The honey possum (Tarsipes rostratus) or noolbenger is a tiny
Australian marsupial weighing just seven to eleven grams for the male, and eight to sixteen grams for the female about half the weight of a mouse. Their physical size ranges from a body length of between 6.5 – 9 cm. They have a typical lifespan of between one and two years.
It is one of the very few entirely nectarivorous mammals; it has a long, pointed snout and a long, protusible tongue with a brush tip that gathers pollen and nectar, like a honeyeater or a hummingbird. Its teeth are fewer and smaller than is typical for marsupials.
Floral diversity is particularly important for the honey
possum as it cannot survive without a year-round supply of nectar, and unlike nectarivorous birds, it cannot easily travel long distances in search of fresh supplies. Radio-tracking has shown, however, that males particularly are quite mobile, moving distances of up to 0.5 km in a night and with utilisation areas averaging 0.8 hectares. Both its front and back feet are adept at grasping, enabling them to climb trees with ease, as well as traverse the undergrowth at speed. Honey possums can also utilise their tail (which is longer than their head and body combined) to grip, much like another arm.
Breeding depends on the availability of nectar and can occur at any time of the year. Females are promiscuous, mating with a large number of males. Competition has led to the males having the largest testes relative to their body weight for any known mammal, being 4.2 per cent. Their sperm is also the largest in the mammal world, measuring 0.36mm. Gestation lasts for 28 days, 2-3-4 young being produced. At birth they are the smallest of any mammal, weighing 0.0005g. Nurturing and development within the pouch lasts for about 60 days, after which they emerge covered in fur and with open eyes, weighing some 2.5g. As soon as they emerge, they are often left in a sheltered area (such as a hollow in a tree) while the mother searches for food for herself, but within days they learn to grab hold of the mother's back and travel with her. However, their weight soon becomes too much, and they will stop feeding off milk at around eleven weeks, and start to make their own homes shortly after this. As is common in marsupials, a second litter is often born when the pouch is vacated by the first, fertilised embryos being stopped from developing - see diapause.