To be successful, an olfactory predator not only must detect the presence of prey but also must be able to determine its location. Following an odor plume, however, is not as easy as following a depositional odor trail. For one thing, it is more difficult for a predator to track an odor plume than the depositional odor trail because an odor plume lacks sharp gradients. If a predator strays from a depositional odor trail, then it can quickly determine its error because it will detect a rapid decrease in odorant concentration. In contrast, a predator following an aerial odor trail is immersed in a cloud of odorants, making it hard to tell where the odorant concentration is greatest. At least for insects, it is much easier to detect an odor plume than to follow it to its source. For instance, many male gypsy moths can detect the sexual pheromone of a female at distances of over 100 m, but few males at such distances can locate the female (David et al. 1983; Elkinton et al. 1987). Nevertheless, both insects and mammals can detect odor plumes over considerable distances and follow them to their sources.