Although members of most wallaby species are small, some can grow up to approximately two metres in length (from head to end of tail). Their powerful hind legs are not only used for bounding at high speeds and jumping great heights, but also to administer vigorous kicks to fend off potential predators. The Tammar wallaby ("Macropus eugenii") has elastic storage in the ankle extensor tendons, without which the animal's metabolic rate might be 30–50% greater. It has also been found that the design of spring-like tendon energy savings and economical muscle force generation is key for the two distal muscle–tendon units of the Tammar wallaby ("Macropus-Eugenii"). Wallabies also have a powerful tail that is used mostly for balance and support. Wallabies are herbivores whose diet consists of a wide range of grasses, vegetables, leaves and other foliage. Due to recent urbanization, many wallabies now feed in rural and urban areas. Wallabies cover vast distances for food and water, which is often scarce in their environment. Mobs of wallabies often congregate around the same water hole during the dry season. Wallabies face several threats. Wild dogs, foxes, and feral cats are among their predators. Humans also pose a significant threat to wallabies due to increased interaction (wallabies can defend themselves with hard kicks, and biting). Many wallabies have been involved in vehicular accidents as they often feed near roads and urban areas. Wallabies are not a distinct genetic group. Nevertheless, they fall into several broad categories. Typical wallabies of the genus "Macropus", like the agile wallaby ("Macropus agilis"), and the red-necked wallaby ("Macropus rufogriseus") are most closely related to the kangaroos and wallaroos and, size aside, look very similar. These are the ones most frequently seen, particularly in the southern states. Rock-wallabies (genus "Petrogale"), rather like the goats of the Northern Hemisphere, specialise in rugged terrain and have modified feet adapted to grip rock with skin friction rather than dig into soil with large claws. There are at least 15 species and the relationship between several of them is poorly understood. Several are endangered. Captive rock-wallaby breeding programs, like the one at Healesville Sanctuary, have had some success and a small number have recently been released into the wild. The banded hare-wallaby ("Lagostrophus fasciatus") is thought to be the last remaining member of the once numerous subfamily Sthenurinae, and although once common across southern Australia, it is now restricted to two islands off the Western Australian coast which are free of introduced predators.