Why Don't Humans Have A Mating Season? Actually, Wait – Do We Have A Mating Season??

Humans, on the other hand, get to be horndogs all year round. Not for us any hormone-induced rut or heat cycle; we reproduce whenever the fancy strikes us. Which raises an interesting question: how come?

Why animals so often have a mating season

Let’s clear something up first of all: there are good reasons for a species to have a mating season. It’s actually the older, more ancestral way of breeding – it’s a behavior we lost, rather than just a different way of doing things.

Why? Well, it takes a fair bit of energy to stay hornt. If you’re a male, you have to keep your testosterone high enough to be ready to go at all times – enough “to maintain the male accessory sex glands, spermatogenesis, muscle mass/bone density, and libido in a fully stimulated adult state,” explained oncologist John Isaacs in a 2023 paper. That can have real health consequences: “the reproductive ‘freedom’ that arises from the loss of seasonal breeding is an increased probability of developing prostate cancer,” Isaacs wrote, “as a result of chronically maintaining a hyperplastic state in the prostate.”

For females, the costs are perhaps even more immediate. Pregnancy is, unsurprisingly, hard on a body: it lowers your blood pressure; increases blood volume; massively increases your resource requirements, which can be a problem if, say, it happens during the winter or a drought; it changes both your weight and its distribution, so good luck running from predators. At some point, you have to give birth, which is messy, dangerous, and painful. And to top it all off, there’s this helpless little being that you’re driven instinctively to care for, even to your own detriment, when it’s all over.

Given all that, seasonal breeding is a sensible solution: it saves all that breeding energy for when the resources and environment are best suited for birthing and caring for offspring. “You want your babies to be born at the optimum conditions”, explained scientist and author Kat Arney in a 2009 episode of The Naked Scientists podcast. 

So you wait until a point “where they're going to survive,” she said, “where there's lots of food to nourish the mother during her pregnancy, when there's a nice warm temperature for the babies to be born in and they're not going to freeze, and you know, when it's not obviously really wet or really horrible.”

It’s evidently a winning strategy. But that just raises a new question: why did humans give it up?

Why humans don’t have a mating season

At some point, the branch on the evolutionary tree of which we are a twig lost this more strict “breeding season” system. We stopped going into heat, and started showing more subtle cues to signal that we were fertile; eventually, we lost even those, becoming the concealed ovulation species we are today.

To understand why, we need to take a look at what makes primates different from other species. We’re comparatively brainy; we grew up in relative abundance, even if we weren’t born there; and we’re extremely social.

That’s more important than you might think. Social creatures need prosocial behaviors: “We […] rely on group members for our survival and reproductive success,” Jacqueline Prime, founder of the environmental nonprofit Prime Earth and a wild gibbon researcher, told HowStuffWorks back in 2017. “That means getting along with each other is of paramount importance.” 

And if bonobos can teach us anything, it’s that sex is one of the most prosocial behaviors there are. “Touch and vocal communications help us solidify our bonds as individuals who get along with each other,” explained Prime. “Humans aren't really different from any other non-human primate on this, we just have different styles and ways of doing things.”

Meanwhile, our big brains, social groups, and jungly ancestral homes have meant that we’re less likely than, say, a Siberian ibex to starve or get eaten due to pregnancy. Our babies have loads of eyes looking out for them; our pregnant females have people who can forage or hunt for them. It’s fairly safe to get pregnant whenever. 

And so we do: female humans – and chimps, for that matter – ovulate every 28 days or so on average, offering no outward signals as to their changing fertility (why this latter point is true is a matter of some debate – the most popular hypothesis is that it stops male humans from killing the offspring, because it might be theirs). We sex each other up willy-nilly; we have new children even when the older ones are still reliant on the parents; we celebrate our birthdays from January to December.

Except. There’s one tiny snag.

The twist

Did you know there’s a most common birthday? Yes, obviously that should be the case in the micro detail, but we’re not talking about a difference of one or two births: there’s one date in particular which, in the US at least, is about 12 percent more likely to be your birthday than would be expected at random.

It’s September 9 – the birthday of Otis Redding, Adam Sandler, Michael Bublé, Hugh Grant, Colonel Sanders (yes, that one) and hundreds of millions more. The second most common birthday is September 19, just 10 days later; third on the list is September 12; next is September 17; then September 10… are you spotting a pattern?

Yep: it appears that, for all we don’t think of ourselves as having a mating season, we still do follow some patterns. For whatever reason – factors have been suggested such as the proximity to harvest; the effect of winter weather on sperm quality; the shorter day length and colder temperatures leading to people spending more time inside, and so on – humans do seem to prefer babymaking in December. 

“Evolutionarily, sometimes we lose the full blown ‘need’ for something but retain it nonetheless,” Anjhula Mya Bais, a relationship expert who specializes in social psychology, told HowStuffWorks. “True, humans have evolved over time in order to give birth year around – which is the ‘highest’ evolutionary purpose that sex would serve – [but] a disproportionate amount of people are born in summer, indicating when most people mate.”

It's a pattern that’s more obvious in rural communities, and in places closer to the poles – specifically, it seems that the farther north you get from the equator, the earlier in the year your birth peak is. In the Caribbean, it’s November, for example; up in Finland, it’s the end of April.

But either way, it’s there. Why don’t humans have a mating season? Truth is, we kind of do. 

“Our tendencies may not be as obvious as other primates,” Bais said. “But they exist”.