Periods, also known as menstruation or estrous cycles, are recurring cycles experienced by female mammals as part of their reproductive system. During these cycles, females experience hormonal changes that prepare their bodies for potential pregnancy. Only certain mammals experience periods, mainly primates like humans and apes as well as some rodents, bats, and shrews.
The most well-known animal that experiences menstrual cycles is the human female. On average, women have their periods every 28 days, although cycle length can vary. During a woman’s period, the lining of the uterus sheds through the vagina along with some blood. This signifies that ovulation has occurred and pregnancy did not result.
Other primates like chimpanzees, gorillas, and baboons also experience menstrual cycles. Their cycles tend to be shorter than humans, averaging 30-40 days. They also experience bleeding but it is less substantial than in human females.
Outside of primates, periods have been observed in species of rodents, bats, and shrews. For example, the spiny mouse native to Africa experiences cycles lasting 4-6 days. Elephant shrews have cycles around 7 days long. Some bat species have unusually long cycles of up to 6 months.
Scientists hypothesize that periods evolved as a way to cleanse the uterus in animals whose bodies could support pregnancy over multiple cycles. Shedding the uterine lining helps prevent infections or buildup of tissue. The blood loss also prompts the body to rebuild the lining, preparing for the next potential pregnancy.
What Causes a Menstrual Cycle?
In female mammals, menstrual cycles are controlled by hormones produced in the hypothalamus, pituitary gland, and ovaries. This is known as the HPO axis:
Hypothalamus – This region of the brain secretes gonadotropin-releasing hormone (GnRH).
Pituitary gland – Produces two important hormones called follicle stimulating hormone (FSH) and luteinizing hormone (LH) in response to GnRH signals.
Ovaries – Secrete estrogen and progesterone in response to LH and FSH. These are key female sex hormones.
Here’s how the cycle works:
Follicular phase – The hypothalamus produces GnRH which triggers the pituitary gland to release FSH and LH. In response, about 20 follicles in the ovaries begin maturing. One dominant follicle emerges and rapidly produces estrogen.
Ovulation – Estrogen levels peak, causing a surge of LH release. This triggers the mature follicle to rupture and release an egg from the ovary into the Fallopian tubes.
Luteal phase – The ruptured follicle transforms into the corpus luteum structure. This releases progesterone to thicken the uterine lining, preparing for potential implantation of a fertilized egg.
Menstruation – If no pregnancy occurs, estrogen and progesterone levels drop sharply. This causes the uterine lining to shed, leading to bleeding. A new cycle begins again.
The length of each phase determines the overall cycle length, which varies by species. Humans tend to have longer follicular phases while other mammals have longer luteal phases.
What Happens to the Uterus During a Period?
During menstrual cycles in humans and other mammals, the uterus goes through a series of transformations:
Menstruation – The uterus sheds its inner mucosal layer along with blood through the vagina. The blood is partly tissue lining and partly blood from arteries damaged in the sloughing process. This lasts about 5 days on average in human females.
Proliferative phase – Under the influence of estrogen, the uterine lining begins to regenerate and thicken again. This is to prepare for a potential pregnancy. More blood vessels develop in this phase as well.
Secretory phase – Progesterone causes more thickening and swelling of tissues and blood vessels. Glands secrete a nutrient-rich substance to supply any implanting embryo.
Ischemic phase – With no pregnancy, necrosis occurs as blood vessels constrict. This cuts off blood supply to parts of the lining, breaking down tissues. The ischemic phase triggers the next menstruation as the lining detaches.
This cyclical buildup and shedding of the uterine lining over a span of about 28 days allows the uterus to maintain a fresh, nutrient-rich environment capable of supporting a pregnancy.
What Are the Signs of a Period?
The most obvious sign of a menstrual period is bleeding from the vagina. In humans, this typically lasts between 3-7 days. Other symptoms related to hormonal fluctuations include:
– Cramps and pain in the lower abdomen, back, or thighs
– Headaches or migraines
– Food cravings
– Breast tenderness
– Bloating and water retention
– Fatigue and low energy
– Acne outbreaks
– Mood swings and irritability
– Trouble sleeping
Signs in other mammals may include:
– Swollen vulva
– Bleeding from vagina
– Restlessness and changes in activity
– Changes in scent and odor to attract males
– Changes in appetite or food preferences
The amount of bleeding and severity of other symptoms varies considerably between species. Some mammals like rodents experience very minor bleeding compared to humans. Others like some bats show no external signs at all.
How Much Blood is Lost During a Period?
The amount of blood lost through menstruation varies:
Humans
– Average: 30-40 milliliters per cycle
– Range: 10 to 80 milliliters
Chimpanzees
– Average: about 8 milliliters
Baboons
– Average: about 10-15 milliliters
For most mammals, even those that experience substantial bleeding, it is generally not enough blood loss to be dangerous or deadly. However, some human females do suffer from menorrhagia or abnormally heavy periods, losing over 80 milliliters per cycle. This can cause anemia if untreated.
Measuring Blood Loss
In human studies, blood loss during menstruation can be measured in a few ways:
– Weighing pads, tampons, or menstrual cups before and after use
– Chemical analysis of iron content in used pads or tampons
– Visual scales comparing used pads or tampons to known volumes of blood
These measurements have found the average blood loss to be 30-40 milliliters across studies. Newer menstrual cups allow for more accurate measurements. Studies of other mammals rely on close visual observation although exact volumes are difficult to determine.
Why Do Periods Cause Cramps and Pain?
