Among donkeys, burros tend to be on the small side. A study of working burros in central Mexico found a weight range of , with an average weight of for males and for females. Height at the withers varied from , with an average of approximately , and girth measurements ranged from , with an average of about . 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 US, 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.