Last time on A Moment of Science we learned about plants that disperse their seeds via zoochory, meaning they've evolved various ways to hitch rides with animals. Some of these plants have hooked fruits that stick to the fur and feathers of passersby. Others display fleshy fruit that entices animals' appetites. A particularly interesting case of this latter form of zoochory is mistletoe, the holiday smooching plant.
Mistletoe plants grow all over the world and their seed dispersal involves a variety of animals, but it's the mistletoe plants of Australia we're talking about. Australian mistletoe has evolved a peculiar interdependent relationship with a bird commonly called the mistletoe bird.
The mistletoe bird survives mostly on the berries of mistletoe plants they follow wherever the plant grows. Mistletoe seeds, for their part, can't germinate unless they pass through the bird's digestive tract. They rely too on what the birds do with the seeds after partially digesting them.
The mistletoe bird's intestinal tract is specialized so that it passes mistletoe seeds within twenty‑five minutes of eating the berries. When the bird excretes a seed, it comes out with a sticky, viscous coating. In order to clean the seed off of its rump, the bird rubs itself against tree branches in a unique fashion some scientists have dubbed its waggle‑dance.
Mistletoe is a parasitic plant that can germinate only on the branches of other plants, so the bird's waggle‑dance, in effect, glues mistletoe seeds exactly where they need to be. In a matter of days, a deposited seed will begin to probe its roots into the host plant's tissue, so that it can access the water and minerals it needs to grow.