It was more than a year into their project before they had enough data to realize their early results were no mistake. The signals of mother and baby bats had diverged because the mothers were carefully ditching their babies in trees while they searched for food.
“We couldn’t imagine that the mother would just leave a pup on a tree,” Dr. Goldshtein said.
Over five years of field work, they discerned a clear picture of what was going on. When Egyptian fruit bats pups are a few weeks old, mothers carry them from the cave at the start of the night, as usual, then fly to a tree and leave them — sort of like day care drop-off, without supervision. The mother returns throughout the night, perhaps to nurse and warm up the pup. When she’s done foraging, she carries the pup home.
The mother uses the same tree, or a few trees, over and over. As the pup gets older and heavier, the mother shifts to a drop-off tree closer to the cave.
Then, when the pup is around 10 weeks old, the mother leaves the cave, alone. The young bat emerges from the cave for its first solo trip — and, though there are thousands of trees nearby, flies straight to its most recent drop-off site. As it grows older, the pup uses the drop-off tree as a starting point for its own exploration.
“We were amazed to see these results,” Dr. Goldshtein said. Somehow, while hanging from their mothers’ bellies, baby bats learn their way around. The authors don’t know exactly how this learning happens. They think it may be by sight, although Egyptian fruit bats can echolocate using clicks of their tongue.
Mirjam Knörnschild, a behavioral ecologist at the Museum of Natural History in Berlin who studies bats, said that the authors had done a “great job” uncovering the poorly understood interactions between mother bats and pups. “The results strongly suggest that mothers actively help their pups with orientation,” she said.
Dr. Knörnschild was surprised that pups can memorize these routes while being carried upside-down and while never flying the routes themselves. “Personally,” she said, “I find it astonishing.”