Prolactin's mediative role in male parenting in parentally experienced marmosets (Callithrix jacchus)

Abstract

Prolactin has been implicated in promoting paternal care behaviors but little evidence of causality has been found to date except for birds and fish. This study was designed to examine the possible causal relationships between prolactin and male parenting behaviors, reproductive hormones, and physical changes in cooperatively breeding common marmosets, Callithrix jacchus. Fifteen parentally experienced fathers were studied over three consecutive infant care periods during two weeks prior and three weeks following their mates' parturition under three-treatment conditions: normal control pregnancy, decreased prolactin and elevated prolactin. The treatments significantly altered the serum prolactin levels in the fathers. Using three methods of determining a father's level of parental care: infant carrying, family effort and responsiveness to infant stimulus tests, we found that only the male response to infant stimuli was altered by the hormone treatments. Lowering prolactin significantly reduced male responsiveness to infant stimuli but elevating prolactin showed the same effect. Hormonal sampling indicated that testosterone levels showed an inverse relationship to prolactin levels during a normal peripartum period and prolactin treatment reduced this relationship. Prepartum estradiol levels were significantly elevated during the lowered prolactin treatment and estradiol was significantly lowered postpartum with the elevated prolactin treatment. Father's weight decreased significantly by the third week of infant care during the normal treatment. Males in the elevated prolactin treatment lost little or no weight from prepartum while in the lowered prolactin treatment showed the most weight loss. The present findings did not distinguish a direct causal relationship of prolactin on behavior in experienced fathers but did find an interaction with other hormones and weight gain.

Figure 1

Mean serum prolactin levels in experienced male marmoset fathers during the prepartum (last two weeks prior to birth) and weeks 1, 2 & 3 following birth. All males under went three succesive treatments during their peripartum periods: lowering prolactin with a D₂ agonist, normal control, and increased prolactin in random order. All males were actively involved in infant care during this time. Prolactin levels were significanly different by treatment (p=0.0001) and by week (p=0.03) in the increased condition.

Figure 2

Mean frequency scores per observation in parentally experienced fathers for their infant directed behaviors under three treatments: lowered prolactin (D₂ agonist), nontreated control, elevated prolactin (human recombinant). Males showed no differences in behavioral frequencies between treatments for their family effort behaviors or percent carrying effort in the family. Males did show significant differences between treatments for their infant responsivenss behaviors, * = p<0.05.

Figure 3

Mean level of estradiol (left panel) and testosterone (right panel) pre and post partum during three treatments: normal, lowered and elevated. Lowering prolactin significantly increased estradiol pre partum while elevating prolactin decreased estradiol. Testosterone was significantly reduced under normal treatment postpartum.

Figure 4

Mean weight (in grams), for 15 marmoset fathers during a normal peripartum period (normal treatment). Father's weight was significantly lower during the third infant carrying week than prepartum (P < 0.05).

Figure 5

Mean weight loss for parentally experienced fathers during the three week postpartum period from the prepartum period for 13 fathers by treatment. Fathers lost significantly more weight in the lowered prolactin condition than in the increased prolactin condition (p=0.01).