Menopause in Nonhuman Primates: A Comparative Study with Humans

Although menopause is a phenomenon predominantly studied in humans or laboratory animals, this chapter discussed the case of nonhuman primates (NHPs), not only with the objective of employing them as study models but also to better understand phylogenetic divergence among species. Those taxonomic differences are reflected in reproductive processes that may be similar to those of human beings, with the presence of a defined cycle or periods of estrus, but perhaps at different ages as well, where menopause plays a crucial role. First, it is important to delimit the concept of menopause by consider- ing its anatomical, physiological, and biochemical parameters, including the cessation of menstrual bleeding or perineal swelling—when present—or follicular depletion and hormonal changes. Thus, the aim of this chapter is to discuss some of the similarities between NHPs and human females, during the menopause period. Studying these phenomena should help us achieve a better understanding of the social, physiological, and environmental factors without adopting any particular cultural view of menopause.


Introduction
Menopause is a process of the reproductive aging [1] manifested in the depletion of ovarian follicles, the reduction of ovarian hormones to castration levels, and the increase in the concentration of serum gonadotropins [2]. In human beings, this process occurs in midlife, heralded by the gradual disappearance of menstrual cycles accompanied by the end of reproductive capacity, which correlates with functional and structural changes in the hypothalamic-pituitary-ovarian axis [3].
This process is not exclusive to humans, for it also occurs in all iteroparous organisms that exhibit declining fertility as a function of general senescence [4]. However, in contrast to human beings, nonhuman primates (NHPs) and even longer lived species like tortoises, elephants, and whales retain their capacity to reproduce until relatively advanced age [5]. Studies in NHPs, such as monkeys and apes, both in the wild and in conditions of captivity, have reported menopause as a physiological phenomena [6][7][8], but they clearly show that the reproductive changes observed in NHPs differ from those of human menopause, at least from a perspective of comparative life history [6]. This is because most of the oldest individuals in all wild species studied showed no signs of ovarian failure, while studies of captive primate species have observed that 67% of old females continued reproducing throughout their lives [7].
It has been suggested that the differences between the human fertility pattern and those of other NHPs are evident in the maximum age of reproduction and mean life expectancy at maturity of both. This refers to the fact that human beings have an early fertility peak that begins to decrease when they are in their mid-1920s, followed by a general decline and then a steep drop that normally begins around age 35; being this age the specific moment fertility functions of NHPs as macaques remain relatively constant over a long period, terminating abruptly only a few years before age death [9].
NHPs are used in medical and scientific research due to their similarities in physiology, neuroanatomy, reproduction, development, cognition, and social complexity to humans, which reflect their close phylogenetic relationship between NHPs and human beings. Primates are divided phylogenetically into strepsirrhines (galagos, lorises, and Malagasy lemurs) and haplorhines (tarsiers and anthropoids). There are three major branches of extant anthropoids or higher primates: the Platyrrhini or New World monkeys (South and Central America) and two groups of Catarrhini (the Cercopithecoids or Old World monkeys (Africa, Europe, and Asia) and Hominoids (Apes and human beings)) ( Figure 1) [10].
The aim of the present chapter is to discuss and analyze some similarities between female NHPs and human females during natural or surgically induced menopause, since expanding our knowledge of this phenomenon in mammals with such a close phylogenetic relationship so to human beings should lead to a more comprehensive understanding of this biological process.

Reproductive cycles in nonhuman primate females and their relation with cycles in human beings
For many mammals, estrus is not only confined to a brief portion of the reproductive cycle that is characterized by an increase in attractivity and in the proceptive and receptive behaviors of females but is also strictly seasonal. In NHPs the reproductive cycles occur for only a few weeks of the year, as occurs among Madagascar prosimians, such as the sifakas. In some New World primates, such as squirrel monkeys, sexual cycles occur only during 3 months of the year [11], but many catarrhine primates do not follow this strict pattern circumscribed by the estrous period [12]. The literature mentioned that apes, human beings, and many monkeys have reproductive cycles that differ in two ways: first, the cycles include menstruation, a cyclical sloughing of the uterine lining. Second, there is greater flexibility in the time of proceptive and receptive behaviors with a longer duration of estrus [13]. Apes and Old World monkeys, meanwhile, exhibit menstrual cycles that range from 25 to 35 days similar to those human females. Also in NHPs, mating activity is not restricted to the periovulatory period as occurs in other mammals since female receptivity is not under strict control of ovarian hormones, but is more closely related to the social context, also as in human beings [14]. Finally, circulating steroid hormones reflect the process of ovulation and ovarian cycling [15].

