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Edwards PD, Toor I, Holmes MM. The Curious Case of the Naked Mole-Rat: How Extreme Social and Reproductive Adaptations Might Influence Sex Differences in the Brain. Curr Top Behav Neurosci 2023; 62:47-70. [PMID: 35301704 DOI: 10.1007/7854_2022_310] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
Research in the neurobiology of sex differences is inherently influenced by the study species that are used. Some traditional animal research models, such as rats and mice, show certain sex differences in the brain that have been foundational to neurobiological research. However, subsequent work has demonstrated that these differences are not always generalizable, especially to species with different social structures and sex-associated roles or behaviors. One such example is the naked mole-rat (Heterocephalus glaber), which has an unusual social structure among mammals. Naked mole-rats live in large groups where reproduction is restricted to a dominant female, called the "queen," and often only one breeding male. All other animals in the group, the "subordinates," are socially suppressed from reproduction and remain in a prepubescent state as adults, unless they are removed from the presence of the queen. These subordinates show little to no sex differences in external morphology, neural morphology, or behavior. However, there are a suite of neurobiological differences between subordinate and breeding naked mole-rats. After naked mole-rats attain breeding status, many of the classically sexually differentiated brain regions increase in volume (paraventricular nucleus, medial amygdala, bed nucleus of the stria terminalis). There are additionally social status differences in sex hormone receptor expression in the brain, as well as other changes in gene expression, some of which also show sex differences - though not always in the predicted direction based on other rodent studies. Data from naked mole-rats show that it is critical to consider the evolved social structure of a species when studying sex differences in the brain.
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Affiliation(s)
- Phoebe D Edwards
- Department of Psychology, University of Toronto Mississauga, Mississauga, ON, Canada
| | - Ilapreet Toor
- Department of Psychology, University of Toronto Mississauga, Mississauga, ON, Canada
| | - Melissa M Holmes
- Department of Psychology, University of Toronto Mississauga, Mississauga, ON, Canada.
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Szafranski K, Wetzel M, Holtze S, Büntjen I, Lieckfeldt D, Ludwig A, Huse K, Platzer M, Hildebrandt T. The Mating Pattern of Captive Naked Mole-Rats Is Best Described by a Monogamy Model. Front Ecol Evol 2022. [DOI: 10.3389/fevo.2022.855688] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Naked mole-rats form colonies with a single reproductively active female surrounded by subordinate workers. Workers perform offspring care, construction and defense of the burrow system, and food supply. Such division of labor, called “cooperative breeding,” is strongly associated with the evolution of monogamous mating behavior, as seen in several mammalian lineages. This association is explained by the evolutionary theory of kin selection, according to which a subordinate adult may help to raise other’s offspring if they are in full sibling relationship. In conflict with this theory, the naked mole-rat is widely considered to be polyandrous, based on reports on multiple males contributing to a colony’s progeny. In order to resolve this contrast, we undertook an in-depth microsatellite-based kinship analysis on captive colonies. Four independent colonies comprising a total of 265 animals were genotyped using a panel of 73 newly established microsatellite markers. Our results show that each mole-rat colony contains a single monogamous breeder pair, which translates to a reproductive skew of 100% for both sexes. This finding, also in conjunction with previously published parental data, favors monogamy as the best-fitting model to describe naked mole-rat reproduction patterns. Polyandry or other polygamous reproduction models are disfavored and should be considered as exceptional. Overall, the empirical genetic data are in agreement with the kin selection theory.
