Koalas have a polygynous mating pattern, where their life-history is characterized by lengthy infant dependence and their exquisite conversion of energy. Marsupial maternal investment is believed to be less depleting compared to placental investment because loss of a joey may have minimal consequences in relation to lifelong successful reproduction. However, the prolonged period of maternal care that is required of marsupials is quite similar to that of placentals. Koalas are pregnant for only one month, but joey emergence doesn't occur until they are 5-7 months old. Koala joeys are only ~2 centimeters long at birth, weighing less than 1 g, their offspring resembles that of a jellybean. Joeys will cling to their mothers until they are 9-10 months, allowing for assisted transportation, at which time they are to grow to ~26% of maternal mass. At around 1 year of age, koala joeys will finally find independence and their mother can re-breed.

 

An extended dependent period among mammals allows for infants to immunize with microbial flora necessary for digestion of poor-quality vegetation. Koala joeys feed on their mothers milk, leaves from the eucalyptus tree and what is known as "pap". Pap is maternal fecal matter that is thought to be very important for a joeys gut biome, introducing the infant to necessary bacteria they will use in order to break down otherwise toxic eucalyptus leaves. A koala's pouch, unlike other marsupials, opens downwards, which allows for their young to access the pap more easily.

 

It has been suggested that the extent of maternal effort among koalas is regulated by energetic factors, limiting reproductive rate and places a premium upon female koalas who allocate their resources to ensure the survival of their young. The sex of the joey is unrelated to the care they receive by their mothers. The energetic demands of nursing joeys is met by increasing leaf consumption and lowering their metabolic rate. Utilizing this strategy means that increasing fat storage is not necessary.

 

It is predicted that female koalas have a reasonably low fertility rate due to the short-term costs of converting an already poor diet into energy for a growing joey. This compensation maximizes the possibility for infant endurance and future reproductive success. Loss of pouch young is not costly for mothers, however, this loss can postpone reproductive output by one year. After analysis of 20 years of captive koala data from the San Diego Zoo, researchers determined that a joeys size at independence does not determine how large they will grow to be when fully matured; females reach maturity around 3-4 years of age, while males need an extra year of growth.