Though to an outside observer, they are by all rights very much human-looking, they do indeed behave as animals. They are filthy and matted with dirt, they stink, and while they are capable of an omnivorous diet, they seem to prefer meat and garbage. Specifically, they are noted to consume nearly everything that is prohibited by the biblical and Levitical food codes. Of the Yahoo, Gulliver states they are: "the most filthy, noisome, and deformed animals which nature ever produced . . . " and they are "restive and indocible, mischievous and malicious." The Yahoos are not simply feral and animalistic, they are man-animals who are naturally vicious and represent the crudest and most corrupt of mankind's primal nature. Swift describes them in deliberately offensive and off-putting terms, frequently drawing them into metaphors with dung. On the evolutionary scale, the words Swift uses to describe the Yahoos are "degenerating by degrees."
Gulliver is painted as being midway between the two opposite ends of this spectrum, both figuratively and literally. He shares some similarities with both races, in some ways like the hyper-rational, and innocent Houyhnhnms, and in other ways like the filthy, emotion-driven Yahoos. At first, Gulliver refuses to admit having anything at all in common with the Yahoos, or vice-versa. He is shown to detest them, and is horrified upon further examination by the Yahoos' similarity to him. Yet he still lacks the humility to see himself as any sort of Yahoo, instead his pride leads him to the absurdity of trying to become a horse. Gulliver will endeavor with admirable determination to improve upon himself; he will strive to change himself into a more horse-like state, but he will obviously fail. To his dismay, he is quite clearly more of a Yahoo than a Houyhnhnm.
Yahoos are stronger and faster than ordinary humans, and even their infants are able to move quickly and swim.
While Yahoos tend to be aggressive by nature, red-haired Yahoos of both sexes are be more aggressive and sexually-active than other Yahoos. Nonetheless, if they feel attracted towards another Yahoo, they will try to force the other to have sexual intercourse with him or her.
Yahoos make their homes (known by Gulliver as "kennels") by digging deep holes at rising slopes with their nails, and the dens dug by their females are larger, large enough to contain two to three young Yahoos.
Yahoos have a tendency to dig for gems and colorful stones found in the mud, and they hoard their gems in their dens and fight against other Yahoos who seek to steal those gems.
Interaction with other species Edit
The Houyhnhnms despise the Yahoos for their savage behavior and for damaging their farms, that the name applied towards them became an adjective for anything bad.
Houyhnhnms sometimes use Yahoos to carry carts, in an unknowing inversion of humans' usage of horses as beasts of burden. They keep Yahoo servants or slaves at huts next to their homes. They also kill Yahoos and cut their skin to make leather and sails.
Although Gulliver himself was a human, he was so disgusted with the Yahoos and their behavior, that it resulted in him preferring to be with horses instead of his fellow humans. Gulliver also refers Yahoo children as "cubs".
At one time when Gulliver took a bath by a river, a black-haired female Yahoo was attracted by his naked form and tried to rape him until his Houyhnhnm companion repelled her.