Deer have dichromatic (two-color) vision with blue and yellow primaries; humans normally have trichromatic vision. Males, therefore, can only be dichromatic, while females can be either dichromatic or trichromatic. Pigeons and doves may be sexually monochromatic or dichromatic. Some populations are melanistic and sexually dichromatic. In fluorescence microscopy, shortpass filters are frequently employed in dichromatic mirrors and excitation filters. Howler species are dimorphic and can also be dichromatic (i.e. "Alouatta caraya"). The gular is dichromatic in bullfrogs, with dominant and fitter males displaying yellow gulars. Hawkfishes are generally not sexually dichromatic, meaning the sexes cannot be distinguished by coloration alone. Males and females are often strikingly sexually dichromatic. The former group are sexually dichromatic, with the females having prominently barred plumage. Horses have two-color, or dichromatic vision, which is somewhat like red-green color blindness in humans. They are sexually dichromatic, as body colour depends on different sexes. Like most mammals, though not primates, they are dichromatic. This means that cattle are dichromatic, as are most other non-primate land mammals. Most mammals are dichromatic, and dogs and horses are often thought to be color blind.