Did you know that humans, mice, monkeys, and other mammals experience two puberties?

It's the second you're no doubt aware of. In a process that initiates in the early teens, boys and girls are flooded with hormones produced by their sex glands. The result is a mental and physical transformation. Boys grow facial hair, develop a deeper voice, and double their skeletal muscle. Girls' breasts enlarge and their hips widen. Both boys and girls grow pubic hair, become sexually fertile, take on an "adult" body odor, and generally annoy their parents.

But more than a decade before this second puberty, the one chronicled again and again in coming-of-age movies, there is a first puberty, a so-called "mini-puberty." Originally described back in the 1970s, scientists are still working today to understand it. Here's what they know: Roughly one to two weeks after birth, newborn boys and girls endure a rush of hormones, a process known as the Postnatal Endocrine Surge. Luteinizing hormone and testosterone dominate male mini-puberty, while follicle-stimulating hormone and estradiol (estrogen) hallmark female mini-puberty. The process lasts around four to six months in boys and a little longer in girls, concluding with hormone levels abating to typical childhood levels.

Exactly why mini-puberty happens is unclear, but the bodily changes it causes are quite clear. In a commentary recently published to the journal Pediatrics, University of Oklahoma pediatricians Kenneth Copeland and Steven Chernausek described a few of them.

"Resultant effects on the reproductive organs include testicular, penile, and prostate growth in boys, uterine and breast enlargement in girls, and sebaceous gland and acne development in both sexes."

According to a recent study, the testosterone surge that boys experience during this phase likely accounts for why adolescent boys are a little taller than adolescent girls. The surge also explains roughly fifteen percent of the height difference between adult men and women.

Research is increasingly showing that mini-puberty is a sensitive time of development, acting as a "stress test" of sorts for the endocrine system, revving it up to ensure lifelong function. It also primes target tissues -- particularly in the reproductive system -- for growth and maturation later in life. Lastly, and more controversially, mini-puberty may be a time where sexual orientation and behavior are imprinted, with hormones playing a defining role. Multiple studies evince this claim in animals, but research in humans is lacking.

Mini-puberty is not nearly as conspicuous as its older, more mature sibiling, but its effects may be just as consequential. Further research will undoubtedly reveal its outsized role in human development.

(Sandra J. Milburn/The Hutchinson News via AP)