The Rain And The Mist (Net)
August 25, 2019 by anon
No matter how long I lived in La Selva I knew I would never take being woken by a chorus of birdsong for granted. I knew I’d never stop enjoying falling asleep to the sound of crickets, or watching the omni-present peccaries snuffling around, or even having a gecko for a roommate. That gecko was rather good company, though I never actually saw him. He always announced his presence every night at eight or so, then hunted voraciously so I was never troubled by insects in my bedroom. This I appreciated – the hard work, the early mornings, the never-ending bicycling and walking in the forest, the excitement of field work and all the new people I was meeting ensured that I when I went to bed I slept like the dead. Hilda was close to submitting her thesis and we had one flight cage up and running. We were ready to begin our experiment.
The experiment itself was based on a fairly straightforward idea. When looking for food, animals could either associate a cue – a point in space, a colour, a shape, a sound – with the presence of food, or they could learn a searching rule, such as “stay here until the food stops arriving, and then go somewhere else”. Our plan was to carry out an experiment called the ‘serial reversal learning task’ to see which of these strategies the bats used. Each bat would be assigned two flowers to play with. First, they’d receive nectar at only one of the flowers whenever they paid it a visit. After fifty visits, this flower would stop yielding rewards and another one would start. After another fifty visits the possibility of reward would switch again between these two flowers, and so on every fifty visits. When the bats didn’t get rewarded at a flower that had previously been giving them nectar, how quickly would they start visiting the other? Would they associate only the first flower with nectar and be unable to look elsewhere? Or would they learn our searching rule of ‘stay if you’re rewarded, change if you’re not’? We planned to find out.
For several days Hilda had been leaving bait out at one particular spot: the site of an old free-flying bat experiment about one and a half kilometers into the forest, just off the main trail. This, she explained to me, was to get the wild bats to visit the spot so we could set up mist-nets there to catch them.
Our flight-cages were up and running now, which meant we could finally do the thing I’d been lusting for the most ever since I found out I was coming to La Selva: bat-catching in the forest
“Have you ever seen a mist-net before?” Hilda asked me one morning after breakfast, as we walked to the office from the mess.
“Nope.” It was a glorious morning and there were dozens of hummingbirds out among the flowers, but I was in a foul mood. I had just finished a cup of coffee, drunk it to the dregs, only to discover a dead wasp at the bottom of the cup.
“Well, you’ll see this afternoon. They are really thin and fine and the bats can’t detect them, so they fly right in and get caught. Then we take them out, and if they’re right for the experiment, we keep them.”
“Ooh!” I perked up. I had read about using mist-nets in one of Gerald Durrell’s books, Golden Bats and Pink Pigeons, appropriately enough in the context of bat-catching. Durrell and his team had baited their prey with “a galaxy of fruit” on one of the islands of Mauritius, spent the night in the forest being devoured by mosquitoes and returned to their net at dawn to find a horde of angry, tangled bats in there. They had actually had to release some bats back into the forest, so I was sure this was going to be easy.
Famous last words.
Our bats were nectar-drinking animals, so instead of a galaxy of fruit, we used a simple solution of sugar in water. We poured this into a chicken-feeder and hung it at the spot we wanted to catch the animals. As the former site of an experiment, it was in fact a tiny clearing in the forest with a shack in bad repair and a power-supply line. Outside the shack was a corrugated metal sheet, corroded with rust and held up by metal poles to form a sort of roof without walls. The chicken-feeder hung under this roof, which we simply called “the hut”. Every evening we biked into the forest and set up the bait around dusk, when the bats woke up, stretched their wings, and headed out in search of food. The next morning we would collect the empty feeder, count it as a good sign that it was completely empty, clean the dead bugs out of it and bring it back to the office to be washed. An empty feeder meant that the bats had drunk it dry the previous night, which meant they were now well-trained to return to this spot every night. Several days of bicycling back and forth on Hilda and Alejandro’s spanking new bike had left me with a very sore bottom and growing impatience. When were we going bat-hunting already?
