The mechanism for monogamy in mammals involves the reward pathway of the brain

Scientists believe that mating-induced partner preferences involve reinforcing properties connected with mating which are then associated with the partner. This associative learning in partner-preference formation can be viewed as a natural example of reward learning, as the process involves a considerable amount of dopamine (Aragona, 2003). During mating, the simultaneous activation of partner recognition and reward pathways in the brain results in convergent V1a (vasopressin) and D2 (dopamine) receptor activation in the ventral forebrain (more specifically, the ventral pallidum), leading to an association between the pleasing nature of sex and the olfactory signature of the partner. This leads to the development of a conditioned partner preference (Lim, 2004). Essentially, mating instigates the release of a euphoric drug in the brain (dopamine), and this excited state becomes associated with the partner, which leads to monogamy.

Ventral Palladium
Oxytocin and vasopressin receptor distribution pattern in prairie and montane voles. Prairie voles have higher oxytocin receptor density in the Nucleus accumbens and higher vasopressin receptor density in the Ventral pallidum. These regions are associated with a dopaminergic reward pathway. Nair and Young.

Hormones and Neurotransmitters: Love is like a Drug

The reward pathways of the brain involve dopamine and the V1aR neurotransmitters, oxytocin (OT) and arginine vasopressin (AVP). Although OT and AVP act as hormones in the peripheral nervous system, neuroanatomical mapping and tracing studies show that they also act as neurotransmitters in the central nervous system. This means they work in the brain as well as certain parts of the body to influence behavior (Nair and Young, 2006).

Both hormones induce pair bonding formation in prairie voles; OT induces the behavior in females, while AVP induces it in males. OT and AVP receptors in the prairie vole brain are concentrated in the reward pathways mentioned earlier. They activate the reward pathway of the brain, which then releases dopamine. The neurotransmitters are so heavily concentrated in these reward pathways in the ventral pallidum after mating that are likely to lead to the conditioning of partner preference formation.

The montane vole, a closely related species, confirms this. In females, OT receptors are absent in these regions of the montane vole but are found in regions important for non-social behavior. Thus, OT release with copulation in the montane vole would not be expected to induce a partner preference, as the neurotransmitters are not found in the reward pathway. It is unsurprising then, that montane voles do not exhibit monogamous behavior. (Insel and Young, 2007)

Parent with Baby Vole
Male (left) and female prairie vole (right) hovering over a pup (middle).
Credit Geert De Vries.

Parental Behavior

In addition to pair-bond formation, AVP facilitates – and an AVP antagonist blocks – paternal behavior such as pup licking. Sexually inexperienced males given AVP were recorded to spend more time grooming pups than those injected with saline solution (Wang et al., 1993). OT is also needed for maternal behavior, as it is necessary for lactation and shown to correlate with other maternal behaviors.

Thus both hormones are not only necessary for pair bond formation, but also parenting behaviors. The hormone is only released when the animal meets a future mate in his/her environment, and before offspring are born. If these hormones aren’t released, the behavior does not occur. Stimulus in development is important for this and other social behaviors.