Dec 12, 2015
I caught my first group of common vampire bats and brought them to the field station. It was important that all the females I captured came from the same roost. At 5:52 pm on Dec 12, 2015, I started observing the entrance (a 1 meter high triangular hole) to a large hollow-tree vampire roost. One or two Saccopteryx bilineata roosted just inside the entryway to the left. Further up and right, there were easily more than a hundred vampire bats staring back at me. The floor of the roost was a pool of black liquid, a mixture of urine and digested blood. The overwhelming ammonia fumes bleached the bats’ fur and made it impossible to breathe when sticking one’s head inside.
My plan was to catch the bats as they exited the tree by trapping them in a large screened area directly in front of the entrance. Then I would scoop them up in hand nets and toss them in a small cage. My worry was that the bats would detect the trap from the entrance, then fly back in the tree or never leave. So I decided to let the bats fully investigate the screen trap and fly through it on the first night. Bats must have to habituate to sudden changes to the area in front of their roost, when for example, a large tree branch falls in front of the exit. After a brief investigation with no negative experiences, they should become accustomed to these obstacles such as fallen tree branches or bat gates. The roost was large enough that I did not fear them switching to another.
My other goals were to note what time the bats started to emerge, to see if they emerge in one large group or small clusters, and to record social calls and echolocation from the emerging bats.
I positioned a 4 x 4 meter screen tent (with two opposite-wall doors) at the entrance and tied the first door directly to the roost entrance. I opened the second door (4 meters directly in front of the entrance door). Bats could fly straight through both tent doors, but those turning as they exit would encounter mesh screen walls. I positioned an Avisoft microphone 2 meters directly in front of the exit allowing me to get high-quality recordings from flying bats as they flew directly towards the microphone at a distance of 0-6 meters. Vampire bat calls are low intensity, so the microphone gain was set to max. I was positioned about 7 meters farther back where I could monitor the recording and record events with an infrared-sensitive camcorder.
At about 6:40 pm, the two Saccopteryx entered the tent and flew back in repeatedly. At 6:55 pm one roosted on the side wall, then it flew out the door. The first vampire bat came out at 7:06 pm; it entered then flew back in to the roost. For the next hour, 1-3 vampires entered the tent about once every 1-10 minutes.
This was good news: it meant the bats were coming out gradually, allowing us to maybe catch one or a few at a time in hand nets. Of the bats that entered the tent, 50-75% flew out and then looped back into the roost, but this activity may have been one or a few of the same individuals. Sometimes they crashed into the side walls. The remaining bats flew out the door. At least one bat landed and crawled a bit on the mosquito netting wall and I could observe it echolocating back and forth across the tent interior. I also observed 2 bats flying back through the tent and into the roost from the outside. It was too difficult given the poor infrared lighting to get an accurate count of bats entering or exiting the tent. Finally, I noticed that at least one bat was circling me while I sat watching.
From 8:11 until 9:11 pm, I counted 9 singles, 3 pairs, and 3 triplet flights into the tent (24 total), and these led to 6 crashes, 7 exits from the tent, and 1 return flight from the outside. Five bats then exited without circling or crashing at the following times: 9:11, 9:13, 9:20, 9:21, 9:22. I then stopped observing. I wanted to make sure that the remaining bats would have time to feed during the night. So at 9:30 pm, I detached, untied, and moved the tent away from the tree (about 15 meters away) and left.
To me, it seemed that a few bats were checking out the tent and then eventually leaving. There were many bats still inside the tree when I peered inside at 9:30 pm. Based on what I saw, I thought it was likely we could catch 20-40 females before 9 pm. With a 5-h drive, this would put us back home at 2 am.
But I was wrong…
Dec 13, 2015
PhD student Victoria Flores, her partner Michael Le Chevallier, my wife Michelle Nowak, and I set up the screen tent outside the roost and waited. This time the second door was zipped shut. A storm sounded like it was approaching, so I decided to not wait for dumping rain and to flush the bats out by crawling inside. I had a bit of trouble squeezing in. Michelle (my very brave wife) entered first after putting a garbage bag over her upper body. I crawled in afterwards. It was the most disgusting place I’ve ever been. I’ll just put it this way: it was like crawling into the rectum of a vampire bat.
Unfortunately, the living wall of bats skittered upwards and could not be reached from the floor inside the hollow. The ammonia fumes also meant we could not spend time inside the tree without poisoning ourselves. We waited outside.
After disturbing the bats so much, it was clear that they knew we were there and they were not coming out. I decided to switch capture strategies at 9:30 pm. We took down the screen tent trap, covered the roost entrance, and quickly set mist nets in a U-formation around the entrance. We then backed away behind the tree and entrance, then turned off all our white lights.
We began consistently catching male vampire bats. Many of them were coming into this roost. Next, we began catching a mix of male and female bats exiting the roost. Many seemed younger based on their appearance, and the females often did not yet have a bare patch around the nipple, meaning they had not nursed a pup yet. We caught maybe 2-3 males for every female. I suspect most of the adult females stayed inside the roost, and came out after we left at 12:38 am.
We caught 19 males and 22 females and took them on the 6+ hour drive back to Gamboa. All the bats survived this grueling all night drive. Victoria and Michelle took turns driving, because only they knew how to drive standard. We kept them in a rabbit cage and a bird cage lined with plastic mesh. I kept worrying that the bats would escape their cages and begin feeding on us in the car. (One of the doors actually became slightly ajar, but no bats escaped–thankfully).
We arrived back at 730am, and the were left alone with thawed cow blood at 8am. Thick black plastic was draped over the cage to make them feel less stressed and encourage feeding.
I was surprised at how many male bats were coming to visit this tree early in the night. I had observed this same pattern of early male visits near Lamanai, Belize. I set nets outside a maternity colony in some ancient ruins and instead of catching females coming out, I caught many males going in. Also in Belize, I noticed that early in the night, we caught only males. Then at 2 am, we began catching all adult females. In his field studies in Costa Rica, Jerry Wilkinson had previously observed that males not only compete over access to large female groups in hollow trees but also visit the female roosts during the night (presumably before or after the female leave to forage).
Thanks so SO much to Victoria, Michael, and Michelle for all their help. The bats are hopping around in the flight cage and we are finally ready now for our behavioral experiments after a long delay.