The Bat Came Back

Reversing Species Declines

Bats are a fascinating, though often overlooked, group of mammals. This is despite the fact that they are the second largest taxonomic mammalian Order, behind only rodents, and make up about 20% of known mammal species (over 1,200 in total). They are the only mammals capable of sustained flight (flying squirrels and gliding possums can only glide for relatively short distances). They also fill important ecological roles including pollinating flowers, dispersing seeds, and controlling insect populations. We’ve discussed bats briefly before, in the final story of Season 2 on vampires, but today we’ll be looking at a different species as an example of a conservation success story.

The gray bat, Myotis grisescens, is native to southern Appalachia, west to Missouri, and south through Georgia and Alabama (i.e. much of the non-coastal southeastern US). There are 16 species in its genus native to the US and it is one of the largest, about 3–4 inches long. One unique aspect to its physiology is that its wing membrane connects at the ankle, while in all other species of Myotis it connects at the toe.

Myotis grisescens is as close to a stereotypical bat as one could get. It roosts in caves and nowhere else, unlike other bat species that will use a variety of roosting sites (even artificial ones like bat houses). It is a nocturnal insectivore that uses echolocation for foraging, navigation, and communication. Like all microbats (the group of bats who use echolocation, as opposed to fruit bats or flying foxes, which have lost it), its vision is poor, but it is not, in fact, blind.

Because they need caves for roosting and hibernation, gray bats are especially vulnerable to any activity that disrupts these cave environments. This includes the building of dams leading to the flooding of upstream caves. Gray bat populations began declining in the 1960s and the decline reached 50% by the 1980s. It was added to the US Endangered Species Act in 1976 and listed as Endangered by the IUCN beginning in 1982.

These declines were primarily caused by direct human disturbance and vandalism of gray bat caves. Many bats were caught for entertainment or to be used for pranks. Entire colonies were destroyed out of fear that the bats carried rabies. Spelunkers could inadvertently harm the bats just from their presence, especially in winter. Even a small amount of light or sound can disturb a roosting bat. If it is hibernating, that disturbance can use up valuable energy reserves required to survive. Each arousal from hibernation can use up enough energy to last an individual bat 10–30 days of hibernation. If the bat must then leave the cave too soon, especially before its insect food sources are available, survival chances are greatly diminished.

Another important factor in gray bat population declines is toxicity from pesticides, specifically DDT. This pesticide is best known for its contribution towards raptor population declines such as the bald eagle, Haliaeetus leucocephalus, and the peregrine falcon, Falco peregrinus. These effects were pointed out by Rachel Carson in her famous book Silent Spring. While bird declines were often a result of eggshell thinning, gray bat mortality likely occurred by ingestion of contaminated prey by adults leading to a lethal dose delivered from lactating females to flightless juveniles. DDT was banned in the US for non-public health uses in 1972.

In the early 1980s, the US Fish and Wildlife Service began efforts to protect and restore the gray bat. These included implementing cave grating and other measures to limit human access to gray bat caves. In this way, disturbance of hibernating bats has decreased and gray bat populations are beginning to recover. Although it is still listed as Endangered by the USFWS, it has recently been upgraded to Near Threatened by the IUCN. This is mainly because its total population is now slowly increasing, though there is still the possibility for declines to resume.

One of the current reasons for concern regarding gray bats is a fungal infection known as white-nose syndrome. It has been compared to chytridiomycosis, the amphibian fungal disease which I mentioned two weeks ago. However, it has only been known for a decade, but it has still been responsible for at least 5–7 million bat deaths in that time. Myotis grisescens is one of the bat species that has been documented presenting the signs of white-nose syndrome.

The disease is named for a distinctive fungal growth around the muzzle of the affected bats. Symptoms include abnormal behavior such as frequent or long arousal from hibernation and subsequent loss of energy reserves. The fungus can also damage or scar a bat’s wing membrane. The fungus is spread via contact and can be transported from cave to cave by humans as it can persist on clothing. There is currently no known cure and efforts at prevention are focused on limiting cave access in affected areas to halt its spread to other caves.

While it is certainly not out of the woods, the gray bat can definitely be counted as an initial conservation success. After historic declines, population numbers are now beginning to increase thanks to conservation efforts. It can be easy to lose hope when looking at the stories of some of the species I have shared this season, especially those under higher categories of threat. The gray bat shows that our efforts are not in vain, and that it is possible to help a species rebound from problems created by human expansion and interaction.

We have one more week left in Season 3, and then I will again have a one month break to rest and prepare for the next season. With that in mind, I am currently opening the floor to any and all suggestions that readers might have for season themes going forward. Let me know what kinds of stories you enjoy, and what topics you would like to see me cover. I will give a shout-out to anyone whose ideas I incorporate.