A brief essay in a recent issue of New Scientist magazine called "Infanticide drives female promiscuity and big balls" caught my eye. (The title for the print edition of this essay is "Big balls a sign of mammal infanticide.") The essay is a summary of a research paper published in Science magazine by researchers Dieter Lukas and Elise Huchard with the less flashy title, "The evolution of infanticide by males in mammalian societies."
The original paper is only available to subscribers and was widely reviewed in popular media, including Carl Zimmer's essay in The New York Times called "Unraveling Why Some Mammals Kill Off Infants." Mr. Zimmer writes, "The authors of the new study, Dieter Lukas and Elise Huchard, started by plowing through the scientific literature, looking for evidence of infanticide in a variety of mammalian species. The researchers ended up with data on 260 species, and in 119 of them - over 45 percent - males had been observed killing unrelated young animals." Furthermore, infanticide evolved because "Killing the offspring of other males can free up females for reproduction and widen the window of opportunity for new males, leading to more offspring for them." Forty years ago renowned researcher Sarah Blaffer Hrdy put forth the same idea based on her research on Hanuman langurs in India.
"Great balls of fury"
One aspect of the study that Mr. Zimmer didn't cover concerned the strong relationship between infanticide and testicle size. The New Scientist summary focused on this and began, "Great balls of fury. Large testicles could be a giveaway sign that the males of a species are given to killing their rivals' offspring." It concluded, "A female evolutionary response to infanticide is to become promiscuous, which blurs the paternity of their offspring. That in turn forces males into a race to produce more sperm and boost their chances of fatherhood, so they grow larger testicles." It's highly likely there would be some limiting factors on testicle size and that stabilizing selection would come into play.
Stay tuned for more on the fascinating world of the animals with whom we share our magnificent planet. This essay made me think about Jerry Lee Lewis's popular song, "Great Balls of Fire." I bet he didn't know that he was touching on evolutionary theory.