Endocrinology of parenting has been the subject of considerable study with focus both on human females and males and on females and males of other mammalian species. Parenting as an adaptive problem in mammals involves specific endocrine signals that were naturally selected to respond to infant cues and environmental inputs. Infants across species produce a number of cues to inform caregivers of their needs. These include visual cues, like facial characteristics, or in some species smiling, auditory cues, such as vocalizations, olfactory cues, and tactile stimulation. A commonly mentioned hormone in parenting is oxytocin, however many other hormones relay key information that results in variations in behavior. These include estrogen, progesterone, prolactin, cortisol, and testosterone. While hormones are not necessary for the expression of maternal behavior, they may influence it.
Based on cross species evidence, some aspects of these mechanisms have been phylogenetically conserved from rodents to humans, meaning that these mechanisms are adaptive for mammalian parenting and that the environment of evolutionary adaptedness of some parenting mechanisms may have evolved when mammals first evolved. The importance of these mechanisms are to regulate parental investment and to inform offspring about their environment, primarily those involving responsiveness and sensitivity. These are commonly mentioned in humans as important parenting characteristics that inform their infants about their environments.
Estrogen, progesterone, prolactin, and oxytocin
Nonhuman females Many nonhuman studies can be used as both potential models for humans and to show the phylogenetic conservation of some endocrine signals. Estrogen and progesterone released by ovaries during pregnancy make oxytocin receptors more sensitive in female rats and is associated with the onset of maternal behaviors in other species as well. Maestripieri found a very similar relationship in which estrogen and progesterone are increased during pregnancy whereas oxytocin was increased postpartum. The presence of estrogen, progesterone, and estradiol in pregnant mammals in some species appears to exhibit a correlation to maternal behavior in the mammals before and after the birth of their offspring as well as in interactions with other offspring
However, an increase in hormones influences maternal behavior, but it is not always the cause of the onset of maternal behavior in females. Some studies on primates in which increased estrogen and progesterone have a negative or absent correlation with maternal responsivity are in black tufted-ear marmosets, common marmosets, lowland gorillas, and baboons. Alternatively, one experimental study showed that nulliparous rats, which tend to avoid pups, were transfused with postpartum rat blood that is high in estrogen and progesterone which resulted in responsiveness to the pups' cues. Due to this variation between species, the effects of the hormones listed does not give much weight to the phylogenetic conservation of these neuroendocrine mechanisms; although Saltzman points out that the social structure of some species may be significant. In social species, previous exposure to infants relies less on these hormones to activate mechanisms and more on modulating maternal behavior, because parenting behaviors are not always dependent on hormones.
In non-human primates, specifically lactating females of multiple species, increased estrogen, progesterone, and prolactin are thought to promote parenting behavior. These species include black tufted-ear marmosets, baboons, rhesus macaques, and gorillas. Endogenous signals such as an increase in estrogen compared to a decrease in progesterone, causes an increase in the levels of prolactin, the "lactating hormone," in the bloodstream. Furthermore, exogenous cues from infants such as suckling induces this mechanism. The hormone oxytocin, similarly to prolactin, has been found to increase with an increase in estrogen and the presence of infant cues such as suckling.
Oxytocin has been found in other non-human species to inhibit the rejection of offspring; oxytocin is essential for responsive and sensitive caregiving. Some specific examples include Francis's study on female rats which linked high amount of oxytocin receptors to increased grooming, and another study by Maestripieri which linked oxytocin levels in free-ranging macaques to increased nursing and grooming.
However, experimental results are less conclusive. As aforementioned, nulliparous mice do not respond to pup calls, but when administered with oxytocin, they do. A similar study conducted by Holman and Goy tested nulliparous rhesus females, where their behavior post injection did not elicit a drastic response to infants, however, the adults presented a change in their behavior with a notable increase in proximity and touching. Oxytocin is more often described as a hormone that facilitates bonding and not one that directly increases care. Also, the mice were responding to pup calls and the rhesus macaque infants weren't necessarily providing cues that would induce maternal care and support. Saltzman proposes that this is due to primates living socially and having a slower developmental trajectory, in which learning is more important.
The function of oxytocin may lead to an increase in maternal behavior by subsequently reducing anxiety as it has been found to regulate anxiety, social recognition, and coping with stress. Early studies have found that oxytocin influenced maternal behavior of mother rats depending on the environment in which they were placed. Oxytocin seemed to have an opposing effect on anxiety so that when placed in a novel environment as opposed to a familiar one, mother rats were better able to cope with their higher levels of stress due to their increased oxytocin levels.
