Prenatal Steroid Hormones and Sex Differences in Juvenile Rhesus Macaque Behavior

Abstract

Rhesus monkeys (Macaca mulatta) have been the primary primate model for investigating hormonal organization of juvenile sexually dimorphic behavior, primarily rough play and mounting. Large doses of androgens administered to pregnant females for 75 or more days of gestation masculinized the genitalia and juvenile behavior of female offspring. Unlike in rodents, estrogenic metabolites of androgens do not appear to play a role in behavioral sexual differentiation of rhesus monkeys as the nonaromatizable androgen, 5α-dihydrotestosterone, produced comparable behavioral masculinization in this species. Because prenatal androgen treatments masculinized both behavior and genitalia, some argued that the behavioral changes seen in androgenized rhesus monkey females reflect socialized responses to their genital’s male-like appearance. By varying the timing of prenatal androgen exposure, the effects of androgens on genitals and behavior were separated. Thirty-five-day androgen treatments in early gestation masculinized female genitalia and mounting in rhesus monkeys, but did not masculinize rough play. By contrast, treatments late in gestation did not masculinize genitalia, but masculinized both rough play and mounting, thus separating genital effects of androgens from behavioral effects. Subsequent work with androgens and antiandrogens identified late gestation as a time when behavioral systems are particularly sensitive to androgens. A study of monkey’s preference for human sex-typed toys found sex differences remarkably similar to those reported in children. Since the sex-typed nature of the toys would be unknown to the monkeys, the preference likely reflects a sex-difference in the predisposition for activities facilitated by the toys. Sexually differentiated behavior ultimately reflects both hormonally organized behavioral predispositions and the social experience that converts these predispositions into behavior.

Spotlight Feature: The Implications of Social Environment for Sex Differences in Brain and Behavior

• Mariela Faykoo-Martinez &
• Melissa M. Holmes 

1. Department of Cell & Systems Biology, University of Toronto, Toronto, ON, Canada

   Mariela Faykoo-Martinez & Melissa M. Holmes

2. Department of Psychology, University of Toronto Mississauga, Mississauga, ON, Canada

   Melissa M. Holmes

3. Department of Ecology & Evolutionary Biology, University of Toronto, Toronto, ON, Canada

   Melissa M. Holmes

Naked mole-rats (Heterocephalus glaber) do not fit the traditional framework of mammalian sexual development in that most individuals remain pre-pubertal for the duration of their [extraordinarily long] lives (>28 years) (Buffenstein, 2005; Jarvis, 1981). Native to East Africa, naked mole-rats are small eusocial rodents, living in underground colonies of up to 300 individuals (Jarvis, 1981). Eusociality refers to their rigid social hierarchy where breeding is restricted to one dominant female (the queen) and 1–3 males. All other colony members are socially subordinate and reproductively suppressed. These “subordinates” are remarkably sexually monomorphic for adults of a sexually reproducing species, failing to show many of the sex differences in behavior, gross morphology, endocrinology, and neural morphology (Holmes et al., 2009) that are highly conserved in mammals. For example, subordinates of both sexes participate equally in both prosocial and agonistic interactions (Lacey & Sherman, 1991; Mooney et al., 2015). Male and female subordinates are of similar body size and weight (Dengler-Crish & Catania, 2007; Peroulakis et al., 2002) and, unlike other rodents, there is no sex difference in anogenital distance, with possible male feminization of the genitalia such that the external penis resembles a clitoris in shape and size (Jarvis, 1981; Peroulakis et al., 2002; Seney et al., 2009) (Fig. 2.5). Furthermore, the perineal muscles used in copulation are sexually monomorphic in subordinate naked mole-rats despite showing dimorphism in most mammalian species (Peroulakis et al., 2002). Finally, no sex differences are seen in circulating gonadal steroid hormones in subordinates (Clarke & Faulkes, 1998; Faulkes, Abbott, & Jarvis, 1990; Swift-Gallant et al., 2015; Zhou et al., 2013), and we failed to find sex differences in regional brain volume, cell number, or cell size in reproductively relevant brain regions known to be sexually differentiated in other mammals (e.g., medial amygdala) (Holmes et al., 2007).

