVP) are critical neuropeptides in bonding.

• Monogamy may have evolved from adaptations of maternal bonding mechanisms.

• Prairie voles are a key animal model: they form lifelong bonds due to high oxytocin and vasopressin receptor expression in specific brain regions.

What is the 4 neurochemistry of love (4)

• Oxytocin: promotes trust, maternal care, and partner preference.

• Vasopressin: more critical in male bonding; relates to aggression, scent marking, and territory.

• Dopamine: reinforces bonding through reward pathways.

• Opioids: linked to the pleasurable feelings of attachment.

What are 3 roles of oxytocin

• Promote Uterine Contraction during Labor

• Milk Ejection during Lactation

• Maternal Nurturing and BONDING

• Key point = stimulates partner preference formation

What is the speculation on sex differences in nature of the pair bond

• Female bonding to male partner evolved from adaptation of systems involved in maternal nurturing.

• Male bonding to female partner evolved from adaptation of system involved in territorial behavior

What is the chemistry of pair bonding (3)

1. Oxytocin (OT) and Vasopressin (AVP)

• These neuropeptides are key for the perception of social cues — recognizing and responding to others.

• In mutant mice lacking OT or AVP, there’s a condition called social amnesia — they don’t remember or distinguish familiar from unfamiliar individuals.

• OT is more linked to female bonding, AVP to male bonding, especially in species like prairie voles.

2. Dopamine

• This is the reward chemical.

• It reinforces positive associations with a partner, helping the brain “learn” that this individual is special or rewarding.

• Plays a similar role in addiction, which is why romantic bonding and drug addiction share neural pathways.

3. Opiates (Endorphins)

• Responsible for pleasure and comfort — the “feel good” aspect of bonding.

• When you’re with someone you love, these are the chemicals that make it physically comforting and emotionally satisfying.

Together, these systems interact to make social bonds emotionally rewarding, reinforce attachment through learning, and build recognition and trust — the biological foundation of pair bonding.

What is social loss and depression (3)

• Partner loss in voles leads to behaviors akin to depression in humans.

• CRFR2 (a stress receptor) activation suppresses oxytocin release, mimicking grief or social loss.

• This suggests a biological parallel between addiction, love, and bereavement.

Case study Bosch et al 2009

Blocking CRF receptors prevents social loss induced depression

CRFR2 main points (4)

1. Corticotropin-releasing factor receptor 2 (CRFR2) in the nucleus mediates social loss-induced passive coping.

2. CRFR2 (a stress receptor) activation suppresses oxytocin release, mimicking grief or social loss.

3. Interaction of the CRFR2 and oxytocin systems in mediating the emotional consequences of partner loss.

4. Chronic activation of CRFR2 and suppression of oxytocin signaling following partner loss result in an aversive emotional state that may share underlying mechanisms with bereavement

What is early life experience

• Early separation or neglect affects social behavior and bonding in adulthood.

• Low oxytocin receptor density in the nucleus accumbens (NAcc) makes individuals more vulnerable to stress-induced social issues.

Case study

Lower CSF oxytocin concentrations in woman with a history of childhood abuse

What is the human relevance

• Oxytocin receptor locations in humans align with brain regions activated during maternal and romantic love.

• Genetic and neuroimaging studies support the role of these systems in human social behavior and relationships.