Silverback gorillas are famous for their impressive, bulging physiques and their rather modest genitalia. Now, scientists have uncovered a potential genetic link between these apes' small members and infertility problems in male humans.

Coming in at just 1.1 inches (3 centimeters) long, on average, the penis of the adult male gorilla (Gorilla) is the smallest phallus of all apes. The gorilla's genital size comes with other deficits in its reproductive capacity, such as low sperm count compared to other primates, and sperm with poor motility and a diminished ability to bind to eggs.

Given that these are reproductive issues that can also affect humans, it may seem surprising that all male gorillas share these traits. However, this can be explained by gorillas' mating system, said Jacob Bowman, lead author of the new study and a postdoctoral researcher at the University at Buffalo.

Gorillas operate in a polygynous system, in which a dominant male has near-exclusive access to females in his troop. The silverback's unwieldy physique means it has no problem securing mates, and thus, its sperm doesn't have to compete with that of other males and it can produce offspring without many, highly motile swimmers. The theory is that this lack of sperm competition led to the evolution of gorillas' small genitalia.

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This got researchers "wondering if, at a genetic level, we can find genes associated with spermatogenesis [sperm production] or that we see leading to poor-quality sperm," Bowman told Live Science. Gorillas and humans share the vast majority of the same genes so if the researchers could pinpoint suspect genes in gorillas, they could next turn their attention to the human genome.

Roughly 15% of U.S. couples have trouble conceiving, according to Yale Medicine, and more than half of those cases involve male infertility. Around 30% of infertility cases have a genetic basis, said Vincent Straub, a doctoral student in population health at the University of Oxford who was not involved in the new study. However, the genes involved in male infertility are poorly understood.

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To help unravel those genetics, Bowman and colleagues combed through a database of more than 13,000 genes across 261 mammals. This involved looking at genes' underlying sequences, to see how they changed over time in related animals. The aim was to see if certain genes in the gorilla branch of the tree of life were evolving at dramatically reduced rates, Bowman said.

This can happen when there isn't strong pressure to get rid of genetic mutations that could hinder a population's survival such as those related to gorillas' low-quality sperm. This process, called "relaxed purifying selection," can result in seemingly harmful mutations becoming common in a species.

The data turned up 578 genes in the gorilla lineage that underwent this type of selection. An analysis and existing data suggested that many of these genes are involved in making sperm. However, not all the flagged genes had known roles in male fertility.

To better understand these genes' functions, the team turned to the fruit fly (Drosophila melanogaster), a commonly used genetic model in biology. They systematically silenced each of the genes in male flies to see if they affected the insects' ability to reproduce. In this way, they uncovered 41 new genes that hadn't previously been tied to male fertility.

The researchers then connected the dots back to humans using a genetic database with data from 2,100 men with infertility, who either had very low amounts or a lack of sperm in their semen. They also looked at data from fertile men, focusing on the genes they'd flagged in gorillas. They found that, in 109 of relaxed gorilla genes, the infertile men carried more loss-of-function mutations than did fertile men; loss-of-function mutations reduce a gene's ability to make the protein it codes for.

While it's likely these genes are involved in human male fertility, more research is needed to learn exactly how they work in the body. Straub emphasized that infertility is very complex, and that not all of it comes down to genetics. To fully understand it, scientists need to account for how different genes interact with one another and with an organism's environment and its behavior.

The findings drawn from gorillas open the door to future explorations about how these genes, and others closely associated with them, might influence fertility in people, Straub said. The study was published May 9 in the journal eLife.

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