Tortoises preyed on may be nearly as heavy as the preying vulture. They are opportunistic feeders and can be carnivorous, preying mostly on lizards and small birds. There are records of some male leopards preying on hyenas. Muggers have also been observed while preying and feeding on a python. Groups of female lions usually hunt together, preying mostly on large ungulates. Cases are known of stoats preying on young muskrats. Slavers continued to take captives by preying on less well-protected peoples. In Africa, lions are threatened by pre-emptive killing or in retaliation for preying on livestock. There is considerable potential for human-leopard conflict due to leopards preying on livestock. Avivores are animals that are specialised at preying on birds. European naturalists have observed bats preying upon earwigs. Instances of fishers preying on cats and small dogs have been reported. In Africa, lions are killed pre-emptively or in retaliation for preying on livestock. Numerous native species have been reported as successfully preying on toads. In the dry season it might have resorted to preying on pterosaurs.