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Faykoo-Martinez M, Kalinowski LM, Holmes MM. Neuroendocrine regulation of pubertal suppression in the naked mole-rat: What we know and what comes next. Mol Cell Endocrinol 2021; 534:111360. [PMID: 34116130 DOI: 10.1016/j.mce.2021.111360] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/03/2021] [Revised: 05/20/2021] [Accepted: 06/03/2021] [Indexed: 01/13/2023]
Abstract
Puberty is a key developmental milestone that marks an individual's maturation in several ways including, but not limited to, reproductive maturation, changes in behaviors and neural organization. The timing at which puberty occurs is variable both within individuals of the same species and between species. These variations can be aligned with ecological cues that delay or suppress puberty. Naked mole-rats are colony-living rodents where reproduction is restricted to a few animals; all other animals are pubertally-suppressed. Animals removed from suppressive colony cues can reproductively mature, presenting the unique opportunity to study adult-onset puberty. Recently, we found that RFRP-3 administration sustains pubertal delay in naked mole-rats removed from colony. In this review, we explore what is known about regulators that control puberty onset, the role of stress/social status in pubertal timing, the status of knowledge of pubertal suppression in naked mole-rats and what comes next.
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Affiliation(s)
| | | | - Melissa M Holmes
- Department of Cell and Systems Biology, University of Toronto, Canada; Department of Psychology, University of Toronto Mississauga, Canada; Department of Ecology and Evolutionary Biology, University of Toronto, Canada
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Social Behavior in Naked Mole-Rats: Individual Differences in Phenotype and Proximate Mechanisms of Mammalian Eusociality. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2021; 1319:35-58. [PMID: 34424512 DOI: 10.1007/978-3-030-65943-1_2] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Naked mole-rats (Heterocephalus glaber) are small rodents native to east Africa, living in subterranean colonies of up to 300 individuals. Within each colony, reproduction is restricted to a single breeding female and 1-3 breeding males; all other colony members are reproductively suppressed and socially subordinate unless removed from the suppressive cues of the colony. Due to their striking reproductive skew, naked mole-rats are often considered eusocial mammals. Consistent with this idea, there are behavioral specializations and at least some evidence for morphological distinctions within and between the breeding and non-breeding members of the colony. Importantly, naked mole-rats show plasticity in their behavioral phenotype whereby changes in the social environment influence expression of both type and amount of social behavior. Thus, naked mole-rats provide the opportunity to examine the proximate mechanisms controlling individual differences in social behavior, shedding light on how mammals live in complex social groups.
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Coen CW, Bennett NC, Holmes MM, Faulkes CG. Neuropeptidergic and Neuroendocrine Systems Underlying Eusociality and the Concomitant Social Regulation of Reproduction in Naked Mole-Rats: A Comparative Approach. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2021; 1319:59-103. [PMID: 34424513 DOI: 10.1007/978-3-030-65943-1_3] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
The African mole-rat family (Bathyergidae) includes the first mammalian species identified as eusocial: naked mole-rats. Comparative studies of eusocial and solitary mole-rat species have identified differences in neuropeptidergic systems that may underlie the phenomenon of eusociality. These differences are found in the oxytocin, vasopressin and corticotrophin-releasing factor (CRF) systems within the nucleus accumbens, amygdala, bed nucleus of the stria terminalis and lateral septal nucleus. As a corollary of their eusociality, most naked mole-rats remain pre-pubertal throughout life because of the presence of the colony's only reproductive female, the queen. To elucidate the neuroendocrine mechanisms that mediate this social regulation of reproduction, research on the hypothalamo-pituitary-gonadal axis in naked mole-rats has identified differences between the many individuals that are reproductively suppressed and the few that are reproductively mature: the queen and her male consorts. These differences involve gonadal steroids, gonadotrophin-releasing hormone-1 (GnRH-1), kisspeptin, gonadotrophin-inhibitory hormone/RFamide-related peptide-3 (GnIH/RFRP-3) and prolactin. The comparative findings in eusocial and solitary mole-rat species are assessed with reference to a broad range of studies on other mammals.
