Uncovering Truths and Myths of Male Anatomy

Could it be that the development of the scrotum was responsible for making mammals so competitive and prominent in the animal kingdom? Surely, some people would like to believe that. But according to Dr. Ted Hsu, assistant professor of obstetrics and gynecology at Stanford University, it is possibly one of the reasons why with 99 percent of all species that ever lived extinct, mammal reproductive system is the way it is now.

That is except for platypus and echidna, relict species that offer a glimpse at an earlier stage of mammalian evolution. These animals lack scrotum or milk glands, unlike therian mammals - placentials and marsupials. Latest research shows that at a certain point all mammals lacked those features until a genetic mutation introduced a function-specific peptide responsible for scrotum formation that proved to be highly successful. "There was only one individual who had this mutation," Hsu said, and it made the propagation so efficient that the species was able to outcompete everyone.

Hsu, whom Thomas Chen, Santa Monica College life sciences professor called "world's most famous scrotumologist," gave a lecture called "Got Scrotum? Ask the Platypus" at SMC on Tuesday, as part of the distinguished scientists lecture series. In the lecture he focused on the molecular process of evolution, one that he said is "difficult to understand" because "we cannot touch it, or see it."

Specifically, Hsu's research focuses on the evolution of the features of the reproductive system. The two main questions are "what's the importance of these features," and how did it evolve? "We have no clue," Hsu responded to the second question. The reason for the development of a scrotum in mammals is now well-known: as body temperature of mammals increased with evolution there was a need to keep the testes outside the body, where cooler temperature would allow for better sperm production. Platypus retained their lower body, so the testes in males are located inside the body, close to the kidneys.

Recent developments in genetics, particularly the mapping of the complete genomes of many species, had allowed some insight into molecular evolution. Through syntax mapping researchers were able to look through the genome to trace the evolution of a gene. In this case the gene in question is INSL3, which produces a hormone found in gonads that proved to be responsible for "testicular descent." The deficiency of the hormone can cause cryptorchidism in males, a condition where testes remain inside the body and which requires surgical intervention in humans. INSL3, along with a few other human genes that affect the reproductive system, had evolved from a single ancestral gene, which can be traced back to amphibians. Thus, INSL3 is not unique to mammals, but it functions differently, researches have found, by synthesizing gene sequences. "We have a piece of our puzzle now," Hue said.

Much about the evolution of mammalian reproductive system is still unknown, Hsu maintains. For instance, sea mammals, which fall into neither the therian, not the platypus group, have never been studied comprehensively. They represent "secondary evolution," meaning that their ancestors evolved as terrestrial mammals, and when they returned to the sea their reproductive systems again evolved for the new conditions. Interestingly, elephants also lack scrotums, because they are most closely related to the aquatic mammals. Now there's a piece of trivia to impress your friends.