Ovarian cycle in nonhuman primates
Ovarian cycles in primates begin with a follicular phase during which the follicle matures, follicular secretion of estrogen increases, and the circulating concentration of progesterone (P 4 ) decreases [16]. In most primates, only one follicle ovulates in each cycle. It emerges during the mid-follicular phase and inhibits maturation of other follicles by secreting large amounts of estrogen that, in turn, reduce concentrations of the follicle-stimulating hormone (FSH) below the threshold level required for maturation of early antral follicle [17]. Second, ovulation occurs immediately after the follicular phase, and this maintains high circulating concentrations of estrogens from the mature follicle while exerting positive feedback on the hypothalamus and pituitary that triggers secretion of the gonadotropin-releasing hormone (GnRH), as well as FSH and luteinizing hormone (LH). The increased LH reaches the ovaries where it causes the follicle to rupture [18]. Third, during the luteal phase, the concentration of P 4 rises, but that of estrogens declines. Fertilization can take place during the early luteal phase of the cycle only during the first 24 h after ovulation. This is because the oocyte has a short life span. If the ovum is fertilized, then the corpus luteum does not degrade and continues to secrete P 4 until the placenta develops [19].
Some of these processes are similar to the ovarian cycles in the human beings [20]; however, studies have found that in all primate species studied, follicular development, ovulation, and corpus luteum formation occur spontaneously and independent of mating-induced stimuli [21]. Also, NHPs have been shown to have extended ovarian cycles, especially prolonged luteal phases, compared to those of other mammals [22]. Also, the duration of the follicular and luteal phases differs among NHPs. Cycle lengths vary among different primate groups: in prosimians, from 30 to 50 day; in New World monkeys, from 16 to 30 days; in Old World monkeys, from 24 to 35 days; in lesser apes, from 20 to 30 days; and in great apes-including humans-from 25 to 50 days [14,15,21,23]. In contrast, squirrel monkeys have a mean cycle length of just 7-12 days, with a follicular phase of about 5 days [24].
Menstruation appears to be absent in all prosimians and possibly in tarsiers, presumably associated with the noninvasive form of placentation characteristic of these primates [21]. However, menstruation does occur in most Old World monkeys and apes, as well as in several New World monkeys [25], and prosimians may be considered to have an estrous cycle, because they exhibit distinct cyclical changes in relation to sexual receptivity, with a peak during the periovulatory period. Finally, many New World monkeys do not exhibit either menstruation or strict estrous cyclicity [20].

Ovarian cycle in humans
In human beings menstrual bleeding is the visible sign of cyclicity; it has a length of 3-6 days and occurs at the end of the luteal phase and the beginning of the follicular. While fertile phase has a length of 5 days and is associated with the end of follicular phase and an increase of estradiol (E 2 ) before ovulation, during this period conception can occur. Recent studies have found that human females possess dual sexuality, which consists of a fertile phase where they are more sexually attractive to men and a phase extended (non-fertile), which presents a motivation or interest in sex with the aim of obtaining some benefits, without conception occurring [26].

Reproductive aging
Female reproductive output differs markedly in relation to species and time. As females of many species age, a period of reproductive instability with perimenopausal-like hormonal changes has been observed. Like many other mammals, NHP females show fertility parameters that are related to age [7]. Anovulation, insufficient luteolysis, and impairment of gestation and lactation processes all become more common toward the end of reproductive life [27]. Female reproductive senescence differs among mammalian taxonomic groups. For example, in NHPs, the end of reproductive life is characterized by the loss of the follicular pool, whereas in rodents, variations are seen in the size of the follicular pool that remains at the end of reproductive life. In humans, experiencing follicular depletion early in the maximum life span is not usual; rather, it is the result of an extended period of altered hormonal environments. These alterations may be caused by reduced circulating estrogens, P 4 , and inhibin, resulting in elevated gonadotropin concentrations (GTHs) for a time, followed by their decline [28]. Monkeys and apes also experience follicular depletion and associated hormonal alterations [8,29], but the stage of life at which these occur is generally later than in humans. Some reports on lemurs and callitrichids indicate an age-related decline in reproduction in many species that is reflected in diminished reproductive success [30]. Older female sifakas (Propithecus edwardsi), a Madagascar lemur, show decreased rates of infant survival, and studies have affirmed that this effect can be attributed to the females' deteriorating dentition resulting in inability to support lactation [31]. This indicates that reduced fertility in old age does not, in and of itself, reflect impaired neuroendocrine or gonadal function [20].