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Affiliation(s)
- Clive W Coen
- Reproductive Neurobiology, Division of Women's Health, Faculty of Life Sciences & Medicine, King's College London, London, UK.
| | - Nigel C Bennett
- Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria, South Africa
| | - Melissa M Holmes
- Department of Psychology, University of Toronto Mississauga, Mississauga, ON, Canada.,Department of Ecology & Evolutionary Biology, University of Toronto, Toronto, Canada.,Department of Cell & Systems Biology, University of Toronto, Toronto, Canada
| | - Christopher G Faulkes
- School of Biological and Chemical Sciences, Queen Mary University of London, London, UK
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Noradrenaline concentrations in the hypothalamus of anoestrus ewes following the ram-induced luteinizing hormone release. Neuroreport 2015; 26:438-43. [DOI: 10.1097/wnr.0000000000000367] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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Fabre-Nys C, Kendrick KM, Scaramuzzi RJ. The "ram effect": new insights into neural modulation of the gonadotropic axis by male odors and socio-sexual interactions. Front Neurosci 2015; 9:111. [PMID: 25914614 PMCID: PMC4391029 DOI: 10.3389/fnins.2015.00111] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2014] [Accepted: 03/16/2015] [Indexed: 11/13/2022] Open
Abstract
Reproduction in mammals is controlled by the hypothalamo-pituitary-gonadal (HPG) axis under the influence of external and internal factors such as photoperiod, stress, nutrition, and social interactions. Sheep are seasonal breeders and stop mating when day length is increasing (anestrus). However, interactions with a sexually active ram during this period can override the steroid negative feedback responsible for the anoestrus state, stimulate luteinizing hormone (LH) secretion and eventually reinstate cyclicity. This is known as the “ram effect” and research into the mechanisms underlying it is shedding new light on HPG axis regulation. The first step in the ram effect is increased LH pulsatile secretion in anestrus ewes exposed to a sexually active male or only to its fleece, the latter finding indicating a “pheromone-like” effect. Estradiol secretion increases in all ewes and this eventually induces a LH surge and ovulation, just as during the breeding season. An exception is a minority of ewes that exhibit a precocious LH surge (within 4 h) with no prior increase in estradiol. The main olfactory system and the cortical nucleus of the amygdala are critical brain structures in mediating the ram effect since it is blocked by their inactivation. Sexual experience is also important since activation (increased c-fos expression) in these and other regions is greatly reduced in sexually naïve ewes. In adult ewes kisspeptin neurons in both arcuate and preoptic regions and some preoptic GnRH neurons are activated 2 h after exposure to a ram. Exposure to rams also activates noradrenergic neurons in the locus coeruleus and A1 nucleus and increased noradrenalin release occurs in the posterior preoptic area. Pharmacological modulation of this system modifies LH secretion in response to the male or his odor. Together these results show that the ram effect can be a fruitful model to promote both a better understanding of the neural and hormonal regulation of the HPG axis in general and also the specific mechanisms by which male cues can overcome negative steroid feedback and trigger LH release and ovulatory cycles.
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Affiliation(s)
- Claude Fabre-Nys
- UMR 7247 Physiologie de la Reproduction et des Comportements, Centre National de la Recherche Scientifique, Institut National de la Recherche Agronomique, Institut Français du Cheval et de L'équitation, Université de Tours Nouzilly, France
| | - Keith M Kendrick
- Key Laboratory for Neuroinformation, University of Electronic Science and Technology of China Chengdu, China
| | - Rex J Scaramuzzi
- Department of Comparative Biological Sciences, Royal Veterinary College South Mimms, UK
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Mooney SJ, Peragine DE, Hathaway GA, Holmes MM. A game of thrones: Neural plasticity in mammalian social hierarchies. Soc Neurosci 2014; 9:108-17. [DOI: 10.1080/17470919.2014.882862] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
