Do humans have a mating season? Turns out we do—sort of.

There’s still a controversy among scientists as to why humans seem to follow a certain pattern. 

 

Couple kissing in front of the ocean.
Humans tend to mate and give birth at certain times of the year. Why is that?

Most animals mate at a certain time of year. One of our evolutionary advantages is we can continuously mate and have young. However, if you look at birth patterns, you'll notice that most birthdays tend to occur at certain times of year. For a mammal with no official mating season, it's surprising that the majority of births occur between July and September. September is the most common birth month in the US, according to one Harvard study, with September 16th being the most common day. Count back the months and you'll realize that these babies were conceived around the holidays.


Some call this our “mating season." But in fact, births peak two times a year, around the holidays, and again in late spring to early summer. How do we know this? More children are conceived at these times, more STDs are diagnosed and treated, and more condoms are purchased. Abortions also peak at these same six month intervals. A 2001 study suggests that late autumn and early winter are the best times of the year for sperm health, as they are when men are more likely to have a higher sperm count. While from August to October, sperm counts are at their lowest.

Researchers have also seen a particular pattern in Google searches. More sexually related searches occur during the holidays and in early summer than at other times of year. Dating terms are also more commonly searched, too. Researchers looked at patterns occurring over five years. They examined searches related to topics such as dating, pornography, and even prostitution.

The fact that babies are often born at certain times of year makes scientists think we may have something of a “mating season." But are there biological factors at work or others?

In terms of psychology, during the holidays, when the weather begins getting cold, and we aren't used to it yet, we may long for the physical warmth a partner provides. Women's bodies are designed to keep their core warm, where the womb and organs reside. Ever notice a woman's hands and feet tend to be cold in winter? This is why. As a result, the desire for a partner at this time of year, may be stronger for them. But yearning for physical warmth might be sublimated into a desire for romance, at least according to a 2012 study, published in the Journal of Consumer Research. More romance novels and movies are consumed during late fall and early winter than at any other time of year, the study concluded.

Since the holiday season is the most common time for people to couple up, it's being called “cuffing season," meaning you cuff yourself to someone, in the romantic sense. Around this time of year, we often focus on relationships, be they with friends, family, or someone special. Singles can feel lonely around the holidays. Plus, there are a ton of parties and social events and a lot of people don't want to go alone. Family members too are known to nosily inquire about the love lives of single relatives, which can act as a motivating factor.

Even so, according to scientists, this isn't exactly a mating season. For instance, a woman may be receptive to sex regardless of what time of the year it is. They ovulate not annually, but every 28 days. Evolutionary biologists aren't exactly sure why humans have this unique ovulation mechanism. It conveys an advantage, to be sure. But why it developed is still a mystery. It may have been so the woman can shed her endometrial lining and ward off infection, thus preserving fertility. STDs were a scourge among our early ancestors. Another theory is that it was a way of reducing mating disputes between partners in the band or group.

Several factors may be in play, pushing people together at certain times of year.

Some studies suggest that the high birth rate occurring around June might be due to climate conditions. Sunlight exposure and warmer temperatures can help improve the rate of sperm production, and thus the likelihood of conception. Other studies suggest that hormonal or menstrual changes in women might increase the conception rate at this time. These are hard to prove however, as it's difficult to reproduce seasonal conditions in a laboratory environment.

While humans can mate all year long, other female mammals have an estrous cycle. This is when they're “in heat." Changes in the animal's physiology and behavior occur. It only happens once a year. But a woman's sex drive can be active at any time of year. For this reason, some sex researchers reject the idea that seasonal changes affect humans.

Instead, the temperatures cooling down and people spending more time indoors together may mean couples cuddling up more and so a higher likelihood of things getting steamy. Some research has shown that infidelity is lower in winter months. Even so, on the whole, pregnancy isn't very predictable. Though we may not have a hard and fast “mating season." Being the complex creatures that we are, there could be environmental, social, biological, and psychological factors all working in concert, giving us a tendency to mate and conceive at a certain times of year.

To learn more about the quasi-human mating season(s), click here:

A landslide is imminent and so is its tsunami

An open letter predicts that a massive wall of rock is about to plunge into Barry Arm Fjord in Alaska.

Image source: Christian Zimmerman/USGS/Big Think
Surprising Science
  • A remote area visited by tourists and cruises, and home to fishing villages, is about to be visited by a devastating tsunami.
  • A wall of rock exposed by a receding glacier is about crash into the waters below.
  • Glaciers hold such areas together — and when they're gone, bad stuff can be left behind.

The Barry Glacier gives its name to Alaska's Barry Arm Fjord, and a new open letter forecasts trouble ahead.

Thanks to global warming, the glacier has been retreating, so far removing two-thirds of its support for a steep mile-long slope, or scarp, containing perhaps 500 million cubic meters of material. (Think the Hoover Dam times several hundred.) The slope has been moving slowly since 1957, but scientists say it's become an avalanche waiting to happen, maybe within the next year, and likely within 20. When it does come crashing down into the fjord, it could set in motion a frightening tsunami overwhelming the fjord's normally peaceful waters .

