Penis
Domestic cats have barbed penises, with about 120–150 one millimeter long backwards-pointing spines. Upon withdrawal of the penis, the spines rake the walls of the female's vagina, which is a trigger for ovulation. Lions also have barbed penises. Male felids urinate backwards by curving the tip of the glans penis backward. When male cheetahs urine-mark their territories, they stand one meter away from a tree or rock surface with the tail raised, pointing the penis either horizontally backward or 60° upward.
Evolution of nervous systems
There has been a gradual increase in brain volume as the ancestors of modern humans progressed along the human timeline of evolution (see Homininae), starting from about 600 cm in "Homo habilis" up to 1736 cm in "Homo neanderthalensis". Thus, in general there is a correlation between brain volume and intelligence. However, modern "Homo sapiens" have a smaller brain volume (brain size 1250 cm) than neanderthals; women have a brain volume slightly smaller than men, and the Flores hominids ("Homo floresiensis"), nicknamed "hobbits", had a cranial capacity of about 380 cm, about a third of the "Homo erectus" average and considered small for a chimpanzee. It is proposed that they evolved from "H. erectus" as a case of insular dwarfism. In spite of their threefold smaller brain there is evidence that "H. floresiensis" used fire and made stone tools as sophisticated as those of their proposed ancestor, "H. erectus". Iain Davidson summarizes the opposite evolutionary constraints on human brain size as "As large as you need and as small as you can".