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Differential effects of chronic fluoxetine on the behavior of dominant and subordinate naked mole-rats. Behav Brain Res 2014; 258:119-26. [DOI: 10.1016/j.bbr.2013.10.023] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/05/2013] [Revised: 10/11/2013] [Accepted: 10/14/2013] [Indexed: 11/24/2022]
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Zhou S, Holmes MM, Forger NG, Goldman BD, Lovern MB, Caraty A, Kalló I, Faulkes CG, Coen CW. Socially regulated reproductive development: Analysis of GnRH-1 and kisspeptin neuronal systems in cooperatively breeding naked mole-rats (Heterocephalus glaber). J Comp Neurol 2013; 521:3003-29. [DOI: 10.1002/cne.23327] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2012] [Revised: 02/20/2013] [Accepted: 02/26/2013] [Indexed: 12/18/2022]
Affiliation(s)
- Shuzhi Zhou
- Reproductive Neurobiology; Division of Women's Health; School of Medicine; King's College London; London; UK
| | | | | | | | | | - Alain Caraty
- INRA; Physiology of Reproduction; Nouzilly; France
| | | | | | - Clive W. Coen
- Reproductive Neurobiology; Division of Women's Health; School of Medicine; King's College London; London; UK
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Abstract
Naked mole rats are mouse-sized rodents that have become an important animal model in biomedical research. They play a unique mammalian role in behavioral and ecophysiological research of life underground. This chapter studies the general physiology, anatomy of organ systems, husbandry, and uses in research of the naked mole rats. Naked mole rats belong to the order Rodentia in that they have two incisor teeth on the upper and lower arcade that continuously grow. The skin is loose, wrinkled, and brownish pink in color. The body is for the most part absent of hairs with the exception of tactile hairs that are regularly arranged throughout the body and which are particularly prominent around the face and to a lesser extent on the tail. They are typically housed at 28–30°C, and at 50–60% relative humidity. Because naked mole rats are social and have cooperative behaviors, the study of their conduct has more applicability to people. The chapter describes the models of experimental research on the naked mole rat such as the model of reproductive suppression, model of somatosensory processing, model of bone elongation, and model of aging.
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Holmes MM, Seney ML, Goldman BD, Forger NG. Social and hormonal triggers of neural plasticity in naked mole-rats. Behav Brain Res 2010; 218:234-9. [PMID: 21130812 DOI: 10.1016/j.bbr.2010.11.056] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2010] [Revised: 11/22/2010] [Accepted: 11/28/2010] [Indexed: 11/25/2022]
Abstract
Naked mole-rats are eusocial rodents that live in large social groups with a strict reproductive hierarchy. In each colony only a few individuals breed; all others are non-reproductive subordinates. We previously showed that breeders have increased volume of several brain regions linked to reproduction: the paraventricular nucleus of the hypothalamus (PVN), the principal nucleus of the bed nucleus of the stria terminalis (BSTp), and the medial amygdala (MeA). Breeders also have more large motoneurons in Onuf's nucleus (ON) in the spinal cord, a cell group innervating perineal muscles that attach to the genitalia. Here, we sought to determine triggers for the neural changes seen in breeders. Specifically, we compared four groups of animals: subordinates, paired animals that did not reproduce, gonadally intact breeders, and gonadectomized breeders. We find that pairing alone is sufficient to cause breeder-like changes in volume of the PVN and cell size distribution in ON. In contrast, increases in BSTp volume were seen only in animals that actually reproduced. Those changes that were seen in successful breeders appear to be independent of gonadal steroids because long-term gonadectomy did not reverse the breeder-like neural changes in the PVN, BSTp or ON, although a trend for gonadectomized animals having larger MeA volumes was detected. Thus, neural changes associated with breeding status in naked mole-rats may be triggered by different aspects of the social and reproductive environment; once changes occur they are largely independent of gonadal hormones and may be permanent.
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Affiliation(s)
- Melissa M Holmes
- Department of Psychology, University of Toronto Mississauga, 3359 Mississauga Road North, Mississauga, ON, L5L 1C6, Canada.
