Patients' perspectives on healthcare factors and symptom course in persistent somatic symptoms: findings from a cross-sectional survey in four European countries.

Parental behavior, defined here as any post-fertilization behavior that increases offspring survival or fitness (), has evolved repeatedly and independently and is likely a precursor to more complicated social behavior such as monogamy and cooperative breeding (; ). Across taxa, the bedrock of parental behavior is an association between parent and offspring. These associations can range from simple maternal tolerance of offspring such as that observed in lizards (; ) to the multiyear investments made in offspring by our own species. The taxonomic breadth and diversity in parental behavior has led to a significant research effort investigating both the ultimate and proximate mechanisms responsible for its repeated evolution (). The nonapeptide hormone arginine vasotocin (AVT—often referred to as vasopressin in mammals; but see ) is a signaling molecule that serves as a proximate mechanism underlying nearly all aspects of vertebrate social behavior (; ; ; ), including parent–offspring associations (; ). For example, increased AVT activity, either through natural variation in receptor density or experimental supplementation, leads to more grooming, nursing, and pup-carrying in rats, mice, and voles, while AVT antagonists inhibit these same behaviors (; ; ; ). In marmosets, increases of AVT lead to greater responsiveness to infant cues (), and specific differences in the promotor region of the human vasotocin receptor gene are also linked to maternal behavior (). In zebra finches, blocking AVT in prospective parents reduces nest-building behavior (), and biparental cichlid fish have greater AVT activity during nest construction (). Conversely, in the poison frog, , AVT supplementation decreases egg attendance (). In the only reptile study to investigate the role AVT plays in parental behavior, found that blocking AVT disrupts mother–offspring association in a viviparous rattlesnake, (). Together, this research suggests that relatively small changes in AVT can underly and influence a range of parental behaviors, even over relatively long time periods (eg, days to weeks), either via downstream actions on other hormones (eg, ) or through organizational effects on the brain during sensitive developmental periods (eg, ). Most previous studies have largely focused on exploring the role that AVT plays in mediating the nature and extent of parental behavior in systems where care is complex and obligate. We argue that AVT may also play a role in the emergence of parental behavior. In these systems, parental behavior is represented by an increase in the extent of social association (or grouping) between parents and offspring. Crucially, AVT has been shown to facilitate social grouping in vertebrates. For example, blocking AVT receptors in normally gregarious finches decreased preference to spend time in larger groups (), and AVT supplementation elicits positive phonotaxis in frogs (). It is therefore plausible that AVT initially played a key role in mediating these early parent–offspring interactions before being co-opted to regulate the more complex forms of parental behavior seen in species with derived care. To test this idea, we need to explore the role that AVT plays in mediating simple parent–offspring grouping in systems where parental behavior is nonobligate (ie, the offspring’s survival is not dependent on the presence of a parent). Examining the extent to which AVT underpins parental behavior in such systems will ultimately provide a broader view of the mechanistic control of these behaviors.