23282584-2.less

.animal(@color: whatever, @imgWidth: null) {
    &   when (iscolor(@color))     {color: @color}
    img when (isnumber(@imgWidth)) {width: @imgWidth}
}

@media screen and (max-width: 20em) {
   .animal(@color: red, @imgWidth: 3em);
}

@media screen and (min-width: 21em) {
   .animal(@imgWidth: 5em);
}

@media print {
   .animal(@color: black);
}