When female dogs are in heat, no one is safe. At any moment, an extremely aroused male dog may try to mount your leg.
And when roe deer are in rut, the bucks go wild. They mark their territory, chase away rivals, and make hoarse barking sounds.
Most animals have a defined mating season. Outside of it, the females cannot become pregnant.
Why aren't humans like that?
Have sex all year round
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Humans have sex with each other all year round.
So do pigs. Sows go into heat about every 21 days all year long. Their cycle lasts between 18 and 24 days, and the period when they are receptive to mating usually lasts two to three days.
Humans, however, generally have sex whenever they choose. We are not restricted by either the seasons or our reproductive cycles.
Thomas Haarklau Kleppestø is an associate professor at NTNU's Department of Psychology.(Photo: Paul S. Amundsen / NTNU)
"One thing that makes humans unique is that we can have sex at any time of the year," says Thomas Haarklau Kleppestø, a researcher at NTNU.
Women have a menstrual cycle that continues throughout the year, and men produce sperm continuously.
This means humans can conceive and have children at any time of the year.
Women do not reveal when they are fertile
Women have what researchers call concealed ovulation, meaning they do not advertise when they are fertile.
"In species with a clear breeding season, females become visibly fertile for short periods. Humans, by contrast, do not display strong external signs indicating when the time is right," Kleppestø explains.
So while ovulation may be concealed, it is somewhat noticeable.
Kleppestø says there are several theories about why women want sex outside the fertile window.
Human babies are helpless
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"One evolutionary theory is that having sex throughout the year helps maintain a long-term partner," he says.
According to this theory, the reason lies in what kind of babies humans have.
"Human babies are premature in many ways, and it takes a very long time before they become independent," says Kleppestø.
While fawns get up and walk straight after birth, albeit a little shakily, and ducklings swim just a few hours after hatching, human babies are completely helpless.
They do not start walking until they are about a year old and are entirely unable to feed themselves. They require extensive care from both parents and other adults.
This may be related to natural selection and the size of the human brain.
More intelligent individuals are better able to adapt, causing our brains to become progressively larger over thousands of years.
Because of our large heads, babies have to be born earlier in order to fit through the birth canal.
“That's why researchers believe women have sex all year round – to encourage greater monogamy and stronger cooperation with men in raising children until they could survive independently,” says Kleppestø.
There may still be seasonal patterns
That does not mean human reproduction is completely independent of the seasons.
For example, more babies are born during the summer. In Norway, 5,250 children were born in both July and in August in 2025, while the number of births was below 5,000 per month during the rest of the year, according to the Norwegian Institute of Public Health.
At the same time, April 21 is the most common birthday in Norway. In fact, April is the month with the highest number of birthdays overall, according to Statistics Norway.
Kleppestø believes these small seasonal patterns are mainly due to the fact that many people are on holiday, the weather is warmer, and couples simply have more time to have sex during the summer.
"It probably has very little to do with biological adaptation," he says.
Perhaps that is what truly makes human reproduction unique: rather than being governed by a biological rhythm, it's shaped primarily by the social and cultural rhythms of our lives.
