'data.frame':	500 obs. of  4 variables:
 $ year   : int  2001 1987 1989 1979 1987 1987 2000 1981 1987 1987 ...
 $ genus  : Factor w/ 3 levels "Dipodomys","Neotoma",..: 2 2 2 2 2 2 2 2 2 2 ...
 $ species: Factor w/ 3 levels "albigula","fulviventer",..: 1 1 1 1 1 1 1 1 1 1 ...
 $ weight : int  280 278 275 274 270 269 265 264 260 260 ...

        genus     species
1     Neotoma    albigula
197  Sigmodon fulviventer
267 Dipodomys spectabilis

[1] 24
[1] 38.92857
[1] 198.8667