Considering the taxonomic scale of primates, we can observe the variability in physiological characteristics, like it is reflected in aging process. As much NHPs get closer to human beings, more similarities are found, going through estrous cycles for strepsirrhines (galagos, lorises, and lemurs), to ovarian cycles, and hormonal profiles similar to human being females, in great apes (orangutans, gorillas, and chimpanzees), and Old World monkeys (macacos and baboons). Also in both cases, at the end of their reproductive life, different physiological and hormonal changes occur, which are associated with the loss of ovarian function that are characteristic of aging, where this gives us the opportunity to study in a comparative way different alterations that could be related to the absence of ovarian hormones.

Menopause in Homo sapiens females and nonhuman primates
Menopause has been defined as a series of changes in the termination of reproductive viability, of which the discontinuation of menstruation is but one component. Menstrual bleeding is a marker of the ovarian and neuroendocrine phenomena of reproductive viability in humans [32], but not all NHPs exhibit this [24]. Consequently, menopause must encompass hormonal, physiological, and biochemical changes that play essential roles in the cessation of ovarian cyclicity, regardless of whether menstrual bleeding is present. However, Walker and Herndon [1] have defined menopause in NHPs as the permanent, non-pathological, age-associated cessation of ovulation, so to infer this event would require considering such biological parameters as menstrual bleeding, perineal swelling, follicular depletion, and hormonal changes.
Some species of NHPs seem to present processes that are quite similar to what human females experience during menopause, but differences also exist, such as the shorter postmenopausal life span and differences in the timing of hormonal changes during the menopausal transition [33]. It is important to consider the time of menopause relative to the average and maximum life span of individuals. For example, humans may be unique among primates in that they have a long post-reproductive survival potential [34]. In human females, the reproductive function does not begin with puberty nor does it end with menopause at a certain chronological age. Instead, both of these are dynamic periods for the reproductive axis, during which development or senescence occurs relatively rapidly. In fact, the reproductive axis ages to a nonfunctional state (menopause) much earlier than other organs, while the reproductive system reaches the point of failure at a relatively young average age of 51 + 8 years [35]; considering that the maximum span for humans is around 80 years, they spend nearly 35% of her life in a post-reproductive state and in very special cases to 60% (122 years). Also, there are significant differences between species of NHPs and humans in terms of life span. For example, the life span of animals after menopause is short compared to humans, as they usually die not long after menopause [1].
Human females are born with a finite number of oocytes; thus, reproductive aging entails the steady loss of these oocytes through atresia and ovulation, processes that do not necessarily occur at constant rates [36]. Peak fertility in humans occurs in the mid-20s, after which it declines steadily until a steep decrease begins after age 35 [37]. This decline in fertility occurs despite normal hormone secretion by the ovaries of "older" reproductive-age humans, which continues until 3-4 years prior to menopause [38].
In spite of the wide age range at which ovarian dysfunction and reproductive failure occur in these species, the sequence of terminal events is fairly predictable. At the beginning of the process, the menstrual cycle length is shortened due to early follicular development and ovulation [39], which reduces fertility (premenopause). This is followed by disruption of regular menstrual cyclicity (perimenopause) and, finally, complete ovarian failure (menopause).
Studies have observed that perimenopause is an indication that the number of remaining ungrown ovarian follicles has dropped below a critical threshold [40]. The period of transition from the reproductive phase to the nonreproductive state is called climacteric. Finally, postmenopause is the period following climacteric and occurs when the hormonal instability that characterizes perimenopause is replaced by the relative stability of the post-reproductive life stage when the reproductive function has ceased [41].
Declining fertility with age is manifested more commonly in monkeys and apes, to the point that some females cease to reproduce altogether before they die. Some reports on Old World monkeys in the wild mention that old toque macaque (Macaca sinica) and gray-cheeked mangabey (Lophocebus albigena) females no longer breed, perhaps due to increasingly long birth intervals that terminate with death or the cessation of ovulation [42]. In contrast, NHP females living in captivity may show life cycles marked by irregular and lengthened menstrual cycles, reduced estrogen levels, very long birth spacing and, in a few cases-such as chimpanzeestotal cessation of ovulation [8,43]. In captive rhesus monkeys (Macaca mulatta), menstruation ends at approximately 25 years of age [44], and their maximum life span is around 30 years [45]; thus, this species may have a maximum post-reproductive life span of approximately 20%, similar to what happens in chimpanzees (Pan troglodytes). There are also differences in life span among species of NHPs and humans. For example, the life span of other animals after menopause is short compared to humans, since they usually die after only a short time, while humans have an extended postmenopausal life expectancy [1].
