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Flying Rodents They’re Not

FOF November 4, 2014 Armchair Interpretive Walks

by Patrick Higgins

It was dark. Glen Stacell and I were rolling slowly down Janes Scenic Drive with lights dimmed in his pick-up truck. There was just the soft crunch of tires on gravel. The silhouettes of cabbage palms and cypress contrasted against the evening sky.  Each of us had an arm out a side window holding a small black device aloft listening for clicks.  We were reconnoitering our upcoming new moonlit tram tours and searching for some of Fakahatchee’s bats.

Eastern pipistrelle, Fakahatchee's smallest bat. Photo by Dr. J. Scott Altenbach

Eastern pipistrelle, Fakahatchee’s smallest bat. Photo by Dr. J. Scott Altenbach

Eight of Florida’s 13 species of bat frequent the Fakahatchee; the Seminole bat (Lasiurus seminolus), Big brown bat  (Eptesicus fuscus), Brazilian free-tailed bat (Tadarida brasiliensis), Evening bat (Nycticeius humeralis), Eastern pipistrelle or Tricolored bat (Pipistrellus subflavus), Northern yellow bat (Lasiurus intermedius), Rafinesque’s big-eared bat (Corynorhinus rafinesquii)  and the endangered Florida bonneted bat (Eumops floridanus).

The Seminole bat, Northern yellow and pipistrelle are solitary roosters utilizing clumps of Spanish moss, dried palm fronds and old tree cavities.  The others are colonial roosters. The Brazilian free-tail and Big-eared have now adapted to man-made structures.  The Brazilian likes bridges and picnic shelters, and is particularly partial to the crevices under barrel tile roofs, a penchant shared with bonneted bats. The Big-eared strays less far from its preferred forest habitat and is more likely to use old cabins and sheds. The evening bat is less picky and sometimes found in the folds of patio umbrellas.  All are slow reproducers with typically a single pup born each spring.

Like all bats they are the subjects of many misconceptions, even being referred to as flying rodents. Bats are in fact in an order all of their own, Chiroptera, from the Greek aptly meaning hand-wing, and may be closer related to primates than rodents.

They are a diverse group accounting for almost 20% of all known mammal species and can be divided into two suborders; the megabats or flying foxes of the old-world tropics, sometimes referred to as fruit bats, and the ubiquitous microbats which include all of Florida’s species.

Our bats are small; ranging from our smallest, the Eastern pipistrelle with a 9” wingspan but weighing in at a mere ¼ oz, to our largest, the Florida bonneted bat with a 20” wingspan but, still only about 1 ¼ oz weight. Despite their diminutive body mass bats are extraordinarily long-lived. Some bats weighing less than an ounce have lifespans of 30 years.

Flying foxes in particular put paid to the myth ‘as blind as a bat’. They have excellent binocular 3D color vision.  But even the smaller eyes of our microbats have highly refractive lens and are densely packed with rods. They have good night vision, but nobody goes around saying, “as sharp as a bat’s eyes at night.”  Microbat eyesight is however made largely redundant by their ability to echolocate: to use reflected sound waves to map their environment in the same manner as dolphins.  This remarkable sixth sense allows them to determine distance, size, shape, texture, speed and direction.

Not only are bats not the harbingers of evil portrayed in horror films, they are highly beneficial, providing many important ecosystem services. Not least of these in Florida is that our species are all insectivorous typically consuming their own body weight in insects every night.  Depending on the size of the bat and size of the insect this can total several thousand ‘bugs’, many of which may be agricultural pests.  In other parts of the world they provide pollination services as well.

As Glen and I proceeded, our scanners periodically rippled to life, with the distinctive slower pulses of bats in the search mode, followed by increasingly frantic chirps as they closed on their targets, culminating in a feeding buzz which ends abruptly when the insect is scooped by wing or tail membrane into its mouth.  Occasionally we also picked up insect noise when it intruded into the bats’ frequency range, but this was a like a low muffled roar and easily distinguishable.

We think of nocturnal predators as being quiet, but bats are noisy, broadcasting at 110-120 decibels, which is the same level as standing next to a jackhammer. So it’s a good thing that most of their calls are ultrasonic and well above 20 kHz, the upper limit of human hearing.  It makes perfect sense being so loud if you’re trying to resolve a moving object as small as a mosquito.

Our Swiss bat detectors are only the size of cigarette packs but quite sophisticated, as well as quite expensive, although I won’t say quite how much in case my wife reads this.  They automatically scan the full chiropteran echolocation frequency range, which varies by species. They then transform the chirp into our audible range, modulate its volume, and display its frequency. This provides a clue to identification, but only a clue as the call ranges of many bats overlap.  However, armed with this and the knowledge of what bats are likely to be found in any given area, one can make a pretty good guess.  Even more sophisticated bat scanners can record wave patterns that can be downloaded onto a computer for analysis and positive identification.

But this aerial combat is not all one sided.  Many insects can detect the bats signals and take evasive actions or apply countermeasures.  Some moths will fold their wings and suddenly drop to the ground. Tiger moths take a different tack. Their larvae feed on toxic plants that make them taste nasty so they emit a series of clicks to warn off an approaching bat.

Earlier in the day Glen had a sugared a few trees along our route with a homemade concoction of rum, molasses and brown sugar.  I would rather have drunk it, but he painted it on several trunks in the hope of attracting moths. The idea was to see what the bats might be feeding on.  It worked well up north, but we didn’t have any luck with it that night, so will revert to the time-honored tradition of a white sheet and backlight for our tours. We had better success with the fireflies and were able to attract a few towards us using a keychain LED flashlight, but you have to get the flash pattern just right for the particular species.  We look forward to sharing our new skills with our moonlight tram riders and boardwalk-after-dark strollers.