To Hibernate, or To Migrate?Bats in Mexico

By Julie Etra

Mexico is well-known for hosting migrating birds and butterflies on their seasonal journeys north and south. Bats? Maybe not so much, but it’s hard to tell. In cooler climates, the majority of bats just hibernate for the winter. It’s apparently very difficult to track bat migratory patterns, so there’s only one bat that’s well known for migrating south to winter in Mexico.

Bats have been getting a lot of bad press these days, given that they were the most likely source of the spillover, the technical term for pathogens jumping from animals to humans, of the COVID-19 outbreak that started near Wuhan, China last year. Bats were also responsible for the SARS virus outbreak in 2002 and are notorious vectors of rabies. Bats carry a huge assortment of viruses to which they are not susceptible. Spillovers generally occur when we humans encroach on a wild animal’s habitat. It can happen in reverse as well, as COVID-19 is known to have recently passed from humans to the mountain gorillas of the equatorial African rainforest in Uganda, Rwanda, and the Democratic Republic of the Congo.

Humans have passed a nasty fungus to bats, called the white nose syndrome, most likely from Europe, that has led to huge die offs of this essential mammal, particularly when they are hibernating and vulnerable. This is unfortunate, because bats are extremely important in many ecosystems. They consume insects that would otherwise damage crops, and pollinate numerous species of plants, including agave, or maguey, as it is called here in Mexico. Besides insects, nectar, pollen, and fruit, some species also eat vertebrates. According to science writer David Quammen, “A single colony of big brown bats in the American Midwest, by consuming 600,000 cucumber beetles in a year, prevents 33 million cucumber beetle larvae from feeding on the next year’s crop. Mexican free-tailed bats eat cotton bollworm moths in Texas. By one estimate, from 2011, bat predation on insects was saving $23 billion annually for agriculture in the United States.”

Bats are a hugely diverse group of mammals, varying in habitat, behavior, diet, morphology, longevity, you name it. They are the second most diverse group of mammals following rodents (mice, rats, rabbits, and other chewing animals). There are over 1,400 species of bats – among them is the much maligned but well named common vampire bat, Desmodus rotundus, which occurs from Uruguay to Mexico, including our lovely Parque Nacional de Huatulco. The rotundus part of the name, which means “portly,” comes from the fact that they get so fat after drinking blood they can’t fly again until they pee away a substantial amount of urine.

The two traits in combination that uniquely characterize bats are that they have “colonized” the air and they are nocturnal; they fly and feed at night. Bat species that eat insects have an extraordinary capability – they hunt by “echolocation,” that is, they emit high frequency sounds that bounce off their prey (e.g., swarms of mosquitoes) and bounce back to the bats’ highly sensitive ears.

Bats in Huatulco

The National Commission on the Protection of National Areas finds that here on the Oaxacan coast, and more specifically in the Parque Nacional de Huatulco, have six species of bats in the park.

Great fruit-eating bat (Artibeus lituratus). Obviously, this bat eats fruit, and occurs from Mexico through southern Brazil, and on some islands in the Caribbean. They are polygamous with groups called harems, one male and two to five females. They change their feeding behavior with the position of the moon, decreasing feeding time when it is full, most likely to avoid predators that hunt by moonlight, like owls.

Jamaican, common, or Mexican fruit bat (Artibeus jamaicensis). This is a large, stout bat that roosts in caves, hollow trunks, and under palm leaves. Its range is Mexico to northwestern South America. It loves figs, which don’t grow in the Parque Nacional in Huatulco, but does eat other fruit and vegetation. Because it carries its food all the way back to its roost, it is an important seed disperser. The Mexican fruit bat also has harems, and can live as long as nine years.

Little yellow-shouldered bat (Sturnira lilium) is another frugivore, critical for seed dispersal. It is opportunistic in its eating habits, feeding on whatever is available.
Palla’s long-tongued bat (Glossophaga soricina). This nectar-feeding bat is super interesting. It has the fastest metabolism ever recorded in a mammal, similar to that of a hummingbird. Although it uses 50% of its stored fat over a day, over 80% of its energy comes directly from simple nectar sugars as soon as the bat consumes them. Its tongue is are powered by bloodflow and the tip can increase by over 50% in length.

Vampire bat (Desmodus rotundus), described above.

The fishing or greater bulldog bat (Noctilio leporinus) occurs from Mexico to northern Argentina and on most Caribbean islands. It uses echolocation to detect waves made by fish, its prey.

As for the bat that migrates, it’s the Mexican (or Brazilian) free-tailed bat, which likes to live in caves, although it will make do with a bridge underpass if it has to. In the summer, it lives – and breeds – in the Carlsbad Caverns in New Mexico; in the winter, it moves to southern Mexico and Central America. The Mexican free-tailed bat makes a formidable migrator: if they get a tail wind, they can cruise along at 60 miles an hour, and they’ve been tracked at an altitude of 10,000 feet.

A 2013 study in Ecosphere, the journal of the Ecological Society of America, located winter cave roosts for the Mexican free-tailed bat in Hidalgo, Michoacán, Jalisco, Querétaro, and Chiapas, but who knows? In Huatulco, a popular cocktail-hour pastime in Santa Cruz is to take your margarita and beach chair to sit on the greenspaces atop the Sector E canals. Bats about the size and color of the Mexican free-tailed bat emerge in droves at sunset.

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