Sexual Dimorphism in the Brain of the Monogamous California Mouse (Peromyscus californicus)

Abstract

Sex differences in behavior and morphology are usually assumed to be stronger in polygynous species compared to monogamous species. A few brain structures have been identified as sexually dimorphic in polygynous rodent species, but it is less clear whether these differences persist in monogamous species. California mice are among the 5% or less of mammals that are considered to be monogamous and as such provide an ideal model to examine sexual dimorphism in neuroanatomy. In the present study we compared the volume of hypothalamic- and limbic-associated regions in female and male California mice for sexual dimorphism. We also used tyrosine hydroxylase (TH) immunohistochemistry to compare the number of dopamine neurons in the ventral tegmental area (VTA) in female and male California mice. Additionally, tract tracing was used to accurately delineate the boundaries of the VTA. The total volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA), the principal nucleus of the bed nucleus of the stria terminalis (BNST), and the posterodorsal medial amygdala (MEA) was larger in males compared to females. In the SDN-POA we found that the magnitude of sex differences in the California mouse were intermediate between the large differences observed in promiscuous meadow voles and rats and the absence of significant differences in monogamous prairie voles. However, the magnitude of sex differences in MEA and the BNST were comparable to polygynous species. No sex differences were observed in the volume of the whole brain, the VTA, the nucleus accumbens or the number of TH-ir neurons in the VTA. These data show that despite a monogamous social organization, sexual dimorphisms that have been reported in polygynous rodents extend to California mice. Our data suggest that sex differences in brain structures such as the SDN-POA persist across species with different social organizations and may be an evolutionarily conserved characteristic of mammalian brains.

Effects of kappa opioid receptors on conditioned place aversion and social interaction in males and females

“…In addition to considering short term and long term effects of KOR signaling, it will be important to examine the behavioral effects of KOR agonists in females of different rodent species. Male and female California mice exhibit sex differences in brain anatomy (Campi et al, 2013), immune responses (Klein and Nelson, 1997), and aggressive behavior (Trainor et al, 2011) that are similar to domestic rats and mice. However sex differences in body size (Williams et al, 2013), parental behavior (Bester-Meredith and Marler, 2003) and glucocorticoid levels (Harris et al, 2012) that are commonly observed in domestic rodents are less pronounced or absent in California mice.…”

“…Images of the left and right side of each brain area were imaged with a Zeiss AxioImager based on a mouse brain atlas and previous descriptions of the California mouse NAc and BNST (Campi et al, 2013). The background for each image was normalized by adjusting the exposure time.…”

The size of non-hippocampal brain regions varies by season and sex in Richardson’s ground squirrel

“…It may seem incongruous that we failed to find many sex differences when so many behaviors in RGS are sexually dimorphic (Michener, 1998), but sex differences in brain region sizes, and the magnitude of those differences, are not always consistent across species or studies (Hines et al, 1992;Noonan et al, 1998;Galea and McEwen, 1999;Franklin et al, 2000;Spring et al, 2007;Iaskin, 2011;Kingsbury et al, 2012;Campi et al, 2013;Sawada et al, 2013). Indeed, there are several examples of brain regions that are sexually dimorphic in size in one study and not in another.…”

“…Examples of this include the song system of songbirds (MacDougall-Shackleton and Ball, 1999;Tramontin and Brenowitz, 2000;Ball et al, 2008) and, in mammals, the hippocampus (Yaskin, 1994;Iaskin, 2011), prefrontal cortex (Kingsbury et al, 2012), medial amygdala (Hines et al, 1992), white matter tracts (Noonan et al, 1998;Franklin et al, 2000) and various other limbic and hypothalamic structures (Spring et al, 2007;Campi et al, 2013;Sawada et al, 2013). Functionally, sexually dimorphic volumes of brain regions are associated with sex differences in behaviour.…”

“…Functionally, sexually dimorphic volumes of brain regions are associated with sex differences in behaviour. Typically, the sex that exhibits the larger volume exhibits improved ability in a task (Sherry et al, 1992;Reboreda et al, 1996;Ball et al, 2008;de Vries and Sodersten, 2009;Costa et al, 2011;Krohmer et al, 2011;Campi et al, 2013;Keeley et al, 2013). Many studies have demonstrated that some of these sex differences arise from the effects of endogenous variation in steroid hormone concentrations on brain anatomy (Williams et al, 1990;Vanhaaren et al, 1990;Roof and Havens, 1992;McEwen, 2002;Isgor and Sengelaub, 2003;Korol, 2004;Schoening et al, 2007;Ball et al, 2008;Brenowitz, 2008;Mitsushima et al, 2009;Mizuno and Giese, 2010).…”