Sexual dimorphism only occurs in ______.

Sexual dimorphism refers to the differences in appearance between males and females of the same species, which can include variations in size, coloration, and other physical traits. In the context of waterfowl, ducks often display pronounced sexual dimorphism, especially in species such as the mallard where males exhibit vibrant plumage for attracting mates, while females tend to have more subdued colors for camouflage purposes.

While geese and swans also exhibit some degree of sexual dimorphism, the most striking examples come from a variety of duck species. The differences observed in ducks are generally more pronounced and diverse compared to those in the other categories. This distinct characteristic sets ducks apart in terms of sexual dimorphism within the larger grouping of waterfowl.