Marsupials typically have a very short gestation period and give birth to highly-altricial young. These young further mature during a prolonged lactation period, usually within a pouch. Why marsupials have not evolved a prolonged pregnancy has remained a mystery.
We examined testis development in the fat-tailed dunnart (Sminthopis crassicaudata). This species of dunnart has one of the shortest gestations of any mammal, just 13.5 days. They give birth to around 10 young which weigh just 12mg each. We found that testis cord formation in this species occurred right after birth, similar to the timing of testis formation in the tammar wallaby. However, the tammar – although still precocial at birth for a mammal – has a 26.5 day gestation and young weigh around 600mg . This is almost 50 times the size of a dunnart neonate, and tammar young are far more advanced in terms of their development and organogenesis. In fact, testis formation seems to be developmentally tied to the timing of birth in all marsupials examined, regardless of their size or developmental stage. Together with our previous work, which has shown that estrogen can drive female development in XY gonads in marsupials, we propose that circulating maternal estrogen may prevent testis formation occurring during gestation. Therefore, the unique marsupial mode of reproduction may have evolved to enable the offspring to complete sex determination outside of the maternal environment.