Answering Questions about Mammalian Reproductive Cycles
Shana Lee McAlexander
What does it mean to “breed like rabbits”? Why is my cat in heat and howling during the night? Why do more deer tromp through town during the fall? Answering your students’ questions about mating and reproduction can be challenging. Understanding the concepts of mammalian reproduction is key for broader discussions of evolution, biodiversity, ecology, and animal behavior. The brief study in hormone physiology at the high school level generally covers fluctuations of LH and FSH as well as ovulation and pregnancy, but students often have unanswered questions. A more robust discussion of mammalian reproductive cycles may help answer these questions.
Although various species exhibit distinct differences in their reproductive strategies, mammals generally share a number of reproductive similarities. Through the union of sperm and egg, mammals reproduce sexually. Males, once mature, produce viable sperm that can fertilize the egg (oocyte), released by mature females during a process called ovulation. The fertilized egg can develop into offspring. If unfertilized, the egg(s) and lining of the uterus break down. Hormones, including follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrogen, testosterone, and progesterone, primarily regulate mammals’ reproductive cycles.
The frequency of cycling varies, but it is important that males and females of the same species are fertile at similar times. The breeding season may span several weeks, months, or the entire year. There may be 1, or more than 1, breeding season each year; there may be 1 cycle per breeding season (monoestrous) or multiple cycles per breeding season (polyestrous).
Fertility is more tightly regulated in females than males. Female placental mammals have 1 of 2 types of reproductive cycles: an estrous cycle or a menstrual cycle. In both types of cycles, physiological changes occur with a period of fertility interrupted by a period of infertility. Cycling is discontinued if the female becomes pregnant.
Most female mammals undergo estrus. The period during this cycle when the female is fertile and sexually receptive is sometimes referred to as “being in heat” because of the associated rise in body temperature. In estrus, if the female does not become pregnant, the lining of the uterus is reabsorbed and there is little to no bleeding. Humans and a few other primates have menstrual cycles. In contrast to estrus, during the menstrual cycle, the lining of the uterus is shed from the body as menses instead of being reabsorbed.
In many mammalian species, breeding cycles have adjusted through evolution by natural selection. Timed mating and gestation ensure offspring born into optimal seasonal conditions. For example, female sheep are receptive only in the fall and winter, so lambs are born in the spring and summer, with fairer weather conditions and a more abundant supply of food.
How does it work?
The logical question follows: How are animals aware of the seasons? One factor is the changing photoperiod (ratio of light to dark), which influences mating. The endocrine activity of melatonin, a hormone secreted by the pineal gland during low light, may regulate sexual receptivity. For some animals, low levels of melatonin (long-day breeders) induce the breeding period; for others, high levels of melatonin (short-day breeders) induce breeding. Long-day breeders are reproductively active during summer months when nights are shorter. Siberian hamsters are a common example of long-day breeders; their reproductive rates dramatically decline during winter months. Domestication or holding these animals in captivity often hinders the animal’s ability to sense the seasonal photoperiod, which causes fluctuations in breeding patterns.
Siberian hamsters are seasonally polyestrous animals. Like horses, sheep, and goats, they are fertile during a particular season but have multiple cycles within 1 breeding season. In contrast, seasonally monoestrous breeders only have 1 cycle per breeding season. Examples include bears, foxes, wolves, and white-tailed deer. Other mammalian reproductive cycles do not seem to be linked to seasonal cycles at all. Cows and pigs, if not pregnant, are fertile several times each year. Humans and other primates have a continuous year-round cycle.
There are also mammals that do not have true cycles at all. For example, rabbits are receptive in 4- to 6-day intervals, but they do not ovulate until after mating has occurred. This induced ovulation also occurs in, but is not limited to, the domestic cat, ferret, mink, raccoon, and llama. As you might expect, induced ovulation significantly increases the probability of fertilization.
What about the guys?
Many males are fertile year-round, but numerous male species demonstrate seasonal reproductive changes too. Goats, deer, and camels experience a heightened sexual state 1 to 2 months per year. During this period, called “rut,” males display more aggressive tendencies and seek out fertile females. Male deer are more active and less cautious during rut, making them more visible to humans during this time. Due to heightened levels of sex pheromones during rut, males often give off a distinct pungent odor to communicate their sexual readiness to females.
The realm of mammalian mating and reproduction is both fascinating and integral to biology. This review only skims the surface of the vast field of reproductive biology. Challenge your students to research the reproductive strategies of animals around them. They may also want to interview veterinarians, ranchers, or zoo keepers to learn more about specific animals in a professional context. And to keep the conversation going in your classroom, check out the Altay® Human Endocrine Organs model; Endocrine System: Molecular Messengers, Chemical Control DVD; and the Pregnancy Test BioKit®, a Carolina Exclusive.