Steroid Hormones and Monogamy – A Complex Relationship

In the last post, the social implications of the neuropeptides vasopressin and oxytocin were synthesized in order to question their role in the occurrence of monogamy. In this post, the focus will be on steroid hormones such as testosterone and estrogen. These molecules differ from neuropeptides in that they can readily cross the blood-brain barrier. It should be noted some researchers hypothesize that many monogamous behaviors occur due to a combination of effects induced by neuropeptides and steroid hormones. These molecules can also act in feedback loops with each other, sometimes causing researchers to observe the same changes in behavior when manipulating different hormones (for example estrogen and oxytocin) (Cushing, 2016).

Although there are exceptions, (some of which will be brought up in this post) high levels of testosterone and estrogen generally correlate to sexual behavior / mating behavior. This  is likely why in many species, there is a negative correlation between testosterone levels and parental care. An analogous relationship between estrogen and parental care seems to be absent, as estrogen levels remain high in mothers even after giving birth in many animals. It seems however, that estrogen may play an important role in parental care – even in males.

In deer mice (Peromyscus), it has been shown that both estrogen and testosterone can promote parental care and monogamous behavior (Trainor, Bird, Alday, Schlinger, & Marler, 2003; Trainor Brian C. & Marler Catherine A., 2002). Both steroid hormones can influence the same behaviors because estrogen is synthesized from testosterone via an enzyme called aromatase. Therefore, an increase in testosterone could lead to an increase in estrogen, assuming that the individual has a sufficient amount of aromatase for chemical conversion.

The conversion of testosterone to estrogen is necessary for specific parental and mating behavior in deer mice. It has been shown that areas of the brain in this species known to be involved in parental care such as the medial preoptic area upregulate aromatase activity in fathers compared to individuals without pups (Trainor et al., 2003). Progesterone levels were also inversely correlated with parental care (Trainor et al., 2003). This is in line with intuitive thinking regarding human biology, considering progesterone levels drop dramatically after birth. As previously stated however, these steroid hormones are normally inversely correlated with parental care and positively correlated with sexual/mating behavior. One possibility as to why some species don’t follow this general rule could be because they have overlapping periods of mating and parental care. One study done by Trainor Brian C. & Marler Catherine A. (2002) supported this hypothesis, and stated that in species with a postpartum estrous cycle (meaning that the females are fertile after they give birth), males can display testosterone mediated behaviors such as mating (especially extra-pair/non-monogamous) and aggression simultaneously to parental care (Trainor Brian C. & Marler Catherine A., 2002). This just shows how complex the evolution of monogamy is. Although steroid hormones influence mating behavior and parental care, the type of relationship (inversely or positively correlated) that steroid hormones have on behavior is dependent on the species in question, and likely what strategy is most fit for that species.

In prairie voles with distinct phenotypic differences regarding mating strategy, it is true that higher estrogen levels are inversely correlated with monogamous behavior. When comparing voles from Illinois (socially monogamous) with voles from Kansas, (more polygynous) researchers found that the Illinois voles had lower levels of estrogen a receptors in brain regions such as the amygdala and stria terminalis compared to Kansas voles (Kramer, Carr, Schmidt, & Cushing, 2006). In a monogamous song-bird, (blue-headed vireo) administration of exogenous testosterone can decrease parental care in males, and increases song production (a mating behavior) (Van Roo, 2004). This can also happen in animals such as deer mice, implying that regardless of the type of relationship steroid hormones have on mating strategy and parental care, there seems to be an optimal level for these molecules that differs depending on the species, and that “normal” behavior can be disrupted in a variety of species by manipulating the levels of these molecules – whether that be agonizing (increasing amounts) or antagonizing (decreasing amounts) them.

Monogamous behavior is not always associated with less estrogen and less testosterone. Steroid hormones have complex relationships with other chemicals such as neuropeptides, and can have vastly different effects on parental care and mating behavior depending on the species.  Evolution often favors what keeps the offspring alive and what maximizes reproductive output for the parent, which is evidently what happened in the examples above.


Cushing, B. S. (2016). Estrogen Receptor Alpha Distribution and Expression in the Social Neural Network of Monogamous and Polygynous Peromyscus. PLOS ONE, 11(3), e0150373.

Kramer, K. M., Carr, M. S., Schmidt, J. V., & Cushing, B. S. (2006). Parental regulation of central patterns of estrogen receptor α. Neuroscience, 142(1), 165–173.

Trainor, B. C., Bird, I. M., Alday, N. A., Schlinger, B. A., & Marler, C. A. (2003). Variation in Aromatase Activity in the Medial Preoptic Area and Plasma Progesterone Is Associated with the Onset of Paternal Behavior. Neuroendocrinology, 78(1), 36–44.

Trainor Brian C., & Marler Catherine A. (2002). Testosterone promotes paternal behaviour in a monogamous mammal via conversion to oestrogen. Proceedings of the Royal Society of London. Series B: Biological Sciences, 269(1493), 823–829.

Van Roo, B. L. (2004). Exogenous testosterone inhibits several forms of male parental behavior and stimulates song in a monogamous songbird: the blue-headed vireo (Vireo solitarius). Hormones and Behavior, 46(5), 678–683.

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