The perimenopause period is also highly variable in human beings, as age at the onset of this period ranges from the mid-1930s to the early 1950s [46]. This wide range impedes gaining a better understanding of the mechanisms that control the onset of menopause in humans. In NHPs, this is even more difficult, since reproductive cessation occurs so late in their life span that relatively few individuals actually live to those ages. However, there are data that support the existence of a perimenopausal in NHPs [33,[47][48][49], a condition that indicates a transitional stage between fertility and age-associated infertility. Also, it has been reported that patterns of vaginal bleeding and serum hormone profiles of macaques in the third decade of life are similar to those described for peri-and postmenopausal human [29].
Although originally the term menopause was coined in human being context, there are some approaches toward NHPs, which let us build it, considering not only the cessation of menstrual bleeding but also other changes, such as the cessation of perineal swelling, follicular depletion, and hormonal-associated changes. So this term has been adapted focusing in the physiological characteristics of each species. By other hand the life span between species should be considered, because unlike human beings, some species usually transit immediately from the reproductive end to death. Therefore, it is of great importance to know what are the differences between species that could help us identify the age of onset of menopause according to the species of the study, and, since this information, it will depend on whether or not our data can be extrapolated to the human.

Menopause in nonhuman primates in wild versus captive conditions
Specific studies over physiological mechanisms that govern the timing of menopause in wild NHPs are scarce [42,50], because many factors could mask the accuracy of these results, including the ages of subjects-which often must be estimated [51]-predation pressures [52], limited survivability [23], infant mortality [53], food availability and nutrition [54], and social dynamics [55]. Therefore, this information is taken as complementary to data derived from captive animals [1].

Macaques (Macaca spp.)
Hodgen et al. [29] reported that female rhesus monkeys (Macaca mulatta), in captivity and at least 22 years of age, showed true menopause, confirmed by circulating levels of pituitary and ovarian hormones and the pattern of vaginal bleeding. Female rhesus monkeys older than 22 years are considered aged, as the maximum average life span for this species is estimated at 30 years [44,45]. Hence, these females are close to the end of their life span, compared to humans, who are considered as "aged" at around 75 years.
Graham et al. [8] examined the reproductive history and histology of pigtail macaques (Macaca nemestrina) by observing females divided into three age classes (4, 10, and 20 years). They reported that one female over 20 years of age showed functional, hormonal, and morphological characteristics of human menopause (i.e., complete follicular depletion, absence of luteal tissue, amenorrhea, increased LH levels, atrophic uterus, and vagina). Miller et al. [56], meanwhile, reported an age-associated decline in fertility in pigtail macaques, similar to the findings for Macaca sylvanus reported by Paul et al. [57].
Walker's study [47], of 15 female Macaca mulatta aged 8-34 years, was designed to characterize the endocrine and menstrual changes associated with menopause in this species. Findings indicate that females aged 24-26 years were in transition to menopause, evidenced by elevated LH concentrations consistent with a low E 2 concentration and no indication of bleeding menstrual. Also, the histological analysis of their ovaries showed little or no evidence of follicular activity. Finally, the females aged 27-34 years clearly showed a postmenopausal process, marked by high LH concentrations and uniformly low E 2 concentrations. This finding was corroborated by Gilardi et al. [48], who suggest that in female rhesus monkeys menopause does not occur until the second half of the third decade of life. Recent studies have also reported that postmenopausal females show low E 2 and P 4 levels, high indexes of FSH and LH, and a significant decline in the anti-Mullerian hormone and inhibin B. All these findings indicate that these endocrine parameters may be associated with menopause [49]. On the other hand, Johnson and Kapsalis [58] reported a median age >27 years for menopause in free-ranging rhesus monkeys.
