Honey possum: tiny nectar-lovers with enormous testes and giant sperm

Size: Length 68mm (head and body), plus an 83mm tail; weight 5-12g.

Habitat: Honey possums occur in the south-west of Western Australia, where diverse heath vegetation provides a year-round supply of nectar-producing flowers.

Conservation status: Least concern.

Superpower: Honey possums have the largest testes relative to their size and longest sperm of any mammal but produce the smallest young.

Honey possums are extremely specialised Australian mammals that are so different from other species that they are classified into a family of their own. Honey possums are tiny, mouse-sized marsupials that have an elongated snout, three black stripes on the back, a long semi-prehensile tail, and very few teeth. They live in sandplain vegetation in the south-west of Australia where they feed on nectar from plants such as banksias and dryandras.

Honey possums are the only marsupials to exclusively feed on nectar and have several adaptations to facilitate this highly specialised diet. They have a flat, elongated snout with a very long, brush-tipped tongue that allows them to access the pollen and nectar within flowers. Their teeth are few in number and very reduced in structure. Their small body size, semi-prehensile tail, and grasping hands and feet let them climb on tiny twigs to reach flowers at the tips of stems. Their kidneys facilitate filtration of a high-water-content diet; they can produce more than their body weight in urine a day.

A nectar diet is very easy to digest, so honey possums have a very simple gut and because they obtain so much energy from their food, honey possums have the highest metabolic rate for their body size of any marsupial. They consume almost their body mass in nectar and pollen each day. However, such a specialised diet means that honey possums have a restricted distribution; they only occur in sandplain areas in the south-west of Australia where the extremely diverse heath vegetation means there is an abundance of flowers year-round.

Honey possums can use daily torpor to reduce their energy requirements overnight, allowing their body temperature to drop as low as 5°C. However, they can’t hibernate for multiple days and so need almost-continuous access to flowers for a supply of nectar.

Male honey possums have enormous testes, which weigh more than 4% of their body mass and also produce the longest sperm of any mammal. This allows for the dominant females to mate with multiple males and competition between the sperm determines which male sires the offspring. A single litter may have multiple fathers. Females mate 2-3 days after giving birth and the resulting embryo undergoes diapause; its development is put on hold until the current young leave the pouch.

Honey possums have 2-3 pouch young at a time, which remain in the pouch for 55-65 days. New-born honey possums weigh less than 0.005g; they are the smallest of all newborn mammals. Honey possums can breed year-round, but most pouch young are found in winter following good rainfall when nectar-providing flowers are most abundant.

Honey possums are nocturnal and sleep during the day in bird nests or the stems of grass trees. Females are larger than males and are dominate, but most of the time honey possums are solitary. The honey possum plays an important role in pollination of the great diversity of plants, particularly for the Proteaceae and Myrtaceae family that occur in their habitat, as pollen collected on their head and face as they feed from flowers can be transferred between plants. Honey possums are currently not threatened, but climate change, increasing fire frequency, and plant disease such as die-back are of concern, especially considering their restricted distribution.

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