Volume 37, Issue 2, 2018

Early Migration

Adaptation to environmental change

A Mexican free-tailed bat
Courtesy of Ann Froschauer/ USFWS

It’s a sure sign of spring in Central Texas when millions of pregnant bats make their annual pilgrimage to the famous cavern. They arrive to feed on insects and give birth to their pups. Such an event causes the cave’s bat population to soar to upwards of 15-20 million, making it the largest concentration of mammals anywhere on earth.

However, researchers have observed that since the 1990s, this annual migration has shifted approximately two weeks earlier. While that may not sound like much, it might signal a larger trend of warming temperatures in the region. Meteorologists Phillip M. Stepanian and Charlotte E. Wainwright noticed this trend after analyzing historical data from 22 years of nightly quantitative radar monitoring population counts.

In addition to the early arrival of migrating bats, a small population has taken up permanent residency in Bracken—this was not happening two decades ago. A year-round population indicates warming temperatures; the bats are now finding adequate food sources during the winter months. When these food sources usually mean crop pests and other insects, this is not necessarily good news. While researchers are still examining the extent and effect of this shifting migratory pattern, this study has revealed how bats, like many animals, are adapting to environmental change.


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