Human females Like in many nonhuman animals, human mothers go through a period of high progesterone during pregnancy that is followed by a decrease in progesterone and a subsequent increase in estrogen, prolactin and oxytocin near parturition. During pregnancy and postpartum, a high estradiol to progesterone ratio is associated with mothers reporting higher feelings of attachment. High levels of progesterone, which are associated with pregnancy, inhibits prolactin and therefore lactation. Prolactin increases during the initial stages of lactation and can be stimulated by estrogen but not progesterone. Research, however, focuses on the role of prolactin for breastfeeding and less on other behaviors. Prolactin increases with infant suckling, but not other forms of infant contact. Oxytocin on the other hand increases with both suckling, and physical contact in human females.
Oxytocin levels in human females are associated with the degree of physical affection and bonding. Feldman (2010) found that mothers who displayed "high affectionate contact" had increased oxytocin levels post interaction, but not mothers who displayed "low affectionate contact." Oxytocin is believed to provide a feedback loop, meaning that maternal-infant contact increases oxytocin and oxytocin increases maternal behavior and facilitates bonding. In one study oxytocin also played a role on mother reported attachment to her fetus. Studies have shown that plasma oxytocin in pregnant women is higher compared to non-pregnant women.
Nonhuman males Wynne-Edwards and Timonin recognize that paternal care is not primed in the same way as maternal care simply because males do not undergo pregnancy. Therefore, males do not go through the same hormonal changes as women. The simplest way, through natural selection, for paternal care to evolve or be maintained is to use the same or similar pathways as females. Wynne and Reburn (2001) suggest that fathers who are pair bonded and spend time with the soon to be mother may activate paternal pathways through various cues.
Estradiol increases just before their offspring's birth in black-tufted-ear marmosets and dwarf hamsters and possibly activates certain pathways involved in paternal behavior. This is similar to estrogen and progesterone in pregnant females. However, the manipulation of estradiol does not increase or decrease paternal behaviors. This may be similar to the finding that women who do not breastfeed or do not have vaginal births still responds to their infants.
Like expecting and new mothers, fathers in multiple mammals have elevated prolactin levels compared to non-fathers. These species include California mice, Mongolian gerbils, dwarf hamsters, meerkats, marmosets, and cotton-top tamarins.
There are variable results in between the effects of oxytocin on paternal care between males of different species. Oxytocin levels are unchanged in California mice before and after becoming fathers, but the amount of paternal exposure to rats is associated with an increase in oxytocin and increased care. However, multiple studies on biparental species show an association between paternal care and oxytocin.
Human males In human mothers, oxytocin is associated with high physical contact and affection. However, studies on fathers show that oxytocin is related to high stimulatory contact and exploratory play. This supports three hypotheses: Mothers and fathers play different roles. Mothers and fathers have similar pathways. That there are parallels to animal models, assuming that a high amount of OT receptors in nonhuman animals is associated with a high amount of oxytocin
One study exhibited the proposed effects that oxytocin had on Tsimane men who had been hunting for varying periods of time. Once the men returned home, it was found that oxytocin levels were higher in those men who had hunted for longer periods of time. As a result of the longer time period spent hunting, the increased levels of oxytocin were thought to be interconnected with familial social contact dating back to humans' evolutionary past.
Comparison of estrogen, progesterone, prolactin, and oxytocin between species and sex Across multiple species and in some cases across sexes, there is evidence for the phylogenetic conservation of parental hormones. These include the relationships between the hormones estrogen, progesterone, prolactin, and oxytocin. In males across species, including humans, increased prolactin levels are associated with fatherhood.
In both non-human primates and humans, the increase in estrogen and progesterone during pregnancy is often followed by a decrease in progesterone and an increase in prolactin, postpartum. One study utilized Silastic implants containing estradiol and progesterone which were implanted in nulliparous rats that either maintained the function of their pituitary glands or underwent a hypophysectomy to determine the length of time for maternal care to ensue. The rats with hypophysectomies exposed to these conditions for 5-6 days were not affected by the treatment and consequently did not induce any maternal behavior. However, it was found that those rats with functioning pituitary glands exhibited maternal behavior within about 3-4 days.
In some studies on females across species, estrogen and progesterone prepartum is also related to oxytocin. Although, the relationships between these hormones is similar across species, there is variation in the degree to how oxytocin effects behavior. For example, in some species, like rats, an increase in oxytocin greatly increases interactions with infants, but an increase in oxytocin in macaques only mildly increased interactions. However, the importance here is that oxytocin increased interactions in all of the relevant cited studies for females, as well as the majority of studies cited for males.