Herbivores are consumed by omnivores or carnivores. Herbivores, omnivores and carnivores are all heterotrophs. Within the subclass Lepidosauria there are herbivores, omnivores, insectivores, and carnivores. Carnivores, Omnivores and herbivores are all basic examples of guilds. Heterotrophs can be classified by what they usually eat as herbivores, carnivores, omnivores, or decomposers. Today, many pterosaurs groups are thought to have been terrestrial carnivores, omnivores or insectivores. Dogs have been described as carnivores or omnivores. Today wolves (carnivores) and bears (omnivores) are endangered. Carrion is an important food source for large carnivores and omnivores in most ecosystems. Most livestock are herbivores, except for pigs and chickens which are omnivores. These nutritional requirements place the dog part-way between carnivores and omnivores. Some are herbivorous, others carnivorous, including seed-eaters, fish-eaters, insectivores, and omnivores. Vulcans are vegetarians by choice and were omnivores in ages past. A 2011 Canadian study found that both omnivores and vegetarians perceived vegetarians as less masculine. They eat the dung of herbivores and omnivores, and prefer that produced by the latter.