Recent studies have concluded that reproductive senescence correlates with overall health [23]. Gore et al. [59], for example, reported that neuroendocrine changes in senescent rhesus monkeys are consistent with those reported in humans [60] and that ovarian changes are related to menopause [61], thus suggesting that these NHPs undergo ovarian changes as a function of aging, similar to humans [40] and chimpanzee [62]. A study of Japanese macaques (Macaca fuscata) reported that in free-ranging individuals, fertility rates diminish at around 25 years of age [63], but those normal menstrual cycles continue when they are in captivity, despite a loss of fertility [64]. Finally, recent studies of cynomolgus monkeys (Macaca fascicularis) have shown an endocrine pattern similar to that of humans during the postmenopause period [65].

Great apes
The menstrual cycles, pregnancy, and genital pathology of common chimpanzees (Pan troglodytes) were analyzed to determine the extent of perimenopausal changes in females with aged approximately 35-48 years. However, those analyses showed no clear evidence of menopause, because several females continued cycling until death [8]. But the authors did observe a reduced likelihood of conception in those female chimpanzees, even though they did not "run out" of oocytes before the end of the maximum life span. They concluded that female chimpanzees aged 35 years of age or more show increased reproductive senescence that is quite comparable to what is seen during human climacteric.
Other studies of common chimpanzees aged approximately 48-50 years and of bonobospygmy chimpanzees (Pan paniscus)-aged approximately 40 years reported that even though the former were extremely aged, they continued to have menstrual cycles and perineal swelling but with increased cycle length. Also, these aged females continue to secrete GnRH in a pulsatile fashion, although the levels of this hormone are higher than younger females [43].
Recent studies by Lacreuse et al. [66] found that many aged chimpanzees continued to menstruate at age 50 or more, but the length of their cycles increased after age 20. Similar results were reported by Videan et al. [67], who concluded that menopause in Pan troglodytes occurs at approximately 35-40 years of age. These data concur with the report on wild chimpanzee by Nishida et al. [50]. These authors reported that the females ceased cycling after 30 years of age. On the other hand, Thompson et al. [68] observed that healthy free-ranging chimpanzees remained reproductively viable well past 40 years. They suggested that in Pan troglodytes, menopause occurs as a by-product of ill health, interpreting that the onset of menopause may be delayed in relatively healthy, long-lived animals. Studies of female chimpanzees have shown that reproductive aging is similar to that seen in human females, including higher fetal loss as a function of advancing age [69] and the age-related depletion of ovarian follicles [62]. Thus, these studies showed that Pan troglodyte females continued cycling into extreme old age, which distinguishes them from human females in terms of menopause.
Other studies in Pan paniscus females, aged at least 40 years, showed no external evidence of menstrual cycling preceding death, and hormone levels consistent with clinically observed amenorrhea, but an exaggerated response to the exogenous GnRH challenge. Histological examination of ovaries showed similar characteristics to those described for senile ovarian tissue in humans [43].
Studies of captive orangutan (Pongo ssp.) females have reported the endocrine characteristics of their menstrual cycle and similarities to the human cycle [70]. These reports considered births and inter-birth intervals across the life span and demonstrated an age-specific decline in the fertility of captive female orangutans (Pongo pygmaeus; [7]). Other studies with wild female Sumatran orangutans (Pongo abelii) failed to document menopause, inferred from increased inter-birth intervals in females of estimated age [51]. Interpreting data from wild animals is difficult because of such countervailing factors as female rank, uncertain age, infant mortality, and food availability [1].
An earlier study that described the reproductive physiology of female gorillas (Gorilla gorilla) mentioned a correlation of perineal tumescence with circulating hormones and reported a pattern of cyclic hormone secretion similar to that of humans [71]. Recently, fecal hormone determination in two captive female gorillas aged approximately 40 years showed evidence of the protracted luteal phases that are typical of aging human females [72].
Information related to the occurrence of menopause in baboons (Papio ssp.) was based on menstrual cycle length, total cessation of cycling that occurs at 26 years of age in captivity [73]. Similar results were reported by Lapin et al. [74], and other studies of wild baboons have reported increased cyclic variability with age and a complete loss of fertility by the age of 25 years. This suggests that baboons undergo age-linked alterations in reproductive function similar to those of humans.
The living conditions of primates have an impact over the animal life span, so the observations in captivity are not always the same as in wild conditions. Although there are some reports about NHP aging process and menopause, they are scarce. Most of the studies report animal physiology and behavior in captivity, because to follow animals in wild by a long period is a very difficult process due to the NHP living conditions.

