Historically, love has always been a subject dealt with in the domain of humanities, in the form of philosophy, art, music, or literature. However, in the 19th century, psychologist William James paved the road for love to enter into the realm of science with his suspicions that emotions had bodily repercussions, and in later years James Papez developed a model of the emotions dependent on the brain. Thus began modern science’s foray into neuroscience. But the question of why humans, or animals, need love arose. Scientists turned to the most important subject of species survival: reproduction.
In order for reproduction to occur, there must be some force that draws complementary sexes together in order to produce the next generation, and a similar force would be needed to maintain a parent-child bond. Without reproduction and offspring survival, a species would die out. Evidence suggests that the emotions that draw parents to each other, children to parents, and other social bonds are the result of the harmonious actions of endocrine and neural factors called nanopeptides, such as oxytocin and vasopressin.
When homeostasis is threatened, the hypothalamus makes both peptides, after which they travel to the pituitary gland, and finally are released into the bloodstream. Nanopeptides are also used in regulating reproductive. Peptides have been shown to affect mating behaviors such as vocalization, receptivity, sensitivity to mating actions, and increase of physical actions such as clasping behavior of male to female in animals such as amphibians, fish, and salamanders.
Oftentimes, such peptides increased male’s sensitivity to a female’s presence, and a female’s receptivity to a male’s advances. Furthermore, peptide expression in the brain can differ across sex. This is true in mammals, where the difference lies in vasopressin is more important for male sexual activity while oxytocin is important for females. Studies of rodents led to evidence of such sexually dimorphic expression.
Research with human implies that both peptides are present in the circulation during sexual arousal; oxytocin levels increased in both sexes during orgasm, but oxytocin and vasopressin were released at different times that contribute to different sexual responses over time. Evolutionary studies of both oxytocin and vasopressin can illuminate the role of love as a humanistic force embodied biologically. One can see this through the presence of ancestral lines of these peptides: isotocin in bony fish and mesotocin in amphibians, reptiles, birds, and marsupials, with mesotocin an intermediate form between oxytocin and isotocin (De? iec 2581).
Mesotocin is also present in lungfish, which is an evolutionary ancestor to amphibians; and arginine vasotocin exists in non-mammal vertebrates and is a precursor to vasopressin. The similarity of these peptides shows that a basic form has been preserved over evolutionary time, causing the ubiquity of these peptides in so many animals, which show that “love”, embodied in peptides, must be a strong factor in survival of species.
The amygdala, which consists of groups of neurons in the brain and processes social signals, projects into the nucleus accumbens and ventral pallidum, two areas that have been implicated in substance abuse and addiction. Oxytocin and vasopressin released during mating could reinforce social signals through linkage to the dopamine reward pathways, therefore setting up a situation in which the animal becomes “addicted to love. ” This can be seen in studies where blocking or clearing the dopamine D2 receptors either impair or enhances prairie vole bonding (De? iec 2583).
However, the positive-feedback loop expressed through peptides and the dopamine reward pathways could foster a monogamous relationship over long periods of time. This loop could also reinforce bonding between parent and child. One way in which this could be tested is to study the dopamine reward pathways and the expression of oxytocin and vasopressin in humans that have little emotional connection to others, and compare these levels to normal humans.