Parenting as an adaptive problem in mammals involves specific endocrine signals that were naturally selected to respond to infant cues and environmental inputs.
The importance of these mechanisms are to regulate parental investment and to inform offspring about their environment, primarily those involving responsiveness and sensitivity.
[1] Estrogen and progesterone released by ovaries during pregnancy make oxytocin receptors more sensitive in female rats[8] and is associated with the onset of maternal behaviors in other species as well.
[4] On the other hand, in non-human primates, specifically lactating females of multiple species, there is an alarming correlation with increased estrogen, progesterone, and prolactin.
[20][21][4][22] Some specific examples include Francis's study on female rats which linked high amount of oxytocin receptors to increased grooming,[23] and another study by Maestripieri which linked oxytocin levels in free-ranging macaques to increased nursing and grooming.
[26][25][27][11] 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.
[28] Early studies have found that oxytocin influenced maternal behavior of mother rats depending on the environment in which they were placed.
During pregnancy and postpartum, a high estradiol to progesterone ratio is associated with mothers reporting higher feelings of attachment.
[35] 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.
[35] 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.
These species include California mice,[38] Mongolian gerbils,[39] dwarf hamsters,[40] meerkats,[41] marmosets,[42] and cotton-top tamarins.
[citation needed] There are variable results in between the effects of oxytocin on paternal care between males of different species.
[44][45][35][46] Since the species in these studies are biparental, excluding rats, it is unclear as to why California mice do not have a change in oxytocin postpartum.
[22][47] This supports three hypotheses: One study exhibited the proposed effects that oxytocin had on Tsimane men who had been hunting for varying 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.
[citation needed] Across multiple species and in some cases across sexes, there is evidence for the phylogenetic conservation of parental hormones.
[22] One study examined the effects that intranasal oxytocin spray administration has in relation to individuals' childhood experiences of punishment by maternal love withdrawal.
[52] It was found that oxytocin effects were absent in individuals who experienced high maternal love withdrawal indicating that the parental behavior associated with withdrawal causes alterations in the genetic expression of endogenous oxytocin levels which affects their children into adulthood.
Contrary to the positive effects of oxytocin on maternal behavior, heightened levels of cortisol postpartum has been linked to a decrease in maternal care in nonhuman species, including the western lowland gorilla,[53] baboons,[54] Japanese macaques,[55] and rhesus macaques.
The functions of prolactin have been extensively studied on rats which has revealed its effects and profound role in maternal care.
The role of prolactin has been found to induce the maternal behavior in nulliparous rats exposed to a hypophysectomized steroid treatment as noted in which prolactin secreting pituitary implants were placed under the kidney capsule which caused a shortened latency to participate in maternal behavior towards foster pups.
[28] The functions of prolactin have been extensively studied on rats which has revealed its effects and profound role in maternal care.
The role of prolactin has been found to induce the maternal behavior in nulliparous rats exposed to a hypophysectomized steroid treatment as noted in which prolactin secreting pituitary implants were placed under the kidney capsule which caused a shortened latency to participate in maternal behavior towards foster pups.
A different study used non-hypophysectomized, steroid-treated nulliparous rats were exposed to a dopamine D2 agonist, called bromocriptine, used to decrease the release of prolactin.
Another experiment also utilized bromocriptine to inhibit the release of prolactin in mother rats who were lactating to their pups during a 2-5-day period.
One study demonstrated that a deficiency in prolactin during the postnatal period in rats has the potential to affect their maternal behavior.
[50] Studies have been conducted that show an interaction between brain circuits that respond to baby-stimuli, such as infant cries, and testosterone and oxytocin pathways.
As such, there is speculation that increasing the availability of testosterone and oxytocin alters the maternal brain to induce a non-aversive response to infant cries.