Attachment / Pair Bonding Study Guide
Behavior: Pair Bonding
In some species, male and female partners form strong pair bonds which is presumably advantageous for division of labor in raising offspring. In ethology, “monogamy” is defined as: pair preference, co-nesting, and co-parenting. Only 3-5% of mammalian species are monogamous by this definition. (Many birds also pair bond, and mechanisms seem to be conserved, but we’ll just discuss mammals).

Assay: Partner Preference Test
Test subject is free to move between a chamber where previous partner is tethered, and a chamber where another potential mate (a stranger) is tethered. Partner preference = % of time spent in previous mate’s chamber. 50/50 means no partner preference. <50% would be stranger preference; 100% would mean absolute partner preference. previous test stranger mate subject potential mate
Correlate Behavior (pair preference) to a Neural Mechanism: Oxytosin/Vasopressin
Prairie voles = strictly monogamous rodent species, mates for life. * Partner preference is induced by a single mating, for both males and females. * Mating causes release of hormone oxytosin (females) or vasopressin (males) * Blocking oxytosin (f) or vasopressin (m) receptors during mating prevents pair preference. * Artificial dose of oxytosin (f) or vasopressin (m) in presence of scent of a stranger potential mate, is sufficient to induce partner preference for that individual without ever having mated. Therefore what matters in mating is the pairing of oxytosin/vasopressin with olfactory cues. * How do these hormones alter behavior? * Female: oxytosin receptors in Nucleus Accumbens (NAcc; a dopamine/reward area)
Blocking oxytosin receptors locally in NAcc during mating prevents partner preference.
Mating causes increase in dopamine in NAcc
Blocking dopamine receptor (systemic) during mating prevents partner preference
Blocking dopamine receptor locally in NAcc during mating prevents partner preference

* Male: vasopressin receptors in Ventral Pallidum (VP; a “liking” or hedonic pleasure area that also gets input from NAcc)
Blocking vasopressin receptors locally in VP during mating prevents partner preference.

Comparative Study Confirms Theory * Montane voles = related species but non-pairing, shows no partner preference. * Montane voles also release oxytosin/vasopressin at mating (?!) * But they don’t have oxytosin receptors in NAcc (f) or vasopressin receptors in the VP (m) * A transgenic strain of montane voles was made with a vasopressin receptor gene from the prairie vole. In the transgenic strain vasopressin receptors were expressed in the VP of males, and the males were behaviorally monogamous (expressed strong partner preference).

Behavior: maternal/neonate bonding
Mammal neonates are helpless and completely dependent on maternal care. Therefore there is a strong selection pressure for neonates to bond to mothers and vice versa, even in species with no pair bonding.
Maternal bonding * Cervical dilation during birth causes surge of oxytosin in mother * Nursing/nipple stimulation also causes release of oxytosin in mother (or non-mother female) * Oxytosin receptors in females in Ventral Tegmental Area (VTA, dopamine-reward) likely promote maternal attachment to newborns * Oxytosin receptors in olfactory bulb may underlie role of scent in maternal attachment * Oxytosin in mother/female induces lactation and maternal nurturing behaviors

Neonate bonding
Mammals have a “sensitive period” for learning positive odor associations at birth.
Neonate brain is “immature” in three relevant ways: * Odor preference learning (in olfactory bulb) is much STRONGER * Locus Coeruleus (LC) is HYPERACTIVE and this enhances odor preference learning * Amygdala, which is essential for fear conditioning / aversion learning, is HYPOACTIVE
Therefore pups are wired to orient strongly toward whatever scents they encounter at birth.
Even if aversive cues are encountered these will not disrupt positive association with those scents.
This supports attachment to mother and also other nest-mates and the nest environment.
*Note that both maternal and neonate bonding mechanisms can be engaged by surrogate parenting.
What about humans?
Not as much known. The behavior is very hard to measure or separate from culture. But we do know: * Humans do have oxytosin (female) and vasopressin (male) * These hormones are released by orgasm (in males and females), childbirth(f), or nursing(f). * Administered oxytosin causes subjects report feelings of safety, lowered anxiety * VTA and other dopamine areas are activated by pictures of one’s partner or children * Oxytosin, vasopressin, and their receptors are concentrated in dopamine circuit brain areas
Study Questions 1. How is pair bonding behavior measured experimentally? 2. In female prairie voles, what hormone is required for pair bonding to her mate? 3. In male prairie voles, what hormone is required for pair bonding to his mate? 4. Where in the female prairie vole’s brain does oxytosin act to bring about pair bonding? 5. What difference between the brains of prairie voles and montane voles explains why one species is monogamous and the other one non-pairing? 6. Name two natural behaviors that cause release of oxytosin in females. 7. Name two of the special properties of newborn mammals’ brains that are thought to support neonates bonding to their mothers.