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.
Smilodon
Another argument for sociality is based on the healed injuries in several "Smilodon" fossils, which would suggest that the animals needed others to provide it food. This argument has been questioned, as cats can recover quickly from even severe bone damage and an injured "Smilodon" could survive if it had access to water. The brain of "Smilodon" was relatively small compared to other cat species. Some researchers have argued that "Smilodon"'s brain would have been too small for it to have been a social animal. An analysis of brain size in living big cats found no correlation between brain size and sociality. Another argument against "Smilodon" being social is that being an ambush hunter in closed habitat would likely have made group-living unnecessary, as in most modern cats. Yet it has also been proposed that being the largest predator in an environment comparable to the savanna of Africa, "Smilodon" may have had a social structure similar to modern lions, which possibly live in groups primarily to defend optimal territory from other lions (lions are the only social big cats today).