skull Although
omnivorous kangaroos lived in the past, these were not members of the family Macropodidae; modern macropods are generally
herbivorous. Some are
browsers, but most are
grazers and are equipped with appropriately specialised teeth for cropping and grinding up fibrous plants, in particular
grasses and
sedges. Modern omnivorous kangaroos generally belong to a different family (for example, the
musky rat-kangaroo). In general, macropods have a broad, straight row of cutting teeth at the front of the mouth, no
canine teeth, and a gap before the
molars. The molars are large and, unusually, do not appear all at once but a pair at a time at the back of the mouth as the animal ages, eventually becoming worn down by the tough, abrasive grasses and falling out. Like many
Macropodiformes, early kangaroos had
plagiaulacoids, but these converted into normal molars in more derived species. Most species have four molars and, when the last pair is too worn to be of use, the animals starve to death. The
dental formula for macropods is . Like the
eutherian
ruminants of the Northern Hemisphere (
sheep,
cattle, and so on), macropods have specialised digestive systems that use a high concentration of
bacteria, protozoans, and fungi in the first chamber of a complex stomach to digest plant material. The details of organisation are quite different, but the result is somewhat similar. The particular structure-function relationship of the Macropodidae gut and the gut
microbiota allows the degradation of
lignocellulosic material with a relatively low emission of
methane relative to other ruminants. These low emissions are partly explained by the anatomical differences between the macropodid digestive system and that of ruminants, resulting in shorter retention times of particulate digesta within the foregut. This fact might prevent the establishment of methanogenic
archaea, which has been found in low levels in tammar wallabies (
Notamacropus eugenii) and eastern grey kangaroo (
M. giganteus). Metagenomic analysis revealed that the foregut of tammar wallabies mainly contains bacteria belonging to the phyla
Bacillota,
Bacteroidota, and
Pseudomonadota. Among Pseudomonadota populations of the
Succinivibrionaceae family are overrepresented and may contribute to low
methane emissions. Macropods vary in size considerably, but most have very large hind legs and long, powerfully muscled tails. The term
macropod comes from the
Greek for 'large foot', as most have very long, narrow hind feet with a distinctive arrangement of toes. The fourth toe is very large and strong, the fifth toe moderately so; the second and third are fused; and the first toe is usually missing. Their short front legs have five separate digits. Some macropods have seven
carpal bones instead of the usual eight in mammals. All have relatively small heads and most have large ears, except for
tree-kangaroos, which must move quickly between closely spaced branches. The
young are born very small and the
pouch opens forward. The unusual development of the hind legs is optimised for economical long-distance travel at fairly high speed. The greatly elongated feet provide enormous leverage for the strong legs, but the famous kangaroo hop has more: kangaroos and wallabies have a unique ability to store elastic strain energy in their tendons. In consequence, most of the energy required for each hop is provided "free" by the spring action of the tendons (rather than by muscular effort). The main limitation on a macropod's ability to leap is not the strength of the muscles in the hindquarters, it is the ability of the joints and tendons to withstand the strain of hopping. with a joey Furthermore, the act of hopping in kangaroos and wallabies is associated with their breathing process. The movement of their feet off the ground helps to expel air from their lungs, while bringing their feet forward for landing replenishes their lungs with air, resulting in greater energy efficiency. Studies conducted on these animals have shown that hopping at faster speeds requires only a minimal increase in effort beyond the energy required to hop in general, which is significantly less than what would be required in other animals like horses, dogs, or humans. Additionally, it has been observed that carrying extra weight requires little additional energy, which is particularly important for female kangaroos and wallabies carrying heavy pouch young. The ability of larger macropods to survive on poor-quality, low-energy feed, and to travel long distances at high speed without great energy expenditure (to reach fresh food supplies or waterholes, and to escape predators) has been crucial to their evolutionary success on a continent that, because of poor soil fertility and low, unpredictable average rainfall, offers only very limited primary plant productivity. Most macropod species have a
polygynous mating system and produce a
mating plug after
copulation.
Gestation in macropods lasts about a month, being slightly longer in the largest species. Typically, only a single young is born, weighing less than at birth. They soon attach themselves to one of four teats inside the mother's pouch. The young leave the pouch after five to 11 months, and are weaned after a further two to six months. Macropods reach
sexual maturity at one to three years of age, depending on the species. ==Fossil record==