A new study from researchers at the University of Maryland reveals the distinct physiological differences in the effects of mate separation on male and female birds. These findings might help pave the way toward a deeper understanding of the ways in which male and female humans process separation from a mate on physical levels.
Published in Hippocampus, the study observed zebra finches in order to determine if their gene expression would change in response to mate separation. Zebra finches are monogamous, biparental avian species that form lifelong pair bonds. They are also known to exhibit many sex differences in both brains and behavior. The research team, who initiated the study while they were at Johns Hopkins University, is led by Farrah Madison, Ph.D., UMD research assistant professor of psychology, and Andrew J. Kesner, researcher at the National Institute on Drug Abuse. Gregory Ball, Ph.D., dean of UMD’s College of Behavioral and Social Sciences and a professor of psychology and of biology, and Beau A. Alward, Ph.D., a postdoctoral fellow at Stanford University, co-authored the study.
The team investigated the effects of mate pair separation by measuring and evaluating changes in gene expression in the brain that are related to the functioning of stress hormones. They measured the effects of mate pair separation on circulating corticosterone concentrations (the main stress hormone in birds), as well as changes in mineralocorticoid receptor (MR), glucocorticoid receptor (GR), and corticotropin releasing hormone (CRH) gene expression in the hippocampus and hypothalamus in both male and female finches. The MR and GR are receptor proteins that regulate the secretion of these adrenal hormones through a feedback system between hormone concentrations in the blood and the brain.
The birds were assessed in three scenarios: the male or female being removed from their respective mate and placed in a cage with a new opposite sex conspecific and stimulus female, the male or female remaining with their mate, but a new stimulus female was introduced, or the subjects were handled but not separated from their mate or the stimulus female.
As previously observed, the researchers found significant increases in plasma corticosterone concentrations in response to both mate pair and stimulus female separation in both males and females. The disruption of the social setting in the birds increased stress hormone secretion.
No effects of treatment on gene expression were observed in the hypothalamus—the region in the forebrain of both birds and humans that links the nervous system to the endocrine system. But in the hippocampus, the region that sits at the top of the brain in birds and plays a significant role in memory, spatial awareness and the regulation of psychological hormone responses to stress in all vertebrates, females exhibited a significant up regulation in hippocampal MR—but not GR mRNA—whereas males exhibited a significant down regulation of both hippocampal MR and GR mRNA in response to mate pair separation. Such decreases in receptor expression in the human hippocampus are associated with depression.
“This sex-dependent response to mate loss suggests that male and female zebra finches may perceive the stressful effects of mate pair separation in different ways, and MRs may influence behavioral flexibility in females,” Madison said. “This is especially interesting, as we already know that in avian species, females are more likely to leave a pair than males.”
“Our results potentially shed light on neurobiological mechanisms that factor into the observed sex differences in how males and females of other species—including perhaps humans—perceive and respond to stressful psychological events such as the death of a spouse, divorce, or marital separation,” Ball said. “For example, in humans, men have been shown to be more apt to demonstrate negative health outcomes in response to the dissolution of a marriage than women, suggesting that women cope with divorce better than men.”
“While it is still too early to extrapolate possible implications of this study on parallel findings in human subjects, our project could potentially lay some groundwork in such an examination,” added Madison.