Many mammals experience pelvic cramps and pain during their periods, including:
– Humans
– Chimpanzees
– Baboons
– Rodents like rats and mice
The reasons are not entirely known but may involve:
– Contraction of the uterus to expel lining
– Vasopressin effects on smooth muscle
– Production of inflammatory molecules called prostaglandins
The cramps help the uterus contract to shed its lining but also cause discomfort. Pain levels vary between individuals and species. Some report minimal pain while others suffer severe monthly cramps.
How Long Do Periods Last?
Menstrual cycles range in length among mammals:
| Species | Cycle Length |
|---|---|
| Humans | 25-35 days |
| Chimpanzees | 30-40 days |
| Baboons | 30-40 days |
| Spiny mice | 4-6 days |
| Elephant shrews | 7 days |
| Bats | Up to 6 months |
The length of bleeding also varies:
– Humans – 3-7 days
– Chimpanzees – 1-3 days
– Baboons – 4-11 days
– Rodents – 12-24 hours
So while a human female may menstruate for 5 days every 28 days, a spiny mouse experiences just 1 day of bleeding every 5 days. Bat species have the longest cycles at 5-6 months but relatively short bleeding of 1-2 days.
Why Do Cycle Lengths Vary?
Cycle lengths depend on the activity in the HPO axis. Some factors that influence cycle variability:
– Species differences in hormone levels and receptor sensitivity
– Environmental cues like light exposure and temperature
– Body condition and percentage of body fat
– Age and sexual maturity
– Social interactions and pheromones
– Illnesses, stress, or nutritional deficits can cause irregular cycles
So a combination of genetic factors, environment, overall health, and lifestyle contribute to the hormonal fluctuations underlying menstrual cycles. This causes variation both within and between species.
At What Age Do Periods Start and End?
Menarche is the beginning of menstrual cycles while menopause is the end. Timing varies by species:
Humans
– Menarche: 10-16 years
– Menopause: 45-55 years
Chimpanzees
– Menarche: 9-10 years
– Menopause: 40-50 years
Baboons
– Menarche: 1.5-3 years
– Menopause: 14-19 years
Rodents like rats
– Menarche: 2-3 months
– Menopause: 15-18 months
The start of menstruation signals sexual maturity and capability for reproduction. The end with menopause occurs when females have declining fertility. Timing relates to average lifespan – longer lived species like humans ovulate over more cycles compared to short-lived mice.
What Triggers Menarche?
The first menstrual cycle begins under the influence of hormones:
– Estrogen levels must reach a critical threshold
– Negative feedback loops in the HPO axis stabilize
– Fat percentage and growth factors play a role
Since it’s dependent on hormonal balance, age at menarche can provide insights into female health. Genetic diseases, poor nutrition, high stress, or obesity may delay periods. Reaching menarche indicates adequate growth and development.
Why Does Menopause Occur?
Cessation of menstrual cycles in mid-life occurs due to:
– Depletion of ovarian follicles over time
– Lower circulating estrogen
– Disruption of HPO feedback leading to irregular cycles
The number of follicles is fixed at birth so cycles cease once the supply is exhausted. Since humans ovulate the most times, they undergo menopause later than other mammals. Menopause marks the end of fertility – an evolutionary adaptation thought to allow females to invest in existing offspring once breeding potential declines.
How Do Periods Affect Mood and Behavior?
Fluctuating estrogen and progesterone during menstrual cycles influence mood and behavior in some mammals:
Humans
– Irritability, anxiety, depression
– Changes in sleep and appetite
– Poor concentration and energy
Chimpanzees
– Increased aggression and dominance displays
– Changes in grooming and affiliation behaviors
Baboons
– Increased scent-marking and mating behaviors
– Swellings on face and rear to signal fertility
Rodents
– Increased activity and exploratory behavior
– Changes in learning and memory capability
Research indicates hormones impact parts of the brain controlling emotions, reward, and social behaviors. These effects are thought to maximize reproductive success at fertile periods. Symptoms vary in degree between individuals and species. Some females experience no behavioral changes during cycles.
Do Periods Really Cause Mood Swings?
Some studies indicate menstrual cycles influence mood:
– Estrogen withdrawal increases sensitivity to negative stimuli
– Serotonin levels may decline during premenstrual phase
– Fluctuating sex hormones impact stress response
However, underlying mental health conditions may also contribute to symptoms like depression, anxiety, or irritability. Causes likely involve a complex interaction between hormones, genetics, and psychology. Tracking mood over multiple cycles can help identify whether symptoms correlate to menstrual cycles.
Why Do Some Mammals Show Heat Behaviors?
In species like baboons, rodents, and dogs, “heat” refers to hormonal and behavioral changes during peak fertility:
– Swellings and scent marking to attract males
– Seeking out and permitting mating
– Restlessness and vocalizing
– Changes in appetite or activity
These effects help advertise, identify optimal timing for breeding, and facilitate mating, thereby promoting reproduction. They demonstrate how in some species, female hormones directly influence sexuality and libido according to the reproductive cycle stage.
Conclusion
In summary, menstruation is a key adaptation experienced by females of certain mammalian species including humans, apes, baboons, rodents, and bats. It involves cyclical shedding of the uterine lining which promotes fertility over an optimal reproductive lifespan. Length, frequency, and duration of periods varies between species under hormonal control. Menstruation provides the foundation for pregnancy and birth among mammals, even though it brings uncomfortable symptoms like cramps and bleeding. Understanding menstrual cycles provides insight into the evolved reproductive biology of mammals.