The Great Mexican Bat Bash

NOTE: No bats were harmed in the making of these movies or during data collection!

This summer we spent developing some projects in Mexico. We mistnetted bats and
recorded their calls using ultrasonic detectors. This methodology is great fun when it works, but we did spend a lot of time fussing with equipment, some of which we never did get to work. As a side topic: last summer I had hired an undergraduate (with a dual major in wildlife science and computer science—note to those of you looking for careers!! These skills are highly valued!!) to work in Delaware with one of my graduate students. She had our program figured out in three days and was instrumental (forgive the pun) in getting our equipment working, but sadly, she wasn’t in Mexico with us. We did, eventually, get the ultrasonic detector working and hooked into the laptop to record bat calls. This detector translates bat calls (which are usually at a frequency too high for human hearing; greater than 20 KHz) into lower-pitched sounds we can hear. Then we can download these calls onto a laptop to view them. These techniques allow us the listen in on bat behavior.

We were in the State of Jalisco along the west coastline. This area is known for its beautiful beaches, great reefs, pozole, a soup with hominy, and ceviche, raw fish or seafood marinated in lime juice. The habitat here is classified as dry tropical forest and most of the trees are bald during the dry season. We were lucky to be there at the beginning of the rainy season, when the trees just start to leaf out. Within two weeks the landscape went from brown and grey to bright green. It was a novel experience for someone used to the ever humid and incessantly green of humid tropical forests, such as the Amazon.

In Mexico, we used mistnets (very fine threaded large nets that are used to trap both bats and birds) to capture bats. We then recorded information about the bats (sex, weight, size, reproductive condition) and turned them loose. Although, we were very careful and gentle in handling these guys, a few became very aggravated! See the first video clip. You can hear Luis and Pamela talking about the bat (Artibeus intermedius) and you can also hear the bat yelling at us. Luis has red color on his gloves; this is not blood, but is bat feces loaded with red-colored cactus fruit juice! If we translate what the bat is saying, it’s something like this: “Hey, you big lout! Let me go! I’m going to call my lawyer! What’s going on here, anyway!”

We hear this bat in the video because bats’ distress and defensive calls are pitched at lower frequencies than their commuting or foraging calls. It makes sense. If a bat wants to scare away a predator, its angry calls should be in the audible range.

{vidavee id=”5256″ w=”320″ }

We also recorded bat calls in the field for several hours per night. We recorded bats at various locations and habitats using ultrasonic detectors. One of the most fun places we went to was right at the beach outside of an open restaurant and bar. In their parking lot was a large calabash tree (Crescentia alata). This species of tree has flowers and fruit that grow directly from the trunk and it is pollinated by bats. As we recorded bat calls, we tried to get some video of bats flying to the flowers, but it was very difficult because they were moving so fast and we were juggling numerous pieces of equipment. We did get this interesting video, though. You can see the flowers on the tree as I pan across it. In the background you hear music from the nearby bar. The loud chirps are the leaf-nosed bats “hitting” a flower for nectar. As they stick their head into a flower while on the wing, they make a very distinctive sound that is different than their approach call. As you watch the video, you will hear a human cough, and then look at the upper left of the frame to see a quick little bat fly by. You also hear a soft hissing sound in the background. That is the ocean, only 100 meters away.

{vidavee id=”5257″ w=”320″ }

It made for a rather amusing scene: I sat in the back of a small pickup holding a laptop and the camera, while Dave held the bat detector close to the tree. The restaurant’s customers drove up into the parking lot, which was patrolled by a couple of large, possibly armed men who had kindly allowed us to be there. Both of us are wearing headlamps and rumpled field clothes, while the parking lot guards wore uniforms. The customers were in linen suits and tiny cocktail dresses and spike heels. We got a lot of curious looks, but it was fun to hear the music while we worked! At least this night they were not playing heavy metal. Generally, the bat detector did not pick up the sounds from the music because the frequency was too low (the detector is tuned to the higher frequencies that humans can’t normally hear). However, car engines and the occasional pop-top have sounds that range into the ultrasonic and these made for some very interesting patterns on the computer screen!

For why bats are important and also at risk, See Tigga’s blog: http://www.pulseplanet.com/sci-diaries/sd_kingston/

For more on the ecology of the area, see:
http://www.worldwildlife.org/wildworld/profiles/terrestrial/nt/nt0217_full.html

By the end of this century, as many as 20% of South East Asia’s bat species are predicted to become globally extinct. For me personally, this a great tragedy, but then I’ve been working with tropical bats for over 17 years, so you might say I’m a little biased! So why should anyone else care about our flying fluffy friends? Well, one of the reasons, is just what an extraordinary loss in species and ecological diversity this represents. To back up a bit — just how many bat species do you suppose there are in the world? I nearly always start off my talks to the general public with this question, because the answer usually comes as quite a surprise to people. I won’t keep you in suspense — the current count is over 1100 species, but it is increasing every year as new areas are explored, and as molecular (genetic) techniques reveal to us much of their hidden diversity. Bats make up a fifth of all mammals, and are found on all continents except Antarctica.

What is so incredible, is that this extraordinary species diversity is matched by incredible ecological diversity. What I mean by this is there is a great range of ways in which bats make a living, from the foods they eat to where they roost, and who they roost with. Although the majority of species eat insects, many depend on plants (fruit and nectar), while there are also fish-eating bats, carnivorous bats and even the infamous blood-eating bats (and just to allay your fears, there are actually only three species of vampire bats in the world, restricted to Central and South America). Overlying this is a huge range of roosting habits and social systems. Some bats just roost on their own under leaves in the forest, others live in caves with over a million individuals. There is pretty much a bat for every occasion (and research interest — which is why so many biologists study them!).

OK, so I’m not supposed to have favorites, but this chap (the Trefoil Horseshoe Bat) here is definitely one of them. This is an insectivorous bat from the study site in Peninsular Malaysia where it typically lives on its own, just roosting under palm or rattan leaves quite low in the forest understorey. Its predominantly a perch-hunter, meaning it dangles from a branch, blasting the area below it with echolocation signals until it detects an insect flying below. It then swoops out to catch the insect before returning to the perch to eat. The elaborate yellow noseleaf helps focus the echolocation signal which is emitted through the nostrils.