Nonhuman primates as a model to study human being menopause
Due to the biological similarities between human beings and NHPs, the latter have been studied in the search for an adequate model of menopause. However, it is necessary to clearly delimit the similarities and differences among reproductive characteristics, perimenopausal and menopausal changes, and the average life span of different species [1]. Establishing similarities with humans during this search requires considering the characteristics of menopausal processes when animals are in captivity versus those who are free-ranging, in order to avoid the confusions that have led to the assertion that menopause is a uniquely human event [1]. Walker and Herndon [1] suggested that a comparative analysis of female reproductive senescence should focus on the anatomical, physiological, and biochemical changes that are essential to the cessation of ovarian cyclicity, regardless of the presence of menstrual bleeding. There are few reports on menopause in New World primates compared to Old World monkeys, but studies of the latter have observed declines in sexual activity and decreased birth rates. Also, reports on captive apes suggest a long post-reproductive life span, though this has not been confirmed in the wild [30].
Among the different primate taxa, menopause is manifested along an evolutionary continuum: in some species-such as cercopithecines and apes-it is followed by an extended post-reproductive life span, while in others it may presage death. In terms of NHPs as models for menopause, the species that have most often been employed are baboons and chimpanzees. Studies of these primates have attempted to simulate all the consequences that characterize menopause, namely, hormonal and cognitive changes, cardiovascular alterations, and osteoporosis.
Until recently, the occurrence of reproductive termination in NHPs was widely questioned. However, numerous studies have reported that this does indeed occur in several species of Old World monkeys and great apes. Most of this research has been conducted with Macaca mulatta [29, 33, 47-49, 59, 61], but other species also experience menopause, including Pan troglodytes [43,62,67] and Gorilla gorilla [27]. For example, the hormonal profiles of peri-and postmenopausal macaques, chimpanzees, and gorillas [1,61,67], as well as the age-related decline in the number of primordial follicle in macaques [61] and chimpanzees [62], share many similarities and occur in a pattern like to that of aging women [40]. On the basis of data from various studies, Fedigan et al. [75] affirmed that "from an endocrinological perspective, reproductive decline may well follow a similar pattern in all primates, and we could use cases of individual post-reproductive monkeys and apes as clinical models of the physiological basis for menopause in human being. However, from an evolutionary perspective, these studies fail to demonstrate similarity between reproductive senescence in NHPs and menopause in the human female. Instead, they highlight the critical differences: female macaques and chimpanzees that cease to cycle very close to age at death, whereas human females cease to cycle in middle age; female macaques and chimpanzees cease to cycle on an idiosyncratic basis, whereas human females universally cease to cycle at the average age of approximately 50 years." In light of these data, it is clear that regardless of the age at the onset of menopause, there are numerous physiological similarities between the females of NHPs and human females with respect to the gradual decline and eventual cessation of reproductive capacity. For this reason, several authors of excellent reviews [1,29,30] have proposed that NHPs provide the most appropriate animal models available for analyzing menopause in human females and the processes associated with it.
Although NHPs present a rich opportunity to study the process of reproductive senescence or menopause (i.e., the permanent, non-pathological, age-associated cessation of ovulation, [1]) and play a unique role in translational science by bridging the gap between basic and clinical research [76], their use as experimental subjects is limited by the lack of available NHPs that are undergoing the perimenopausal transition and natural menopause, their short menopausal compared to that of human being, high costs, and the strict ethical guidelines that researchers must follow when studying them (see Ref. [33,76]).
Despite these difficulties, the use of NHPs as study models has several advantages. Macaques (Macaca spp.), including Macaca mulatta and Macaca fascicularis monkeys, for example, have been particularly useful due to their availability, moderate size, and ability to adapt to laboratory conditions. Also, approximately 95% of the overall genetic coding sequence of macaques is identical to that of humans [77], and many of their physiological systems are comparable. Finally, because they are relatively long lived, they are effective models for studying a number of diseases and conditions that increase in frequency with aging. These factors explain why female macaques have been the preferred model for examining critical health concerns of human beings, including luteal phase deficiencies and hypothalamic amenorrhea [78], obesity and diabetes [79], cardiovascular diseases [80], osteopenia, osteoporosis [81], osteoarthritis [82], cognitive deficits associated with age [76], and-at least potentially-decreased interest in mating [83].
If a single conclusion can be gleaned from this brief summary, it is that a large number of physiological conditions and pathologies that human beings experience during their lifetime appear to be broadly manifested in primate taxa, though information is lacking in other regards, such as the interaction between deficits in cognitive processes and their effect on the modulation of social and sexual interaction.
