Dicromantispa interrupta

Dicromantispa interrupta

Friday, December 24, 2010

The Bat - WNS Arms Race

By now, most of you have surely heard of the catastrophic malady in bats known as White-Nose Syndrome. It has been called the worst disease to occur in North American mammals in recorded history. Its cause is not entirely understood, but a fungus named Geomyces destructans is believed to be the causative or an aggravating agent. The affliction does not kill its victims quickly; it is more of a wasting disease causing infected bats to awaken several times during hibernation to drink water and forage in vain for insects.

G. destructans infests hibernating bats mainly in the colder areas of the continent, although its range is still expanding and we have no way of knowing how widespread it will ultimately become. It is believed to have originated in Europe and then recently introduced to NA via dirty booted cavers or an infected bat stowaway on a transoceanic ship.

The fungus is likely feeding on bat skin, hair and body fluids. The long, deep folds of bat wings held closely to the body additionally trap moisture, creating a perfect incubation chamber for fungi. A knowledgeable caver recently posted on a forum that the pathogen appears to sap moisture from hibernating bats, as evidenced by low weights of infected bats and observations of them drinking thirstily from almost-frozen water sources in mid-winter near hibernaculums.

Our insectivorous bats are quite small and capable of storing only limited amounts of body fat to tide them over the winter. Each mid-winter awakening thus consumes precious energy. It becomes even more critical if the bat is stressed by being injured, ill or dehydrated, or if it didn’t get enough food in the months prior to hibernation.

Wibbelt, et al. (http://www.cdc.gov/eid/content/16/8/1237.htm) have just published a paper documenting the presence of G. destructans in five species of bats in Europe, noting that the affliction does not seem to be causing any deaths over there. They postulate it is present throughout Europe and that the fungus and European bats have co-evolved together. Importantly, they point out that European bats are generally larger than North American bats.

Imagine a species in the wild having to cope with numerous threats, including parasites, predators, competition, a surfeit of food in summer and a shortage in winter, adverse weather (heat, cold, rain, wind and storms), injuries and diseases. Many animals – yea, some entire species – live near or at the edge of their existence. Bats are numerous and thus can be thought of as highly successful, and indeed, they are as a group, but many of them individually are at the perimeter of their envelope. For them, any new stress added to the existing welter of stresses could be the last straw.

The small bodies of northern bats are at a great disadvantage in the cold, so fewer species live there year-round. Those that do tend to exist in smaller numbers and take advantage of several strategies. One such is to hibernate, but that makes it easier for infectious organisms to find them. Another way is to evolve a larger size, as bears, canids and felids have done, but a larger size has serious drawbacks for a bat. It becomes a more noticeable and desirable target to predators; it requires more food, water, roosting space and foraging habitat; its maneuverability on the wing is reduced, adversely affecting its ability to hunt insects; and it is forced to hunt larger, rarer prey. This is a tack taken by European bats, but WNS-susceptible northern North American bats do not currently exhibit it. Evidently, for them, smaller is better.

Perhaps larger sizes in affected bat species will be selected for more forcefully in the future? Perhaps the ranges of affected species will become reduced in size to where they will live only in warmer climes where insects are available nearly year-round and hibernation is unnecessary? Another strategy to avoid WNS and similar diseases is to hibernate in smaller aggregations, or even singly. Indeed, mass hibernators are currently believed to be especially prone to harboring WNS. Still another prophylactic is to migrate south where hibernation is moot or greatly shortened. But, like larger size, each of these also has its drawbacks. Ironically, the federally-endangered gray bat (Myotis grisescens) already migrates south from its northern summer range, but still hibernates and does so in large colonies and is particularly susceptible to WNS.

It is now time for America’s bats to “co-evolve” with Geomyces destructans. I am left wondering how these bats will respond to the epizootic, whether they will have sufficient time to evolve survival mechanisms or if they will become extirpated from the colder portions of their ranges or become extinct altogether. It would be a grand evolutionary experiment were it not for the horror of millions of cruel deaths and the reduction of Earth’s rich biodiversity.

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