A few years ago my friend and neighbor’s youngest son, Nadav, was soon to become a Bar Mitzvah and we were having the first of many festive meals together with friends and family.  I was in their living room chatting with Nadav’s uncle from abroad while gazing outside at the lovely evening.  There hanging upside down from a palm tree outside their apartment, almost within reach, was a bat.  It was grasping a palm frond with its clawed feet, leathery wings rapped around its body, with large brown eyes in a delicate, strange little face, apparently looking back at us.  I often see bats flying around our neighborhood at twilight, and certainly there is plenty of evidence in the form of bat droppings, that bats hang out near our house and eat the carobs from our trees, but that was the only time I managed to see more than a dark shape silhouetted against the darkening sky.

Bats order Chiroptera (Greek for hand-wing) are the only mammals that fly, though some other mammals are able to glide for a limited distance.  Bats' wings are related to our forearms with the thin bones in their wings corresponding to our fingers.  Bats use these ‘fingers’ to change the shape of their wings making them flying acrobats with more maneuverability in the air than any bird.  The more than 1,000 species of bats represent about 20-25 % of all the species of mammals.
   These bat species can be separated into two groups:  the micro bats and the mega bats.  Micro bats are nocturnal, relatively small, mainly feed on insects and use echolocation to navigate.  Mega bats (also known as fruit bats) are also nocturnal, they mostly eat fruit, and most do not have echolocation, although the most common Israeli bat, the Egyptian Fruit Bat does have the ability to navigate using echolocation.  Mega bats, in spite of their name, vary greatly in size, but the big ones are ‘mega’ indeed, for example,
Golden Crowned fruit bats can have a wing span of up to 1.5 meters.

 

Picture of Egyptian Fruit Bat hanging from palm tree by Amram Zabari

Picture of Egyptian Fruit Bat hanging from palm tree by Amram Zabari

 

The bats ability to navigate in compete darkness by means of echolocation was first discovered by an Italian Priest named Lazzaro Spallanzani in the 1800s.  Spallanzani was an impressive biologist who led the way in many fields.  He was interested in how certain nocturnal animals skillfully navigate at night.  Initially he worked with owls.  He hung small bells at the ends of ropes in a large cathedral and found that if owls were forced to fly when there was total darkness they would bump into the ropes.  Next Spallanzani worked with bats.  However, no matter how dark the cathedral was, the bats never touched the ropes and the bells never rang.  Spallanzani blindfolded the bats, still they could navigate.  He then physically blinded them, still they navigated.  Finally Spallanzani seeing that the bats had relatively large ears, wondered if their ears were somehow used in navigation.  To test this he put earplugs in the bats ears and found that with these earplugs, the bats bumped into the ropes making a huge racket of bells ringing.  With earplugs his bats couldn’t navigate at all.  When Spallanzani wrote that bats “saw with their ears” he was ridiculed for this assertion by the prominent scientists of his day.

In the 1800s there was no way for Spallanzani to know that his bats were making loud high frequency bursts of sound beyond the human range of detection.  Just like radar the bats are able to detect these bursts of sound when the sound echoes off surrounding objects.   But, it wasn’t until 1938 that an American Scientist, Donald Griffin, was able to use special microphones to record the high frequency sounds of bats and show that Lassaro Spallanzani was right, bats do “see with their ears”!  Using this ‘radar’ the micro bats are formidable predators against insects.  In one night a bat might eat up to 3000 mosquitoes. 

There was once an amazing diversity of bat species living in Israel - 33 different species in this small area - almost as many as in all of Europe.  However, in the 1950s farmers thought that the most common of these bats, and the bat that I saw outside my neighbor's window, the Egyptian Fruit bat (Rousettus aegyptiacus) was eating their fruit, so bat caves were fumigated with powerful poisons.  The fruit bats were decimated and so were many other species of bat that lived in the same caves.  Not surprisingly these fumigations seemed to have the exact opposite effect than was desired.  By killing off the insect eating bats, the insect population increased and these insects probably caused more damage to agriculture than the fruit bats did.  Since then Dr. David Malkin from Tel Aviv University and others have made a concerted effort to educate Israelis to the true nature of bats, and such fumigations have stopped.  One study to asses the possible recovery of the bat populations is being done in Masua Park by Asaf Tzoar from Hebrew University.  He is using a piece of equipment that is able to record bat calls (a bat detector) and with this is able to remotely identify the species of bats that fly by each night without disturbing what could be an endangered species. 

In 1999 Dr. Carmi Korine from Ben Gurion University of the Negev did an experiment to test whether the fruit bats actually damage commercial fruit production.  He systematically collected bat droppings from bats living in caves in the Carmel Mountain area near several commercial orchards.  He found that only 15% of the bat’s diet was from commercial fruit, while the majority of what the bats ate was either from wild trees or from the fruit in gardens (like the carob trees in my garden).   Even the fruit that was taken by the bats from the commercial orchards was often unsellable overripe fruit. This is the same old story.  Farmers who put their life’s blood into their produce become suspicious that a particular animal is a threat and understandably complain to the authorities, but then instead of doing an objective study the powers that be act impulsively and often cause more harm than good.

In an addition to this story, recently the residents of a neighborhood in the Tel Aviv area complained to the municipality because bats were eating the fruit from the trees lining their streets and causing a mess on the sidewalks and cars parked beneath.  The municipality came back and offered to cut down the trees.  The Tel Aviv area gets quite hot in the summer and the trees provide much needed shade so, thankfully, the people nixed that ‘solution’ before any damage was done.  Less drastic measures are now being considered including injecting the trees with hormones so that they don’t produce any fruit, or discouraging the bats by lighting the streets at night. 

Next time you are annoyed in the morning by what the dive bombing bats have left behind, remember that their cave mates might have eaten 3000 mosquitoes and smile while you clean your car.

 

 

References:

 Carmi Korine, Ido Izhaki, and Zeev Arada.  1999.  Is the Egyptian fruit-bat Rousettus aegyptiacus a pest in Israel? An analysis of the bat's diet and implications for its conservation.  Biological Conservation 88:  301 – 306

 Picture in banner is an Egyptian Fruit Bat, Picture was taken by Adrian Pingstone

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