The US Bat Decline: What is going on?

Daniel Karp
Sep 20, 2019 · 4 min read

By Melissa Alpizar

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Myotis lucifugus, the little brown bat, is one of the most common bat species in North America. It is often used as a model organism for scientific research. By U.S. Fish and Wildlife Service @ https://www.fs.usda.gov/wps/portal/fsinternet/cs/detail

Bats are very unique, mysterious, and underrated mammals. Through evolution, their forelimbs have been modified for flight making bats the only mammals capable of true, sustained flight. They come in a variety of sizes and morphologies. Their diet can include insects, fruit, and few species feed on blood. Bats can be found worldwide with the exception of exceedingly cold regions. Most bats are nocturnal and many undergo hibernation seasonally. Torpor, which is very similar to hibernation, is a state of inactivity and lowered metabolic rate. Many bat species undergo both. Torpor is characterized as a shorter time period, compared to hibernation. These energy saving states are commonly accomplished in caves, mines, rock crevices, or any other dark, cold, and moist areas.

Bats play an important role in many ecosystems. They can serve as pollinators, seed dispersers, as well as lowering or maintaining insect populations. Bats also indirectly benefit humans by consuming large numbers of night-flying insects including mosquito’s which can carry diseases. Bat feces, known as guano, can also serve as suitable fertilizer for plants and lawns.

Recently, multiple species of these incredible creatures living in the United States have been suffering from a massive reduction in their populations, and the death tolls are on the rise.

Fungal Skin Disease

The culprit is a fungal skin disease called Pseudogymnoascus destructans, more commonly referred to as White-Nose Syndrome. Fungal pathogens typically only affect the skin of animals, but it can have disastrous anatomical impacts across a wide range of species. This particular fungus colonizes on bat’s skin around the muzzle, as well as the bat’s wings. It looks like white fuzz on a bat’s face, which is how the name of the disease arose. Essentially, the fungus causes the hibernating bats to become disturbed from their torpid state, increasing their metabolic rates, and increasing their evaporative water loss. This leads to loss of energy and dehydration causing emaciation and ultimately death. The disease is thought to be transmitted from bat to bat because of their close proximities to one another whilst in torpor or hibernation. Millions of bats have been fatally affected by this fungus since the discovery of the disease in 2007 (McGuire et al. 2017).

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This Little Brown Bat found in Great Smoky Mountains National Park shows clear signs of White-Nose Syndrome. By Dolovis U.S. Fish and Wildlife Service Headquarters @ https://commons.wikimedia.org/wiki/File:Bat_with_White-nose_Syndrome_(6847107816).jpg#file

Understanding the Fungus

In a recent study, bats were collected from a hibernating site absent of White-Nose Syndrome and separated into two groups at random (McGuire et al. 2017). One of the groups was administered the fungus, while the other was not. Both groups were kept in identical environmental conditions. Comparisons were made after the two groups of bats were allowed to hibernate. As expected, the bats with White-Nose Syndrome experienced higher torpid metabolic rates which resulted in higher energy use, as well as higher evaporative water loss.

The progressive stage of the disease was also measured using UV fluorescence which revealed the disease on the wings of the bats. Surprisingly though, the study showed that bats with later stages of the disease showed higher water loss, but not necessarily higher metabolic rate. This indicates that disease severity effects water loss, but not metabolic rate. In other words, there is not a direct correlation between water loss and torpid metabolic rate increase, but both are caused by White-Nose Syndrome. Therefore, the damage to the wings due to the spread of the fungus causes dehydration.

Metabolic rate and water loss were measured in both dry and humid air to account for differences in how the disease affects bats in different environments. Higher metabolic rate was seen in the more humid air. Water loss was higher in dryer air. Knowing this, change of humidity inside of caves or other hibernation sites could immensely affect bats with white-nose syndrome.

Saving the Bats

Having a framework for understanding how the disease functions within bats is key for implications to help conserve these species of bats. Currently, only North America is being affected by White-Nose Syndrome, but it has already spread across the country to Washington State, from its origin in New York. There is a strong possibility that the disease may also be transmitted unintentionally by humans carrying the fungus on their attire or gear within different cave systems. Careful management can possibly be implemented to lessen our effect on the spread of the fungus.

Current testing is underway in attempts to remedy the issue through the modification of microclimates at hibernation sites, chemical and biological treatments, and more. A better understanding of the fungal host physiology and disease transmission is necessary to gain effective mitigation strategies. This knowledge will also help us predict the full impacts on bats.

Humans indirectly benefit in many ways from the existence and services of bats. With bats currently declining, the repercussion of their lowered numbers will likely become apparent in the near future. As the disease continues to be studied and further researched, awareness of its presence and effects on bats will hopefully shed light to these night dwellers.

Citations

Mcguire, Liam P., et al. “White-Nose Syndrome Increases Torpid Metabolic Rate and Evaporative Water Loss in Hibernating Bats.” American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, vol. 313, no. 6, 1 Dec. 2017, doi:10.1152/ajpregu.00058.2017.

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