Fish-eating Myotis

Only the finest seafood will do for the Americas’ largest mouse-eared bat

by Christie Wilcox

Binomial
Myotis vivesi

Family
Vespertilionidae

Colony size
~10,000

Weight
20–30 grams

Diet
Mostly seafood (~90% marine fish & crustaceans)

Status
Vulnerable

Region
Islands in Gulf of California and off Baja California

At first glance, Partida Norte Island in the Gulf of California doesn’t look like much of a home for bats. There are no large caves to shelter them from the salty sea spray or the intense sun, and the dry, desolate landscape boasts little in the way of insects or plant life—“maybe a cactus,” says Dr. Luis Gerardo Herrera Montalvo, a physiological ecologist with the National Autonomous University of Mexico in Mexico City. But, just out of sight, are tens of thousands of fish-eating myotis (Myotis vivesi)—the only bat in the world that dines almost exclusively on seafood.

Despite the vulnerabilities that tend to come from island living, Dr. Herrera’s research has discovered these unusual bats are flourishing on the few islands in the world where they exist. He notes we must prevent predators like cats and rats from establishing on the islands where the bats roost, so they can continue to thrive.

Dr. Herrera first began working with these bats a little over two decades ago. He’d just started at the university when a colleague mentioned little was known about the bat species endemic to the islands in the Gulf of California and nearby coasts. Since these bats live only in that area and nowhere else in the world, it makes them vulnerable to a variety of threats. Herrera made his way to Partida Norte Island for the first time shortly after he learned about the bats, and he or his students have gone back every year since.

It’s a long and dangerous trip. The weather can be unpredictable, and there’s no infrastructure on the island, so everything the team needs has to be carried with them.

During the daytime, the bats roost amongst the seabirds that nest on the island’s jagged cliffs, or beneath the rocky rubble that dominates the island. “You can be walking by, and they are under your feet,” Dr. Herrera says. To take measurements, obtain genetic samples, or tag the bats, you first have to extract them from the rocky rubble. That means moving a lot of rocks, as they tend to wedge themselves about a meter down.

“Every year, we think this is the last time we go,” he says. But every year, he finds some reason to return. “All of a sudden, some new idea comes about, or some colleague shows his or her interest in doing something with this bat, and we just keep going,” he says.

A mysterious fisher

Before Dr. Herrera started working with the fish-eating myotis, pretty much all that was known about them was their striking diet. “They’re the only species of bat that regularly eats marine creatures,” he says.

Over the decades, he and his students—particularly the now-graduated Dr. José Juan Flores—examined the bats’ diets in a variety of ways, from sifting through guano to using advanced molecular techniques. All of these methods confirmed that more than 90% of what the bats eat comes from the sea.

“I like to say it’s a marine mammal,” says Dr. Edward Hurme, a behavioral ecologist with the Max Planck Institute of Animal Behavior in Germany, and one of the many researchers Dr. Herrera has taken to Partida Norte Island. While they aren’t what we typically think of as a marine mammal, the bats are unbelievably well-adapted to a seafaring life. They don’t need freshwater to drink, for instance—they can subsist on seawater. And they’re experts at locating seafood buffets.

That’s actually what led Dr. Hurme to study them. For his Ph.D., he wanted to use GPS technologies to track bat movements. “With GPS tracking, you need clear skies for the signal to transmit, and a bat that you can easily recapture,” he explains. “So, a bat that lives on an island and always comes back to the same spot is ideal.”

His research revealed the fish-eating bats are impressive voyagers. “They go on these really long trips out into the open ocean, over these deep, crazy, choppy waters to find prey,” he explains. They’ll fly out up to 50 kilometers (31 miles) from land in a single evening—not bad for a bat measuring about 14 centimeters (5.5 inches) from nose to tail.

But it’s not clear how they choose their flight path. “We still don’t know how they find the fish,” Dr. Herrera says.

They seem to keep an ear out for other fishing bats, Dr. Hurme notes. “If you play the feeding buzzes of one bat, you get a lot of other bats checking you out—even if you’re on land.”

Still, exactly how that first bat spots the meal in the often flat and glassy waters of the Gulf of California remains a mystery.

The bats’ penchant for marine fare explains their most distinctive feature: their humongous feet, perfect for plucking morsels from the sea surface. Each foot is nearly 2.5 centimeters (1 inch), about three times the length of similarly sized insect-eating relatives. And the long, curved claws are so sharp that they tend to snag on anything they touch. “They would always get hooked on my watch or stuck in my shirt,” Dr. Hurme recalls.

But while their toes are ferocious to fish, Dr. Herrera says the bats themselves are sweethearts. “They are very nice—easygoing,” he notes. “You can tell by how many papers we have published on this species that they are very close to both our academic and our human hearts.”