Are we the only species that experiences menopause?
Your Menopause Question: Are we the only species that experiences menopause?
Our Response: Your question prompted a personal anecdote. During a wintry visit to the Toronto Zoo in Canada, we found ourselves alone with the caretaker of the primate compound. When I said that I was a gynecologist interested in menopause, the caretaker pointed to an older female, describing her as withdrawn, moody, but receiving calcium for bone health. More recently, my aging female Australian Shepard/Saint Bernard was slouching on the carpet. With advice from the local veterinarian, I began applying an antifungal/steroid to her irritated vulva. This, however, is a common disorder
of menopausal women. This led me to question: Since all species reproduce, what other species experience menopause after reproduction is over?
The process of human reproduction is a beautifully choreographed process. Entire textbooks are devoted to describing the biochemical and hormonal events that take place during puberty, menstruation, and fertilization to newborn delivery.
Yet, despite these studies that follow roughly 30 years of a woman’s life, our understanding of the biology of the 40 or 50 years after the onset of menopause has received far less attention. Yet, we know that the process originates from a small group of estradiol-sensitive neurons in our brain that from puberty on are responsible for our reproductive cycles, heat control, mood stabilization, and energy expenditure (Hrabovszky, 2014). The loss of that estradiol leads to mood swings, hot flashes, weight gain, bone changes, and cardiovascular risk. Is this process unique to humans?
Veterinary science has two objectives. One is to understand and improve the lives of those in the animal kingdom. But, the second is to use transitional animal models to benefit the health of humans. It is estimated that worldwide, by 2030, over 1.2 billion women will be over the age of 50. Within the United States, by 2020, over 45 million women will be in
menopause (Brenton, 2021). The value of animal models to expand our knowledge of menopause is apparent. But what do we know about menopause in other species?
The answer to this question depends on what measurements one uses (Walker, 2008, and Brinton, 2012). Should they include changes in and then cessation of menses? How about blood tests such as elevations in follicle-stimulating hormone (FSH) as a central brain measure, anti-müllerian activity (AMA) as a determination of ovarian follicle activity, hormone levels reflecting ovarian follicle function, or simply a specified number of years after childbirth? And, does the species being studied normally have regular menstruation, or does that change during their lifespan?
But, to approach this question requires other considerations (Walker, 2008). If the organism lives a long time, it will be more difficult to gather information. Also, the cost of such a biologic trial may compromise the question being asked. And, finally, are there enough members of that species to actually get meaningful data?
Many laboratory models have advanced our knowledge of reproduction, while fewer have focused on the time following reproduction (Atselis, 2008; Herndon, 2012; Brinton, 2021; and Ly, 2021). Perhaps smaller mammals would be more suitable for this type of research, starting with the smallest, cheapest, and most numerous.
- The common worm lives for two to three weeks, and while limited in scope of sexuality (since it involves males and hermaphrodites), this model still provides observations on declining reproduction, which occupies one-third of its lifespan (Lu, 2021).
- Fruit flies live 50 days, but ovarian studies demonstrate three stem cell lines; thus, they are of value for ovarian aging (Lu,2021).
Moving up the organizational chart (Brinton, 2012):
- Mice (with a lifespan of one to three years) and rats (with a lifespan of two and one-half to seven and one-half-years) are a common source of investigative studies. Chemical and surgical ablation of ovarian function result in genetic alterations in ovarian aging and osteoporosis. Studies of the tail of the ovariectomized rat were the first to explain the biology of hot flashes (Kobayashi, 2000).
- Farm animals offer another possible model for menopause. Pigs (with a lifespan of 20 years), sheep (with a lifespan of five to 15 years), horses (with a lifespan of 25 years), and cows (with a lifespan ten to 20 years), provide fertile grounds for cardiovascular changes and bone health following ovarian failure and the end of reproduction.
- Even some whales experience menopause as noted in the orca (killer whale), beluga whale, and pilot whale (Nattrass, 2019).
In the end, many scientists who ponder the biology of menopause ultimately turn to the nonhuman primate. But, the expense and the long observation period of each primate make this approach a challenge. The chimpanzee is the closest nonhuman primate (Herndon, 2012). Early studies using genital swelling and menstrual bleeding suggested a lifespan of approximately 45 years with menstrual cycling up to the last year of life (Graham, 1981). Later studies based on hormone measures showed that some reproductive cycling continues until at least age 60, suggesting that they may be capable of reproducing later than humans (Herndon, 2021) with little or no menopausal period. While there are few data on orangutans, gorillas live to be at least 50 years old but show a decline in reproduction based on fecal hormone studies after age 35 (Atsalis, 2008). Rhesus monkeys cease menstruating at about age 30 and may live to be about 40 years old, but menopause occurs between years 20 to 25 (Walker, 2008). This model produced the first endocrine profile of women’s menstrual cycles in 1980.The menstrual cycles and pregnancies of baboons are similar to those of humans, but it appears that their menstrual cycles begin to vary by age 19 and end by age 26 (Walker, 2008). While most humans live as long in their menopausal years as during the reproductive period, these nonhuman primates appear to reproduce later into their lives but with shorter post-reproduction periods.
One might ask why menopause exists at all? The Grandmother Theory, as an evolutionary event, proposes that aging women (and therefore, certainly in the animal kingdom) continue to offer support to the offspring of their own daughters (Peccei, 2001). A counter argument is that menopause is a byproduct of improved health, leading to increased longevity.
Apart from the debate as to why menopause occurs, credit goes to the many scientists who have devoted their lives to understanding the evolution and life patterns of the animal kingdom. And, what do studies of these laboratory models tell us? We are not alone.
James Woods |