Mother hyena and cubs in Kenya.
Credit: Kay Holekamp's laboratory, Michigan State University
When a mother does her best, she expects a well-behaved child. But for top-dog hyena moms, a hell-raiser is preferred.
Alpha females give a hormone boost to their developing cubs, making them more aggressive when fighting for food and increasing their chances of survival, according to a study in the April 27 issue of the journal Nature.
The extra hormones also inspire young males to mount females early and often, giving them a better shot at performing their tricky mating dance correctly down the road. [Top 10 Swingers of the Animal Kingdom]
You won't believe how hard the act is, and why.
Unlike most mammalian societies, female spotted hyenas run the show and are significantly more muscular and aggressive than males. After studying hyenas in Kenya for nearly two decades, researchers discovered that in the final stages of pregnancy, high-ranking females provide their developing offspring with higher levels of androgen—a male sex hormone associated with aggression—than lower-ranking mothers provide to their developing young.
This is the first study to show that a mother's social status, and not just her genetic makeup, can directly affect her offspring's observable physical characteristics.
Aggressiveness is a good attribute for a creature living in a society where 40 to 60 individuals scrap over food, and especially for females requiring extra energy for developing offspring.
By infusing her developing young with androgen, the mother increases the likelihood that her genetic information will survive.
'Imagine giving birth through a penis'
But providing the extra hormones takes a toll on the mother. The dose of androgen that she received from her own alpha mother damages her ovaries, making it difficult to conceive.
It also causes female reproductive organs to grow. A lot. Her clitoris, which contains the birthing canal, protrudes 7 inches from her body.
"Imagine giving birth through a penis," said study co-author Kay Holekamp of Michigan State University. "It's really weird genitalia, but it seems to work. Although giving birth through a 'penis' isn't a trivial problem."
The clitoris' birth canal is only an inch in diameter, and the tissue often tears as a 2-pound cub squeezes through the narrow opening. The rip can be fatal, as evidenced by the high death rate for first-time mothers.
Practice makes perfect
Because of the female's awkward genitalia, successful mating for hyenas is tricky to pull off. It takes careful positioning for the male to crouch behind her and somehow get his penis to point up and backwards to enter her clitoris.
"Males need practice. After a couple of months of practicing, they get it lined up just right," Holekamp told LiveScience.
Since the sons of alpha females are born hyper-aggressive, they start trying to mount females at just a few months old, giving them a better shot at sealing the deal later in life.