In the vast majority of animals, when food is scarce, the first effects are on the body, which shrinks more and more to run out of fat until it wrinkles. However, there are some parts of the body that are preserved from this “shrinkage,” parts usually considered essential for life. For example, in humans, the head is always the same size because it contains the brain, the primary organ for our survival.

In other animals, this part may be different. In fruit flies, for example, whose life span is only about 45 days and whose purpose of existence is only to reproduce in order to keep the species going, it is the genitals that are considered as the primary organ, one of those that should not be affected by the lack of food and nutrients. When the availability of food is low, in fact, fruit flies keep the size of their genitals constant in order to be more likely to reproduce. Never as in this case, does size really matter.

This is what researchers at the University of Illinois and Loyola University in Chicago have discovered. In fact, scientists have discovered lower levels of a protein in the genitals of these small insects that act as a negative growth factor, called FOXO, a clear sign of maintaining reproductive success. The study, published in Biology Letters, describes how this negative growth factor, which counteracts the growth of other parts of the body due to lack of nutrients and food, acts much less in the genital area.

By artificially increasing the activity of the FOXO protein in the genitals of these malnourished gnats, the genitals themselves began to decrease in size, up to 29%, like other parts of the body.

“Our results suggest that there was significant selective pressure to limit the amount of FOXO in the genitals of the fruit flies, a way to ensure reproductive success, given the female’s preference for mating with males with larger genitalia,” reports Alexander Shingleton, professor of biological sciences and author of the study together with colleague Austin Dreyer.