Primates are mammals distinguished by their large brains, advanced cognitive abilities, flexible behavior, and sophisticated social systems [84]. For example, chimpanzees have the ability to recognize themselves in a mirror [85] and perform tasks involving concept formation [86]. Moreover, the structure and function of human and NHP brains are very similar. In this regard, we can mention nuclear organization, projection pathways, and innervation patterns [87], as well as similar cortical development and organization [88], including visual cortical functional divisions and prefrontal cortex subdivisions [89] that are critical for cognitive processes [90].
In human beings and NHPs, cognitive and reproductive functions decline gradually with advancing age and more precipitously with the loss of circulating estrogen that occurs during menopause. Cognitive deficits in NHPs can be quantified over their life span using a battery of cognitive tests that are similar to, if not the same as, those used with humans [91]. These include the monkey version of the Wisconsin Card Sorting Test (WCST) [92], which is the gold standard for assessing cognitive flexibility in humans. Using a version of WCST (without the numerosity category), executive function deficits have been reported in both middle-aged and older rhesus monkeys [93], as well as in middle-aged menopausal rhesus monkeys [91]. However, the limited availability of animals of adequate age [33] means that studies with monkeys typically involve only a few animals and use premenopausal ovariectomized subjects rather than naturally menopausal females.
Given the dramatic effects of sex steroids on neuronal morphology and brain activity in regions involved in cognition, one might expect that age-related changes in the endocrine milieu will have important consequences for cognitive functions. In effect, data on aged, naturally or surgically menopausal monkeys indicate that estrogen does indeed modulate a broad range of cognitive domains, such as learning and memory. These effects observed appear to be task specific and sensitive to the time that passed without estrogen prior to estrogen replacement. For example, on the delayed response (DR) task-a test of prefrontal functioning-it was noted that performance was impaired in postmenopausal individuals compared to age-matched premenopausal rhesus monkeys [94]. This result suggests that the absence of estrogen, associated with menopause, could be detrimental to prefrontal functioning.
Although the effects of the menstrual cycle, estrogen withdrawal, and estrogen replacement in young monkeys appear limited to non-mnemonic functions, such as attention or aspects of face processing [95], a broad range of cognitive functions, including memory, are sensitive to estrogen deprivation and replacement in older monkeys [92]. Neurobiological data are consistent with such cognitive findings and demonstrate an array of morphological and physiological changes following ovariectomy and/or estrogen replacement in brain areas that are important for cognition.
Although the specific mechanisms through which estrogens may affect cognition remain to be elucidated, it is clear that these hormones have broad effects on areas of the brain that play key roles in cognitive functions [96]. Estrogen receptors are found in the cerebral cortex, hippocampus, and amygdala in both monkeys [97] and human beings [98]. Estrogens alter the neuronal morphology and physiology of some of these areas [99].
NHPs provide valuable animal models that have significantly advanced our understanding of numerous behavioral and biological phenomena in humans and other primates. Their value as models for studying menopause in humans derives from their common ancestry, as well as a series of hormonal, cognitive, and social influences that are similar to those experienced by human beings. The aging process or menopause has been also explored focusing through the neural basis of cognitive functioning, revealing not only alterations over specific neural systems but also differences in the affectation level among brain regions and neurobiological parameters. Therefore, further research into the interactions among hormones and various neurotransmitter systems could potentially produce improved knowledge of the neural and hormonal bases that comprehend the gamma of alterations that human beings suffer before, during, and after menopause.

Anxiety and depression during natural or surgical menopause of nonhuman primates
The decrease in ovarian hormones during natural and surgical menopause is associated with a higher incidence of psychiatric disorders, such as anxiety and depression in vulnerable women, where the decrease of hormones-principally E 2 and P 4 -can induce neural changes that exert affects on both the emotional and affective levels [100]. In this regard, ovariectomies in NHPs have been used as a model of surgical menopause at the experimental level, given that the absence of certain hormones induced by ovariectomy can reproduce the physiological, emotional, and affective change characteristic of menopause.
