Museum specimens reveal shrinking flying foxes in Madagascar, with commercial hunting suspected

Flying foxes are the biggest bats in the world; they look like German shepherd puppies with 5-foot (1.5-meter) wingspans. But Madagascar's flying foxes are shrinking.

In a new study published in the Journal of Mammalogy, scientists showed that these giant fruit bats have been getting smaller over the past several decades, a change that the researchers suspect is related to an increase in large-scale commercial hunting.

Museum specimens reveal a size gap

The paper's authors, both scientists at the Field Museum in Chicago, had noticed that the flying foxes represented by skulls and skins preserved in the museum's mammal collections seemed to be larger than the members of the same species they encountered while doing recent fieldwork in Madagascar. The researchers wondered why the modern bats were smaller than the older ones collected decades earlier.

"I was thinking about a million different angles to explain why the bats were getting smaller. I wondered if it was related to changes in temperature or habitat," says Emily Cornelius Ruhs, a postdoctoral researcher in the Field Museum's Negaunee Integrative Research Center and the paper's lead author.

But when Ruhs talked to her colleague Steve Goodman about it, he had a different hypothesis in mind. Under Madagascar's national legislation, these fruit bats are considered vermin because they consume commercially important fruits. Flying foxes are classified as a game species and are currently hunted for food on a large scale in certain portions of the island nation.

Goodman, the Field Museum's only full-time field biologist, has been living and working in Madagascar for nearly 40 years, and he noticed after looking at late 18th- and 19th-century museum specimens that "many of these animals were of monster size compared to animals we handle today. Hence, the idea was formed that they have diminished in body size as a result of recent heavy hunting pressure and they are not living as long as they did in decades past. This underlines the importance of historical museum material in helping scientists to understand change through time," says Goodman.

A sharp decline around 1980

Goodman then visited the collections of nine natural history museums across the United States and Europe, where he examined 121 Madagascar flying fox specimens that were collected over the course of the past 150 years. He took measurements of the bats' skulls and teeth, and then Ruhs performed statistical analyses of these data to look for overall trends in size.

"We took all those measurements and put them together, and we looked at how they changed over time. We saw a clear drop in size starting around 1980, when we believe bat hunting became more commercialized," says Ruhs. The change was so stark that at first, Ruhs thought it must have been a mistake.

"I thought that there must be something wrong with the analysis; there was no way that the rough estimate we had was correct, but it was."

Larger bats may be easier targets

Ruhs notes that while the study establishes a correlation between the bats shrinking and the commercialization of hunting, the project so far does not explicitly show that commercialized hunting caused the bats to shrink. However, Ruhs says that she suspects that it's no coincidence that the biggest bats were the ones that were disappearing.

"If you're the biggest bat in the sky, you might be the easiest target. You're also going to provide a hunter with the most meat," says Ruhs.

Many of the largest flying foxes identified in this study were also older individuals. If bigger, more mature bats of breeding age are being targeted by hunters, that could affect flying fox populations overall.

However, larger bats being targeted by hunters might not be the only factor in the trend toward smaller bats. "Things like deforestation, climate change, inadequate or shifting timing of the fruit season could all be playing a role in making flying foxes smaller than they were 40, 50 years ago," says Ruhs. "I don't think there's any one cause that explains it all."

Protecting populations under human pressure

But the study's findings suggest that Madagascar's flying foxes are shrinking in size largely due to human-driven pressure. "These bats are an important food source in certain portions of the island for people to feed their families or on the menu in some roadside restaurants, so it is critical to figure out ways to protect the bat populations," says Ruhs.

"This needs to involve higher levels of protection following new national laws, particularly at breeding sites, so that the populations don't collapse."

For Ruhs, the study also highlights the value of museum collections. "We can still get a lot of information from these bat specimens that were collected in the 1800s," she says.