Koalas are sexually and proportionally dimorphic. Determining the impact of sexual selection is difficult and problematic when studying koalas because males rarely engage in agonistic competition, females can control their ovulation after copulation, and copulation is rarely observed in a natural setting.

 

Typically, larger males will challenge each other over seasonal mating rights; these behaviors can be seen across many species, humans being a prime example. Oddly enough, koalas rarely engage in physical conflict and fighting over procreation is no exception, therefore they're sexually dimorphic size differences are not based on sexual selection competition. Instead, it has been proposed that male koalas have evolved as a consequence of endurance rivalry; males use vocal sexual advertisements that are pleasing to female koalas. Female choice may perhaps also play a more significant role when compared to other mammalian species.

 

Size dimorphism can be associated with sexual segregation, niche separation, and dietary differences. Size dimorphism in koalas begins to appear when koala joeys are around 7 months old, at which time they begin to emerge from their mother's pouch. Significant size differences appear when the joey is around 1 years old, and they begin their transition into independence from their mother's. Female koalas invest a substantial amount of time into rearing their joeys, with males providing no parental care after birth. Despite their smaller size, some female koalas would resist their suitors by screaming and striking at their partner, causing the male to abandon mating efforts. Females appear to have the ability to regulate ovulation which could directly regulate male siring success even if copulation occurred.

 

Larger male koalas are better prepared to withstand the rigors of lengthy mating seasons because they carry larger energy reserves which acts as a buffer to loss of mass, a luxury that smaller males do not enjoy. It is hard for any koala, male or female, to create considerable energy reserves based on their nutritional deficient diet. By maximizing the time spent in reproductive activities, larger males can minimize the time spent foraging to replenish themselves. Larger males also emit longer, deeper sounding bellows which effectively communicates to potential partners their size and reproductive fitness, and only males possess scent glands on their chests. 

 

In order to evaluate the role of body size in regulating reproductive output, researchers used microsatellite markers to identify sires. Over a four year study period on Bees Island, Australia, it was found that males were significantly larger than females, with non-sires having equal  body mass to that of females. Each koala was given a score between one and ten which indicated their muscle mass, higher scores corresponding to greater muscle mass. Along with body mass, testes size, head length, and overall condition was recorded. The analysis of 35 male and 35 female subjects produced a correlation between male body mass and reproductive output was identified, whereas in female koalas, body mass did not play a significant role in whether or not they reproduced successfully.