At the behavioral level, ovariectomized primates may exhibit anxiety and depression-related behaviors. For example, long-term ovariectomy may increase anxiety in Japanese macaques (Macaca fuscata), associated with decreases in such behaviors as positive social contact, dominance, and the time spent receiving grooming. Similarly, temperament tests performed on these individuals show an increase in anxiogenic behavior [101]. Furthermore, ovariectomized pigtail macaques (Macaca nemestrina) present higher scratching rates [102], a well-established indicator of anxiety in NHPs, while in Japanese macaques, a reduction in locomotion has been observed after ovariectomy [101], in association with depressive behavior. Therefore, these behavioral alterations are probably due to the absence of ovarian hormones, given that after ovariectomy in rhesus (Macaca mulatta) and pigtail macaques a reduction in E 2 and P 4 concentrations is detected, in relation to increased anxiety [102].
The absence of ovarian hormones in NHPs may also generate neural changes in the brain ( Table 1). Studies of ovariectomized Japanese monkeys have detected downregulation of estrogen receptor beta (ER-β) in the subiculum of hippocampal formation, while postmenopausal monkeys of the same species have shown upregulation of ER-β [103]. On the other hand, in ovariectomized African green monkeys (Cercopithecus aethiops sabaeus), a reduction of synaptic plasticity of the hippocampus was detected [104]. Given that the reduced density of dendritic spines and ER-β in the hippocampus is related to an increase in indicators of anxiety and depression in ovariectomized rodents [105], this is probably occurring as well in nonhuman primates that experience surgical menopause. In addition, the long-term absence of ovarian hormones may impact serotonergic activity. For example, it has been demonstrated that ovariectomy in rhesus macaques reduces expression of the mRNA of the tryptophan hydroxylase-2 (TPH-2) enzyme, increases the expression of MAO-A, and increases DNA fragmentation of serotonin neurons in the dorsal raphe nucleus [106]. These events could lead, on the one hand, to greater serotonin degradation and, on the other, neuronal death and, finally, a malfunction of the serotonergic system.
Furthermore, long-term ovariectomy in Japanese macaques reduces the expression of serotonergic neurons and gene expression of TPH-2, the serotonin reuptake transporter (SERT), and 5HT 1A autoreceptors in the dorsal raphe nucleus [109]. This agrees with data showing that in depressed female of Macaca fascicularis the binding potential of 5HT 1A receptors is reduced in the hippocampus, amygdala, and cingulate cortex [110], three of the structures involved in the pathophysiology of anxiety and depression. In contrast, stress-sensitive female monkeys of the same species decrease levels of Fev (transcription factor that determines whether a neuron is serotonergic), TPH-2, SERT, and 5HT 1A mRNAs in the dorsal raphe nucleus [111]. Thus, in the long term, the reduction of TPH-2, which is important for serotonin synthesis, together with determinant markers for serotonergic function, could generate a higher incidence of anxious and depressive behaviors in NHPs with menopause, as occurred in human beings.
On the other hand, exogenous administration of E 2 or P 4 in ovariectomized primates has the capacity to restore serotonergic neurotransmission [106]. Further, serotonin neurons can express the ER-β protein and ER-β mRNA [112]. And, therefore, estrogens could increase the availability of serotonin in the brain by interacting with its receptor. Thus, the absence of ovarian hormones, such as E 2 and P 4 , has the ability to induce changes at the level of the the central nervous system in primates [103]. This evidence suggests that neural changes could be related to anxiety and depression behaviors, which could indicate some vulnerability in NHPs that experience natural or surgical menopause or suffer changes in different neurotransmission systems in which ovarian hormones participate, all of which could affect the emotional and affective state of these individuals.

Conclusion
Menopause is a natural process that entails the permanent cessation of ovulation. It is associated with physiological and structural changes in aging females. Although it has long been assumed that menopause occurs only in human beings, the search for medical/clinical models to aid in research on this process has revealed that some species of NHPs also exhibit menopause. However, certain differences between human females and NHPs are clear: shorter postmenopausal life spans and variations in the timing of hormonal changes during the menopausal transition. But NHP models allow us to better understand not only several of the processes that occur during human aging-such as cognitive changes, cardiovascular alterations, and osteoporosis-but also similarities among species along the taxonomic scale.
On the other hand, increases in anxiety and depression behaviors may be observed in NHPs that undergo natural or surgical menopause. In a comparative perspective, these findings could improve our understanding of the neurobiological mechanisms that underlie emotional and affective disorders associated with the absence of ovarian hormones, given that experiments have demonstrated that long-term hormonal absence has the ability to affect numerous neurotransmission systems involved in mood disorders. In addition to reproducing various neural changes that can be correlated with depressive and anxious behaviors in NHPs, this might help understand the neurobiological substrate of emotional and affective disorders that can appear in women who experience natural or surgical menopause.