That afternoon Hilda dug out an inoffensive looking plastic bag from one of the storage boxes and took me out to the corridor so we could unravel the mist-net and prepare it to be set up. Then she proceeded to pull yard upon yard upon yard of soft, fine, tangled black thread from the bag.
“What the hell is that?”
“The mist-net of course. Don’t worry -” she added hastily, pulling a few dry leaves out of it, “It’s not as bad as it looks! We’ll untangle it, and set it up, and we can fold it so it won’t tangle again.”
“Is that even possible?” I was appalled. I had been expecting something with a stiff frame that snapped into place, and here was a horrendous long black thing that looked like a wad of hair from a shower drain. It was so hopelessly knotted I thought it might be best for us all to put it out of its misery with a pair of scissors.
“We can do it!” Hilda said firmly. She knelt down and took one end of the net on her knee. “Let’s see what we have here…”
A mist-net is usually made up of fine, polyester threads woven together to form a virtually invisible mesh, held up between two metal poles fixed in the ground. The net hangs in several soft folds between strong horizontal strings that span across the body of the net, making it possible to trap a flying animal and entangle it. For the net to be of any use however, it is vitally necessary that there are no knots in its smooth expanse that could be easily spotted from afar. After fifteen minutes of struggle I was beginning to lose hope, when Alejandro wandered by.
“Untangling this mist-net.”
“Yeah? From in there it looked like you guys were doing yoga. You know -” Alejandro stretched his arms vaguely around himself. “Couldn’t see the net from in there at all.”
Alejandro sauntered off to climb a tree and at last we pulled the net apart.
“Phew!” I sighed. “So now what do we do? We have this big, huge net, how’re we going to take it into the forest?”
“Oh this part is easy! Now we fold it up and it won’t get tangled, I promise.”
“No, you do this.”
She made me hold the loops together, and proceeded to rapidly twist the net around itself until it had shrunk to the size of piece of string
She threaded the loops through one of the handles of the bag and carefully pushed the net into the bag. I had to say I was impressed.
“But won’t it get tangled now?”
“Nope, it should be okay. This was the worst mist-net, usually they’re not so tangled.”
We took the net and its aluminium support poles out to the hut, walking this time. We drove the poles into the soft ground, hung the net, and twisted it up again so nothing would fly into it. We would go bat-catching the next day. I was wild with anticipation, so I really shouldn’t have been surprised when I heard thunder in the late afternoon, and the skies opened in a steady downpour. It wasn’t as intense as tropical showers could be, but by five thirty the next evening I knew it was a no-go.
“Are you sure we can’t go today?”
“It’s not such a good idea to catch bats in the rain,” said Hilda ruefully. “It’s not about us, I mean, we can get wet, it’s no problem. But the bats’ll get wet when we’re taking them to the flight cage, and then they get all cold and shivery, and that’s not nice.”
“I know, I get it. We don’t want to make them sick. We’ll go out tomorrow if it doesn’t rain.”
But it did rain the next day, forcing us to call off bat-catching once again. We stayed in, proof-read Hilda’s thesis, read more papers about reversal learning and prayed to the weather gods to spare us. When the rain started after lunch for the third day running, we knew we were losing the waiting game.
They didn’t call it the rainforest for nothing.
Shambhavi’s stories from the Costa Rican rainforest are published as a four-part series here on Amicable Scientists. This is part three; do check out the other parts as well!
Guest post written by Shambhavi Chidambaram.
Shambhavi (AKA Sam) is a biologist studying animal behaviour and neuroscience at the Humboldt University, Berlin. When she’s not chasing bats or running trails, she’s usually busy hunting for a perfectly-tailored humorous metaphor. Science writing for her is an art; she believes her work is done when anybody who reads her pieces has not only learned something fascinating but feels smarter in the process. While Sam mostly writes about biology and Open Science, she’s always keen for new ideas. You can follow her on Twitter at the handle @Quidestvita.