Both traits point the same way: intense sperm competition. Female honey possums mate with several males in quick succession, so the sperm of different males compete inside the female reproductive tract. Where that happens, selection favours large ejaculates and, in this lineage, unusually long cells, each measuring around 0.36 millimetres, roughly six times the length of human sperm.
The rest of the animal is equally specialised. It survives only on nectar and pollen, taken with a tongue tipped in fine bristles that mops flowers, and its teeth are reduced to a few blunt pegs. Its heart rate and metabolic rate are among the highest of any mammal, and it enters torpor on cold nights to avoid starving before morning.
Because it depends on flowers being available all year, the honey possum is confined to the southwestern Australian heathlands where banksias and related plants bloom in overlapping succession. It is a pollinator of several of those plants, transferring pollen on its snout and fur between visits.