Crucially, subordinate naked mole-rats are neither asexual nor sterile. They can become reproductive if removed from the suppressive influence of the queen, showing the endocrine and behavioral transitions characterized as mammalian puberty. We often see these changes occurring in both males and females. For example, RFamide-related peptide-3 immunoreactivity (the mammalian ortholog of gonadotropin inhibitory hormone) is lower in breeders than subordinates, regardless of sex, in the dorsomedial and arcuate nuclei of the hypothalamus (Peragine et al., 2017). This protein is thought to be a main player contributing to reproductive suppression in subordinates. Similarly, breeders of both sexes have larger regional volumes of the medial amygdala, bed nucleus of the stria terminalis, and paraventricular nucleus of the hypothalamus when compared to subordinates. Thus, for some variables, social/pubertal status is a better predictor than sex. However, for other variables, the release from pubertal suppression seems to trigger the emergence of sex differences. Indeed, sex differences in behavior emerge post-puberty as both males and females begin to display sex-typical reproductive behaviors (e.g., mounting in males and lordosis in females). Urinary progesterone increases in females within days of removal from the colony and the vagina becomes perforate (Faulkes, Abbott, Jarvis, & Sherriff, 1990). As the female begins to breed, she will become longer as her vertebral column lengthens with each litter born (Dengler-Crish & Catania, 2007). Alternatively, the male endocrine transition is marked by an increase in urinary testosterone and, over time, breeding males often decrease in size. This emergence of sex differences extends to the nervous system where differences in gene and protein expression have been reported (Faykoo-Martinez et al., 2018; Holmes et al., 2008; Swift-Gallant et al., 2015; Zhou et al., 2013). For example, female breeders have higher expression of estrogen receptor alpha and progesterone receptor mRNA and higher numbers of kisspeptin immunoreactive cells relative to male breeders and subordinates of both sexes (Swift-Gallant et al., 2015; Zhou et al., 2013), and these sex differences are all directly related to ovulation in female mammals. Not all emergent sex differences are in keeping with mammalian “norms,” however. For example, the breeding female is socially dominant and the most aggressive individual in the colony (Clarke & Faulkes, 2001; Reeve, 1992), pushing and shoving other colony members. This is key as it reveals species-specific patterns of sex differences in brain and behavior as well as at least some dissociation between sex- and gender-typical variables.

All of this is consistent with the idea that sex is “less important” than social/pubertal status for sculpting brain and behavior in this species. More recently, however, we discovered that molecular sex differences exist in the brains of subordinates. Specifically, stress-related genes have higher expression in socially relevant brain regions (e.g., nucleus accumbens) in males compared to females (Faykoo-Martinez et al., 2018), suggesting that reproductive suppression is controlled, at least in part, in a sex-specific way. Thus, on the one hand, socially mediated pubertal suppression might prevent or delay the emergence of sex differences but, on the other, sex differences in mechanism might be critical for allowing pubertal suppression to exist in both sexes. That is to say, sex-specific mechanisms underlying reproductive suppression may serve to compensate for sex chromosomal gene expression, ultimately bringing males and females closer together on many variables (De Vries, 2004).

Studying diverse species like the naked mole-rat allows us to better understand the complex interplay between an organism’s biological sex and sociosexual environment. From an evolutionary perspective, we learn about how species-specific social organization and reproductive strategy is associated with the type and magnitude of sex differences present in a species (e.g., sexual selection). From an organismal perspective, we learn about how social cues influence the development and maintenance of sex differences across the life span, which is hugely important for understanding individual differences in, and plasticity of, sex and gender variables. Employing both perspectives is necessary for understanding the causal relationship(s) between sex differences in brain and sex differences in behavior and can challenge how we think about sex and gender in both human and non-human animals.

Fig. 2.5

Naked mole-rats show reduced sexual dimorphism compared to other mammals. (a) Breeders (BRE), particularly queens, are typically larger and heavier than subordinates (SUB). Subordinates are not sexually dimorphic in overall body size (a) or external genitalia (b and c). The genital mound enlarges in queens and she also develops a perforate vagina (b). The animals pictured here are all young to middle-aged adults ranging between 5 and10 years of age. Their weights are as follows: BRE female = 53.2 g, BRE male = 44.0 g, SUB female = 34.9 g, SUB male = 36.3 g

Funding Acknowledgment

This work was supported by NSERC PGS D and ACM/Intel SIGHPC Computational and Data Science Fellowship (to MFM) and NSERC Discovery Grant RGPIN 2018-04780, NSERC Discovery Accelerator Supplement RGPAS2018-522465, and Ontario Early Researcher Award (to MMH).

Spotlight references

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