"It could happen anytime, but the risk just goes way up as this glacier recedes," says hydrologist Anna Liljedahl of Woods Hole, one of the signatories to the letter.

The Barry Arm Fjord

Camping on the fjord's Black Sand Beach

Image source: Matt Zimmerman

The Barry Arm Fjord is a stretch of water between the Harriman Fjord and the Port Wills Fjord, located at the northwest corner of the well-known Prince William Sound. It's a beautiful area, home to a few hundred people supporting the local fishing industry, and it's also a popular destination for tourists — its Black Sand Beach is one of Alaska's most scenic — and cruise ships.

Not Alaska’s first watery rodeo, but likely the biggest

Image source: whrc.org

There have been at least two similar events in the state's recent history, though not on such a massive scale. On July 9, 1958, an earthquake nearby caused 40 million cubic yards of rock to suddenly slide 2,000 feet down into Lituya Bay, producing a tsunami whose peak waves reportedly reached 1,720 feet in height. By the time the wall of water reached the mouth of the bay, it was still 75 feet high. At Taan Fjord in 2015, a landslide caused a tsunami that crested at 600 feet. Both of these events thankfully occurred in sparsely populated areas, so few fatalities occurred.

The Barry Arm event will be larger than either of these by far.

"This is an enormous slope — the mass that could fail weighs over a billion tonnes," said geologist Dave Petley, speaking to Earther. "The internal structure of that rock mass, which will determine whether it collapses, is very complex. At the moment we don't know enough about it to be able to forecast its future behavior."

Outside of Alaska, on the west coast of Greenland, a landslide-produced tsunami towered 300 feet high, obliterating a fishing village in its path.

What the letter predicts for Barry Arm Fjord

Moving slowly at first...

Image source: whrc.org

"The effects would be especially severe near where the landslide enters the water at the head of Barry Arm. Additionally, areas of shallow water, or low-lying land near the shore, would be in danger even further from the source. A minor failure may not produce significant impacts beyond the inner parts of the fiord, while a complete failure could be destructive throughout Barry Arm, Harriman Fiord, and parts of Port Wells. Our initial results show complex impacts further from the landslide than Barry Arm, with over 30 foot waves in some distant bays, including Whittier."

The discovery of the impeding landslide began with an observation by the sister of geologist Hig Higman of Ground Truth, an organization in Seldovia, Alaska. Artist Valisa Higman was vacationing in the area and sent her brother some photos of worrying fractures she noticed in the slope, taken while she was on a boat cruising the fjord.

Higman confirmed his sister's hunch via available satellite imagery and, digging deeper, found that between 2009 and 2015 the slope had moved 600 feet downhill, leaving a prominent scar.

Ohio State's Chunli Dai unearthed a connection between the movement and the receding of the Barry Glacier. Comparison of the Barry Arm slope with other similar areas, combined with computer modeling of the possible resulting tsunamis, led to the publication of the group's letter.

While the full group of signatories from 14 organizations and institutions has only been working on the situation for a month, the implications were immediately clear. The signers include experts from Ohio State University, the University of Southern California, and the Anchorage and Fairbanks campuses of the University of Alaska.

Once informed of the open letter's contents, the Alaska's Department of Natural Resources immediately released a warning that "an increasingly likely landslide could generate a wave with devastating effects on fishermen and recreationalists."

How do you prepare for something like this?

Image source: whrc.org

The obvious question is what can be done to prepare for the landslide and tsunami? For one thing, there's more to understand about the upcoming event, and the researchers lay out their plan in the letter:

"To inform and refine hazard mitigation efforts, we would like to pursue several lines of investigation: Detect changes in the slope that might forewarn of a landslide, better understand what could trigger a landslide, and refine tsunami model projections. By mapping the landslide and nearby terrain, both above and below sea level, we can more accurately determine the basic physical dimensions of the landslide. This can be paired with GPS and seismic measurements made over time to see how the slope responds to changes in the glacier and to events like rainstorms and earthquakes. Field and satellite data can support near-real time hazard monitoring, while computer models of landslide and tsunami scenarios can help identify specific places that are most at risk."

In the letter, the authors reached out to those living in and visiting the area, asking, "What specific questions are most important to you?" and "What could be done to reduce the danger to people who want to visit or work in Barry Arm?" They also invited locals to let them know about any changes, including even small rock-falls and landslides.

Your genetics influence how resilient you are to the cold

What makes some people more likely to shiver than others?

KIRILL KUDRYAVTSEV/AFP via Getty Images
Surprising Science

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Harvard study finds perfect blend of fruits and vegetables to lower risk of death

Eating veggies is good for you. Now we can stop debating how much we should eat.

Credit: Pixabay
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