Throughout nature, we see seasonal breeding as being a part of species survival.
Most animals mate in the summer-fall to give birth in the spring. Mating during the late fall or winter would push birthing back into the summer months. Doing so would not allow ample time for the babies to develop and become self-sufficient enough to survive and thrive during a cold and long winter.
In the Arctic, many species are forced to mate within a 1-week window in order to give the offspring time to develop and prepare for winter.
Research by Dr. Russ Reiter has suggested that the hormone responsible for these mating cycles is melatonin.
Melatonin is, of course, the master hormone produced in the pineal gland that regulates our connection to the light/dark seasonal or circadian cycles. During the longer nights of winter, we produce more melatonin. The shorter nights of summer trigger the release of less melatonin. (1)
How Does the Pineal Gland Regulate Seasonal Breeding?
The only reliable indicator of the changing seasons is the number of hours between sunrise and sunset. For example, in June, you can have temperatures as high as 85 degrees or as low as 25 degrees in many parts of the US, but the number of hours between sunrise and sunset has been pretty much the same for millions of years.
The precision of these cycles is interpreted by the pineal gland through its production of melatonin. Melatonin then orchestrates the ebb and flow of numerous hormones, particularly sex hormones, that have ensured the survival of our species.
As the days begin getting longer in the spring-summer, melatonin levels are suppressed and reproductive hormone activity is increased. In the winter, when nights are long, melatonin levels surge and reproductive hormone activity is suppressed. (2,3) The increase in winter melatonin was a natural means of contraception for many species, and possibly humans. In fact, in the 1990s, a melatonin birth control pill was under development. (3)
Since melatonin production is linked to light exposure, using melatonin as a birth control agent would likely be effective – as it was in traditional cultures, when sleep was initiated by sunset and activity by sunrise. However, it would be a pretty unpopular birth control pill, as the fine print may have to read, “Only effective if sleep and total darkness is initiated just after sunset.”
In the end, The Dietary Supplement Health and Education Act of 1994 classified melatonin as a dietary supplement, thus, the development of melatonin as a birth control pill was squashed.
Sex and the Seasons
For millions of years, animals have synced up their breeding schedule with this predictable light/dark cycle. Melatonin is also responsible for changes to the color and thickness of the coats of animals, migration patterns and breeding. (1)
Dr. Reiter’s first studies evaluated hamsters. Hamsters have testicles that literally shrink during the winter months. In the spring, they swell to a considerably large size in proportion to their body. If the hamster-sized testicles were on humans, the testes would weigh about eight pounds. This interesting characteristic makes the evaluation of testicular atrophy during the winter months extremely obvious. (1)
Dr. Reiter and his associates discovered that the hormone involved in the shrinking or shriveling of the testicles each winter was melatonin. An increased amount of melatonin linked to longer nights associated with long, cold winters translated into an evolutionary trait that decreased sex drive and paused mating activities. (3)
In the 1890s, Eskimos were studied before their exposure to the modern world and artificial light. In one of these cultures, during the long winter months with no or little light, women would completely stop menstruating. Their melatonin levels were also surging during these months. Come spring, when it was lighter for longer, melatonin levels would be suppressed and reproductive hormones would surge… and mating season would begin! (5)
In Northern Finland above the Arctic Circle, there is an 8-week period during the summer months where there is a significant surge in conception rates, and an associated boost in spring births. Researchers discovered that during this surge there is a significant drop in melatonin levels. (4)
There is also a drop in melatonin production around the time of ovulation during a normal menstrual cycle. According to Dr. Russ Reiter, it is possible that this dip in melatonin production during ovulation is linked to increased fertility during that time. (1,4)
Our excessive exposure to artificial light after sunset has resulted in a disconnection to the light/dark cycles. Many adult humans are producing less melatonin year-round than they normally should.
Premenstrual syndrome (PMS) symptoms such as headaches, weight gain, cravings, insomnia and irritability have all been linked to a decrease in the production of melatonin. Studies show that women with PMS produce less melatonin than other women, particularly a week before the cycle starts. (1,6)
Many experts believe that humans were once seasonal breeders like all other mammals.
As artificial light was first introduced in the form of fire, humans stayed up longer into the night, slowly altering the natural circadian production of melatonin and other sex hormones.
Soon, the long nights of winter that suppressed conception rates were replaced with more artificial light, resulting in less sex hormone suppression. Over thousands of years, we traded our seasonal breeding instincts to be able to breed year-round, as we do today.
That said, there are still significantly higher birth rates in the spring in the colder climates compared to any other time of the year. (4,5)
What do you think… A coincidence? We would love to hear your thoughts! Share in the comments section!