The Place: The Tasman Sea, 5 miles off the coast of New Zealand. The Time: 30 Million Years Ago. A Spring typhoon crawls eastward toward the coast, drinking the ocean as it goes, putting limbs of lightning steadily one before the other. The loud darkness to the West has eclipsed the sun, but over the island the purple dusk is settling and mixing with the grey smoke overhead. The wind is intolerable. It whips the seawater into a green froth. Yet up there, at the crest of the storm front, you can see the specks of life, flotsam picked up by the wind on its march. A pink pigeon is sucked into the vortex and plummets into the water, breaking its neck. A flock of ibises, or what is left of them, fights the wind but ultimately falls to it. And there, the tiniest speck of all, flapping like a bit of cloth in this squall, is a miraculous bat. She started out his journey with a thousand roost-mates, of which she is the only one left. Exhausted from nearly a thousand miles of traveling atop the wind, she falls into the ocean. But she doesn’t die; instead, she starts swimming with her wings in a clumsy breaststroke over to a piece of driftwood and clings to it with her fragile thumbs. Shivering and gasping, but alive, she hugs the battered wood as she did her mother when she was a bee-sized infant. In the morning, the typhoon is past, and she wakes up shipwrecked on the shore of an unknown land, full of fern trees and ten-foot tall birds, giant centipedes and carnivorous parrots. Bone-wet and near death, she crawls up the beach to the safety of the saltweed. It is a world without mammals. She is the first. She won’t be for long.
Let’s say you want to win a getaway vacation to a remote South Pacific island. Consider all the minute details that go into the winning of that prize. First, you have to pass the most basic qualifications: You have a name and an address, and be able to write them. You have to mail in the form. Perhaps there’s an essay involved. Then comes the probability factor: you must be chosen at random from a few hundred thousand participants. If fortune smiles on you, there’s still the problem of getting to the island. You have three options: flying, boating, or swimming. But the island is a thousand miles away, and the only ferry is a raft without sails or rudders, and the only plane that flies to the island used to contain the skeleton of Amelia Earhart.
Now suppose you actually get to Skull Island and decide to stay forever. To gain citizenship, you must marry someone there. However, no one on the island speaks English, or is otherwise compatible with you in the slightest. The only way for you to find a mate you can stand is to wait for another tourist from your country to win their own getaway sweepstakes. At this point, the probability is slim-to-none.
This silly analogy more or less describes what ecologists call sweepstakes dispersal. Except that, since getting to the island isn’t the goal for most individual terrestrial animals, it’s less like winning the lottery and more like getting marooned on a deserted island. Sweepstakes dispersal has another name: waif dispersal. It pretty well describes our wind-swept bat, or the small reptiles and spiders that might float from the mainland to the island on a raft of vegetation, or the unfortunate mammals that might end up on the shore of an uncharted desert isle after a three hour tour.
One of the most vexing ecological mysteries of our age.
The key to sweepstakes dispersal is probability. The roulette wheel of a hurricane is what any terrestrial animal must pass through before making it over the deep blue sea. But you can improve your odds. Being small helps, if you’re going to raft there, which explains why rodents are far more common on South Pacific Islands than kangaroos. If you’re large, your best bet is to swim… which is why there were elephants on the island of Sicily, among other places. (Other mammalian megafauna, like polar bears, either swim to islands or are brought there by dharma. But the easiest way to get across a large body of water is to fly. Birds do it with relative ease, but when you’re a bat, which needs to consume its body weight in insects every night, and which isn’t naturally inclined to go adventuring over the high seas, you’re going to need a little push from a divine wind.
So if you’re small enough to survive the journey, and if you’re lucky/unlucky enough to get picked up by an errant storm, and if your raft or wind drops you off on dry land hospitable to your species, then you’ll want to turn your dispersal into colonization. If you didn’t travel with a mate, and didn’t arrive pregnant, you’ve got to find another of your kind that was also brought to the island by similar circumstances, which is a pretty tall order. But the Earth is old and large, and lightning strikes twice somewhere every day.
The Lesser Short-Tailed Bat is one of two species of mammal endemic to the islands of New Zealand, the other one being the Long-Tailed Bat. (Hawaii, the most isolated landmass on Earth, boasts a single species of bat as its only indigenous mammal.) As prehistoric New Zealand was populated entirely by those animals that could make the crossing from Australia or South America, the birds and insects evolved to fill the niches left unoccupied by all the mammals that weren’t there: kiwi birds evolved to hunt worms and dig like badgers, while ten-foot tall moas became grazers and browsers, the deer and elephants of the island. So when the lesser short-tail arrived, it found that, not only could the New Zealand ecosystem accommodate a bat, it could also become something more. There was also a job opening for a mouse or shrew.
Yes, the lesser short-tail can fly to catch insects, but it can also crawl on its wrists to find insects in the leaf litter. If the kiwi, another nocturnal insectivore, is any indicator of what happens to winged animals on New Zealand, the Lesser Short-Tailed Bat may be the world’s first flying mammal that is evolving towards flightlessness. Or, at least, it was until we showed up with the cats, rats, and dogs that are driving it extinct.
It also evolved to eat nectar, and has become a major pollinator of the New Zealand’s parasitic wood rose. They’ve even gained their own symbiont, a non-parasitic insect called the batfly. It lives in the bat’s fur and eats its guano. It is so well-adapted to life on the bat that its appendages have become shaped to “swim” through the bat’s fur. Batfly nurseries in the guano deposits are guarded by males which emit a high-frequency buzzing sound which deters the bats from entering to consume the grubs.
So in a way, that first bat to not only make it to the island, but successfully colonize it with a mate, really did win the sweepstakes. She found a new habitat devoid of other mammals (even the slow-breeding marsupials) with unexplored niches in both the air and the forest floor, and could develop symbiotic relationships with other species, which generally helps with survival. From an evolutionary standpoint, it was an island paradise. And she arrived by sheer luck, being the individual mammal with the one-in-a-billion chance of winning. But the universe, that compulsive gambler, was bound to get a mammal to New Zealand somehow after billions of tries with tens of thousands of storms and trillions of bats. To the Adam and Eve bat of Christchurch, they got lucky. To the island, life was inevitable.
Just as every deserted island has a niche for volleyballs, demanding to be filled.