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Kalamatianos T, Faulkes CG, Oosthuizen MK, Poorun R, Bennett NC, Coen CW. Telencephalic binding sites for oxytocin and social organization: A comparative study of eusocial naked mole-rats and solitary cape mole-rats. J Comp Neurol 2010; 518:1792-813. [DOI: 10.1002/cne.22302] [Citation(s) in RCA: 56] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
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Holmes MM, Goldman BD, Goldman SL, Seney ML, Forger NG. Neuroendocrinology and sexual differentiation in eusocial mammals. Front Neuroendocrinol 2009; 30:519-533. [PMID: 19416733 PMCID: PMC2748139 DOI: 10.1016/j.yfrne.2009.04.010] [Citation(s) in RCA: 61] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/03/2009] [Revised: 04/21/2009] [Accepted: 04/23/2009] [Indexed: 12/21/2022]
Abstract
Sexual differentiation of the mammalian nervous system has been studied intensively for over 25 years. Most of what we know, however, comes from work on relatively non-social species in which direct reproduction (i.e., production of offspring) is virtually the only route to reproductive success. In social species, an individual's inclusive fitness may include contributions to the gene pool that are achieved by supporting the reproductive efforts of close relatives; this feature is most evident in eusocial organisms. Here, we review what is known about neuroendocrine mechanisms, sexual differentiation, and effects of social status on the brain and spinal cord in two eusocial mammals: the naked mole-rat and Damaraland mole-rat. These small rodents exhibit the most rigidly organized reproductive hierarchy among mammals, with reproduction suppressed in a majority of individuals. Our findings suggest that eusociality may be associated with a relative lack of sex differences and a reduced influence of gonadal hormones on some functions to which these hormones are usually tightly linked. We also identify neural changes accompanying a change in social and reproductive status, and discuss the implications of our findings for understanding the evolution of sex differences and the neuroendocrinology of reproductive suppression.
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Affiliation(s)
- Melissa M Holmes
- Center for Neuroendocrine Studies and Department of Psychology, University of Massachusetts, Amherst, MA 01003, USA
| | - Bruce D Goldman
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06269, USA
| | - Sharry L Goldman
- Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT 06269, USA
| | - Marianne L Seney
- Center for Neuroendocrine Studies and Department of Psychology, University of Massachusetts, Amherst, MA 01003, USA
| | - Nancy G Forger
- Center for Neuroendocrine Studies and Department of Psychology, University of Massachusetts, Amherst, MA 01003, USA.
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Holmes MM, Goldman BD, Forger NG. Social status and sex independently influence androgen receptor expression in the eusocial naked mole-rat brain. Horm Behav 2008; 54:278-85. [PMID: 18455726 PMCID: PMC2630401 DOI: 10.1016/j.yhbeh.2008.03.010] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/24/2008] [Revised: 03/12/2008] [Accepted: 03/14/2008] [Indexed: 10/22/2022]
Abstract
Naked mole-rats (Heterocephalus glaber) are eusocial rodents that live in large subterranean colonies including a single breeding female and 1-3 breeding males; all other members of the colony, known as subordinates, are reproductively suppressed. We recently found that naked mole-rats lack many of the sex differences in the brain and spinal cord commonly found in other rodents. Instead, neural morphology is influenced by breeding status, such that breeders, regardless of sex, have more neurons than subordinates in the ventromedial nucleus of the hypothalamus (VMH), and larger overall volumes of the bed nucleus of the stria terminalis (BST), paraventricular nucleus (PVN) and medial amygdala (MeA). To begin to understand how breeding status influences brain morphology, we examined the distribution of androgen receptor (AR) immunoreactivity in gonadally intact breeders and subordinates of both sexes. All animals had AR+ nuclei in many of the same regions positive for AR in other mammals, including the VMH, BST, PVN, MeA, and the ventral portion of the premammillary nucleus (PMv). We also observed diffuse labeling throughout the preoptic area, demonstrating that distribution of the AR protein in presumptive reproductive brain nuclei is well-conserved, even in a species that exhibits remarkably little sexual dimorphism. In contrast to other rodents, however, naked mole-rats lacked AR+ nuclei in the suprachiasmatic nucleus and hippocampus. Males had more AR+ nuclei in the MeA, VMH, and PMv than did females. Surprisingly, breeders had significantly fewer AR+ nuclei than subordinates in all brain regions examined (VMH, BST, PVN, MeA, and PMv). Thus, social status is strongly correlated with AR immunoreactivity in this eusocial species.
