Among animals in isolation, and sometimes in freedom--though this can less often be observed--it is well known that various forms of spontaneous solitary sexual excitement occur. Horses when leading a lazy life may be observed flapping the penis until some degree of emission takes place. Welsh ponies, I learn from a man who has had much experience with these animals, habitually produce erections and emissions in their stalls; they do not bring their hind quarters up during this process, and they close their eyes, which does not take place when they have congress with mares. The same informant observed that bulls and goats produce emissions by using their forelegs as a stimulus, bringing up their hind quarters, and mares rub themselves against objects. I am informed by a gentleman who is a recognized authority on goats, that they sometimes take the penis into the mouth and produce actual orgasm, thus practicing auto-fellatio. As regards ferrets, the Rev. H. Northcote states: "I am informed by a gentleman who has had considerable experience of ferrets, that if the bitch, when in heat, cannot obtain a dog she pines and becomes ill. If a smooth pebble is introduced into the hutch, she will masturbate upon it, thus preserving her normal health for one season. But if this artificial substitute is given to her a second season, she will not, as formerly, be content with it."[179] Stags in the rutting season, when they have no partners, rub themselves against trees to produce ejaculation. Sheep masturbate; as also do camels, pressing themselves down against convenient objects; and elephants compress the penis between the hind legs to obtain emissions.[180] Blumenbach observed a bear act somewhat similarly on seeing other bears coupling, and hyenas, according to Ploss and Bartels, have been seen practicing mutual masturbation by licking each other's genitals. Mammary masturbation, remarks Féré, is found in certain female and even male animals, like the dog and the cat.[181] Apes are much given to masturbation, even in freedom, according to the evidence of good observers; for while no female apes are celibates, many of the males are obliged to lead a life of celibacy.[182] Male monkeys use the hand in masturbation, to rub and shake the penis.[183]