I actually like Valentine's Day, but this photo is still the stuff of nightmares
By Madeline McCurry-Schmidt
To quote nearly every tv sitcom cynic in nearly every Valentine’s Day episode: “What’s sooooo special about one day in February?! Ugh!”
And while that sitcom character goes on to learn the value of love, I’m going to argue that the anti-Valentine’s gripe has a biological basis.
You see, humans breed (and fall in love, I guess) year ’round. Though studies show humans have slightly more sex in the winter (being stuck indoors and all that), we’re technically “continuous breeders.” This continous drive to breed makes it unnatural to focus all romance on just one day a year.
The once-a-year concept would make more biological sense if humans were “seasonal breeders.” Species that are “seasonal breeders” don’t have sex on just one day, but they do wait for a specific time of year.
Seasonal breeders include lemurs, horses, groundhogs, sheep, marine iguanas, moose, hamsters, most insects, most birds and some crabs. Seasonal breeding is why animal shelters see tons of kittens in the spring, and why those birds in “Bambi” acted so twitter-pated. As I’ve mentioned before, even earwigs are seasonal breeders.
It’s not like the animals are telling each other “I’m not in the mood” those other times of year. Many animals have evolved hormonal signals that control sex drive. These hormonal signals are often influenced by “photoperiod,” meaning the length of day. Days are shorter in the winter and longer in the summer, and the animal’s brain senses that. 
In mammals, the hypothalamus controls production of a hormone called gonadotropin-releasing hormone (GnRH). From the “gonad” part of the name, you can guess that this hormone has something to do with sex. When the hypothalamus releases GnRH, the pituitary begins releasing hormone called gonadotropin LH, which regulates reproductive function.
In non-scientific language, the mammals get the signal: “DO IT!”
“Short day” breeders, like sheep, get a rush of hormones in the fall, which drives the females to go into estrus. After a winter of gestation, sheep give birth in the spring when food is plentiful. Some sheep do have a mutation that makes it so they can breed all year. But that mutation hasn’t really caught on, probably because having a lamb in the dead of winter isn’t a good plan. Have a baby when food is easy to get, and there’s a better chance it will survive.
Seasonal breeding can also be influenced by other animals in the ecosystem. On the coast of southern Africa, biologists have found that three species of seasonal-breeding crabs stagger their breeding to maximize survival. By emerging at different times of year, the larval crabs avoid inter-species competition for resources. (Meanwhile, a fourth, smaller species of crab breeds continuous to “maximize reproductive output.”)
Others have to line up breeding with migrations. Geese in the north migrate south in the winter, so they lay eggs in the spring to give their goslings enough time to mature and prepare for the flight.
Like every aspect of evolution, the time for breeding is about fitting into a niche–making sure offspring are born when resources are available. Everything has to line up just right.
Too bad climate change has thrown things out of whack. When it rains sooner or gets warm earlier in spring, plants and animals get out of sync. In some regions, photoperiod is no longer a good indicator of temperature. Babies may be born before food becomes plentiful.
For example, many robins (a symbol of springtime in America) migrate from low-altitude habitats in the winter to higher-altitude habitats in the spring. Weirdly, biologists in the Rocky Mountains have observed robins arriving sooner in the springtime than they used to. The biologists think that because the temperature at the lower altitudes is getting warmer sooner, the robins think it’s time for spring sooner, and they migrate. Unfortunately, their habitat in the Rockies is still covered in snow.
Because of climate change, the robins and their food supply get out of sync. Changing temperatures lead the robins to the right place, but the timing is wrong.
As with everything in science, more research is needed. But for seasonal breeders, it seems like climate change is a problem. Some animals, like those mutant sheep, will adapt, but others won’t be as lucky.
How Animals Do It 