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Affiliation(s)
- Melissa M Holmes
- Center for Neuroendocrine Studies and Department of Psychology, University of Massachusetts, Amherst, MA 01003, USA.
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Holmes MM, Rosen GJ, Jordan CL, de Vries GJ, Goldman BD, Forger NG. Social control of brain morphology in a eusocial mammal. Proc Natl Acad Sci U S A 2007; 104:10548-52. [PMID: 17556547 PMCID: PMC1965550 DOI: 10.1073/pnas.0610344104] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2006] [Indexed: 11/18/2022] Open
Abstract
Social status impacts reproductive behavior in diverse vertebrate species, but little is known about how it affects brain morphology. We explore this in the naked mole-rat, a species with the most rigidly organized reproductive hierarchy among mammals. Naked mole-rats live in large, subterranean colonies where breeding is restricted to a single female and small number of males. All other members of the colony, known as subordinates, are reproductively suppressed. Subordinates can become breeders if removed from the colony and placed with an opposite sex partner, but in nature most individuals never attain reproductive status. We examined the brains of breeding and subordinate naked mole-rats of both sexes, including several regions linked to reproduction and shown to be sexually dimorphic in other mammals. Stereological analyses revealed that neural morphology depends on status, such that breeders, regardless of sex, had more cells than subordinates in the ventromedial nucleus of the hypothalamus and a larger volume of the bed nucleus of the stria terminalis, paraventricular nucleus, and medial amygdala. Several other brain regions examined were unaffected. Surprisingly, males and females did not differ on any measure. These findings provide evidence that a change in social status triggers considerable neural remodeling and indicate that status, rather than sex, has a predominant role in determining neural structure in this remarkably social mammal.
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Affiliation(s)
- Melissa M Holmes
- Center for Neuroendocrine Studies and Department of Psychology, University of Massachusetts, Amherst, MA 01003, USA.
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Seney M, Goldman BD, Forger NG. Breeding status affects motoneuron number and muscle size in naked mole-rats: recruitment of perineal motoneurons? ACTA ACUST UNITED AC 2006; 66:1354-64. [PMID: 16967507 DOI: 10.1002/neu.20314] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
Naked mole-rats live in large colonies and exhibit a strict reproductive hierarchy. Each colony has one breeding female and one to three breeding males; all other individuals are nonreproductive subordinates. Subordinates show a remarkable lack of sex differences in behavior and anatomy, but can become reproductive if removed from the colony. We recently reported that the striated perineal muscles and their innervating motoneurons, which are sexually dimorphic in all other mammals examined to date, are not dimorphic in subordinate naked mole-rats. Here we asked whether sexual differentiation of this neuromuscular system occurs when a subordinate becomes a breeder. The size and number of cells within Onuf's nucleus (homologue of the rat spinal nucleus of the bulbocavernosus) as well as perineal muscle volume were examined in subordinate and breeding naked mole-rats of both sexes. Sex differences in perineal motoneurons were not observed, regardless of social status. To our surprise, however, counts of motoneurons in Onuf's nucleus were increased approximately 30% in breeders of both sexes. This was accompanied by a reciprocal decrease in cells in Onuf's nucleus that were characterized by small soma size, and lacked a clear nucleus or nucleolus. Although not exhibiting typical motoneuron morphology, some of these small cells were positive for the motoneuron marker, SMI-32. The neuronal changes correlate with increased perineal muscle volumes in breeders. We propose that small, relatively undifferentiated cells are recruited to the pool of large Onuf's nucleus motoneurons when subordinate naked mole-rats become breeders.
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Affiliation(s)
- Marianne Seney
- Department of Psychology and Center for Neuroendocrine Studies, University of Massachusetts, Amherst, Massachusetts 01003, USA.
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