<P> Throughout the world, working donkeys are associated with the very poor, with those living at or below subsistence level . Few receive adequate food, and in general donkeys throughout the Third World are under - nourished and over-worked . In temperate climates the forage available is often too abundant and too rich; over-feeding may cause weight gain and obesity, and lead to metabolic disorders such as founder (laminitis) and hyperlipaemia, or to gastric ulcers . </P> <P> In the Iberian Peninsula and the Americas, a burro is a small donkey . The Domestic Animal Diversity Information System (DAD - IS) of the FAO lists the burro as a specific breed of ass . In Mexico, the donkey population is estimated at three million . There are also substantial burro populations in El Salvador, Guatemala, and Nicaragua . </P> <P> Burro is the Spanish and Portuguese word for donkey . In Spanish, burros may also be called burro mexicano (' Mexican donkey'), burro criollo (' Criollo donkey'), or burro criollo mexicano . In the United States, "burro" is used as a loan word by English speakers to describe any small donkey used primarily as a pack animal, as well as to describe the feral donkeys that live in Arizona, California, Oregon, Utah, Texas and Nevada . </P> <P> Among donkeys, burros tend to be on the small side . A study of working burros in central Mexico found a weight range of 50--186 kilograms (110--410 lb), with an average weight of 122 kg (269 lb) for males and 112 kg (247 lb) for females . Height at the withers varied from 87--120 cm (34--47 in), with an average of approximately 108 cm (43 in), and girth measurements ranged from 88--152 cm (35--60 in), with an average of about 120 cm (47 in). The average age of the burros in the study was 6.4 years; evaluated by their teeth, they ranged from 1 to 17 years old . They are gray in color . Mexican burros tend to be smaller than their counterparts in the USA, which are both larger and more robust . To strengthen their bloodstock, in May 2005, the state of Jalisco imported 11 male and female donkeys from Kentucky . </P>

Where does the term donkey's years come from