How autumn changes animal sexual behavior—including that of humans

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: In the Northern Hemisphere, days are getting shorter and temperatures are dropping—autumn has arrived. A common sign that winter is coming is trees losing their leaves.

But did you know that autumn also sparks changes in animals' sexual behavior—including ours? For example, humans actually have a breeding season. The hormone testosterone surges in autumn, particularly in males, linked to a decrease in daylight and increased social activity indoors.

This leads to more conceptions around November and December, with a subsequent peak in births the following August and September, nine months after the hormone surge. No wonder the most common birthday worldwide is Sept. 26. Here are five more seasonal changes in sexual behavior in the natural world.

Red deer spend most of the year in separate male and female herds. However, the autumnal increase in testosterone, typical of animals that mate during the autumn, prompts male red deer to become more aggressive and begin the "rut." This is a time when the males engage in antler-locking fights to win access to groups of females known as harems. The holder of a harem gets to mate with the females in his group and father offspring.

Hence, competition is fierce. Mating during the autumn ensures that the females give birth in late spring, when food is abundant and the weather is more favorable, increasing offspring survival. Red deer are also responding to climate warming, giving birth almost two weeks earlier.

Although heterosexuality is prevalent during the reproductive season, autumn brings a return to homosexual behavior for many species. This "seasonal bisexuality" has been observed anecdotally in species including red foxes, red squirrels and bats, with many species appearing to switch fluidly between heterosexual and homosexual couplings. Interestingly, this behavior is not due to a lack of partners of the opposite sex.