The animal on the cover of Erlang Programming is a brush-tailed rat kangaroo (Bettongia penicillata). The brush-tailed rat kangaroo is a small mammal found in western and southern Australia. It is a cross between a rat and a small wallaby, and although some of its features are reminiscent of a rat, it is not a rodent and is instead classified as a marsupial. In south Australia, they are found in semi-arid scrublands and grasslands; in western Australia, they prefer eucalyptus forests containing a vegetative layer of tussock grass, low woody scrub, and occasional bare patches of ground. They once inhabited more than 60% of the Australian mainland, but now they inhabit less than 1%.

Brush-tailed rat kangaroos have an unusual mammalian diet that consists of bulbs, tubers, seeds, insects, resins, and underground fungi; they do not drink water or eat green plants. Although fungi are not considered a good food source for mammals in general, they provide the nutrients necessary for the brush-tailed rat-kangaroo’s health. The kangaroos’ coats are yellowish-gray in color, their feet are pale brown and have hairs that bristle, and their long tails have a prominent black crest. Their tails are also useful: brush-tailed rat kangaroos are able to curl their tails to carry bundles of material to build their nests. They are relatively slow-moving creatures, but are able to hop away
quickly when disturbed.

Brush-tailed rat kangaroos are extremely nocturnal. During the day they rest in well constructed, hidden nests made up of grass and shredded bark. They appear to be solitary except when ready to mate. Mating occurs year round, and females give birth to one young after a gestation period of 21 days. The newborn remains in the mother’s pouch for about 98 days, and then stays in a nest until a new infant is born. As with many other kangaroos, the brushtailed
rat kangaroo mates shortly after giving birth and can keep embryos in a state of dormancy until they are needed.