They then form a bud, which becomes genitalia. The cells that turn into either a penis or a clitoris start on opposite sides of the embryo.Īs the embryo forms, and it changes from a flat sheet to a 3D embryo, these cells meet. In snakes and lizards, the cloaca is located closer to the lateral plate mesoderm, the same tissue that makes the paired limbs, receives the signal.īut, in mammals, the cloaca is closer to what’s known as the ‘tail bud’, found at the bottom of the animal’s trunk, or torso. The cloaca's location determines which tissues receive these signals first. The embryonic cloaca - which eventually develops into the urinary and gut tracts - sends signals that tell nearby cells to form genitals. Snake and lizard genitalia is derived from the same tissue that creates hind legs, while mammalian genitalia derives from the tail bud. Geneticists recently studied cloaca (shown) to see how external genitalia forms and why it grow in different regions of the body in different animalsĪnd in November, a team of geneticists at Harvard Medical School's Department of Genetics discovered not only how such external genitalia forms, they identified why the genitals grow in different regions of the body in different animals. Researchers at Stanford University in California found a molecular mechanism that could explain why we lost a piece of non-coding DNA that influences the expression of the androgen receptor gene involved in hormone signalling. The chimpanzee penis has spines on it, designed to temporarily injure females when they mate in order to delay her from mating with other males too soon.Ī study in 2011 discovered evidence to suggest humans once had spines on their penises, but evolved to make them smooth. He added that the differences don't just stop there though. 'Or perhaps it acts as a radiator, releasing heat when we walk and helping to keep our bodies cool' 'Why is ours the longest? It may be because we walk upright on two feet, making it more conspicuous to play a role in attracting more females. Chimps have more competition so produce larger numbers of sperm, but gorillas have a hierarchy so they don't need as muchīut he said there wasn't a discernible reason why there is so much variation in penis size between humans and great apes. For testicles, this difference is explained by how societies are organised. Gorillas (left) have small penises and small testicles, while chimps (right) have medium-sized penises and large testicles. He said that if a female is 'receptive to multiple males in each reproductive episode', it maximises her chances of getting high-quality DNA. 'It all comes down to the amount of competition the chimps, and their sperm, have when they mate with females.' 'Why the difference? Well for the testicles it's pretty straight forward. 'Amazingly the size of the reproductive organs, especially the testicles, are a reflection of how the apes are organised socially,' explained Professor Curnoe. Gorillas have small penises and small testicles, while chimps have medium-sized penises and large testicles.īy comparison, human males have the longest penis and medium-sized testicles. The relative size of the male reproductive organs across these species was discussed by evolutionary biologist Darren Curnoe in his latest 'How Did We Get Here?' video series for the University of New South Wales, Australia. Gentlemen, if you've ever been concerned about the size of your penis, take comfort in the fact that no matter how small it seems, it is one of the largest in the primate world.Īn evolutionary biologist has compared the sizes of genitals of humans to gorillas and apes to look at why there is such a discrepancy between the species.Īnd he concludes the human penis may have grown so long to make it more conspicuous to potential partners, or even as a way of cooling the body down. An evolutionary biologist has compared the sizes of human genitals to gorillas and apes to look at why there is such a discrepancy between the species
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